Sisson's anatomy (veterinary)

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Dr. Waqas Nawaz
PMAS arid agriculture university rawalpindi.pakistan.

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Sisson's anatomy (veterinary)

  1. 1. 1
  2. 2. THIS BOOK IS DUE ON THE DATEINDICATED BELOW AND IS SUB-JECT TO AN OVERDUE FINE ASPOSTED AT THE CIRCULATIONDESK. iAUG 2 1 1974 m 6198? .> •"0 JUN30 1!76 NOV 2 3198 MAR 2 7 1991 4i9?? Hm 2^ lis •3 1 i7^ m 5 f ?«
  3. 3. THE ANATOMY OF THE DOMESTIC ANIMALS SEPTIMUS SISSON, S.B., V.S.PROFESSOR OF COMPARATIVE ANATOMY IN THE OHIO STATE UNIVERSITY, COLUMBUS, OHIO MEMBER OF THE AMERICAN ASSOCIATION OF ANATOMISTS FELLOW OF THE AMERICAN ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE WITH 725 ILLUSTRATIONS MANY IN COLORS SECOND EDITION, ENTIRELY RESET PHILADELPHIA AND LONDON W. B. SAUNDERS COMPANY
  4. 4. Copyright, 1910, by W. B. Saunders Company Reprinted June, 1911 Revised, Entirely Reset, Reprinted AND ReCOPYRIGHTED SEPTEMBER, 1914Copyright. 1914, by W. B. Saunders Company Reprinted September, 1917 Reprinted March, 1921 Reprinted October, 1927 Reprinted September, 1930 PRESS OF W. B. SAUNDERS COMPANY PHILADELPHIA
  5. 5. TO MY WIFE KATHERINE OLDHAM SISSONIN GRATEFUL RECOGNITION OF CONSTANT INSPIRATION AND ENCOURAGEMENT
  6. 6. PREFACE This book supersedes the authors Text-book of Veterinary Anatomy. Acomparison of the two will show the new title to be justified by the extent andcharacter of the changes which have been made. The number of illustrations has been increased from 588 to 725. More thanthree hundred new and original figures have been prepared. Nearly all of theseare reproductions of photographs, most of which were taken by the author. Thepreparation of the prints for reproduction has been executed by Mr. W. J. Norriswith unusual care and skill. Continued observations of well-hardened material and frozen sections have ledto a considerable number of modifications of statement. It is scarcely necessaryto say that the recent literature, so far as available, has been utilized. Many changes in nomenclature have been made. Most of the synonyms havebeen dropped or relegated to foot-notes. Exceedingly few new names have beenintroduced. Nearly all eponyms have been eliminated, on the ground that theyare not designative and are usually incorrect historically. The changes made inthis respect are in conformity with the report of the Committee on Revision ofAnatomical Nomenclature which was adopted by the American Veterinary Med-ical Association two years ago. Progress in the direction of a simplified and uni-form nomenclature is much impeded by the archaic terminology which persists toa large extent in clinical literature and instruction. The author is under special obligation to Professors EUenberger, Baum, andSchmaltz, and to their publishers, for permission to use or to copy figures fromtheir excellent works. Their generosity in this matter has been of great value tothose who are unable to use the German literature. A few illustrations have beentaken from other sources and credit has been given in each case. For helpful suggestions and for assistance in the reading of the proof-sheetsthe author is much indebted to his colleague, Dr. F. A. Lambert. Great credit is due the publishers for their determination to spare neitherpains nor expense in attaining a high degree of typographical excellence. Septimus SissonThe Ohio State University, Columbus, Ohio.
  7. 7. PREFACE TO THE FIRST EDITION The lack of a modern and well-illustratedbook on the structure of the princi-pal domestic animals has been acutely a long time by teachers, students, felt forand practitioners of veterinary medicine. The work here offered is the expressionof a desire to close this gap in our literature. The study and of material which has been hardened by intra- of frozen sectionsvascular injection of formalin has profoundly modified our views concerning thenatural shape of many of the viscera and has rendered possible much greater pre-cision in topographic statements. The experience of the author during the lastten years, in which almost all of the material used for dissection and for frozensections in the anatomical laboratory of this University has been hardened withformalin, has demonstrated that many of the current descriptions of the organs inanimals contain the same sort of errors as those which prevailed in regard to similarstructures in man previous to the adoption of modern methods of preparation. While the method of treatment of the subject is essentially systematic, topog-raphy is not by any means neglected either in text or illustrations; it is hoped thatthis will render the book of value to the student in his chnical courses and tothe practitioner. Embryological and histological data have been almost entirelyexcluded, since it was desired to offer a text-book of convenient size for the studentand a work of ready reference for the practitioner. It is believed that the use ofblack type for the names of important structures and of small print for certaindetails or matter of secondary importance will prove useful in this respect. Veterinary anatomical nomenclature is at present quite chaotic in EngUsh-speaking countries. In this work an attempt is made to eliminate some termswhich do not appear to the author to fulfil any useful purpose, and others which areclearly erroneous or otherwise undesirable. In many cases the terms agreed uponby the Congresses at Baden and Stuttgart are adopted either in the original Latinor in anglicized form; otherwise these terms are added in parenthesis. Theauthor favors the substantial adoption of this terminology, but considered itdesirable to offer a sort of transitional stage at present. The original illustrations are chiefly reproductions of photographs, many ofwhich were taken by Mr. F. H. Haskett. The preparation of the pictures forreproduction was carried out by Messrs. J. V. Alteneder and W. J. Norris. Theauthor takes pleasure in expressing his appreciation of the care and skill exercisedby these gentlemen in this often difficult task. The author is under great obligation to Professors Ellenberger and Baum inDresden, to Professor Schmaltz in Berlin, and to their pubUshers for permission touse or to copy figures from their most excellent works. Their generosity in thismatter has made it possible to supply this text with a larger number of high-classillustrations than is to be found in any other. A few figures have been taken fromother sources, and proper credit has been given in each case. For checking over certain data and for assistance in the correction of the proofsthe author is much indebted to his associate, Dr. F. B. Hadley. The author desires to express his high appreciation of the determination andconstant effort of the publishers to do all in their power to render the book worthyof favorable reception by the profession for whom it is intended.Ohio State University, Columbus, Ohio Septimus Sisson.
  8. 8. CONTENTS INTRODUCTION OSTEOLOGY p^obThe Skeleton 20 Structure of Bones 21 Development and Growth of Bone 23 Composition and Physical Properties of Bone 24 Descriptive Terms 25 The Vertebral Column 25 The Ribs and Costal Cartilages 27 Costal Cartilages 27 The Sterjium 28 The Thorax 28 The Skull 28 Bones of the Thoracic Limb 29 Bones Limb of the Pelvic 30Skeleton of the Horse 32 Vertebral Column 33 Ribs 45 Sternum 48 Bones of the Skull 49 Cranium 49 Face 63 The Skull as a Whole 73 The Cranial Cavity 81 The Nasal Cavity 82 The Paranasal Sinuses 84 Bones of the Thoracic Limb 86 Bones of the Pelvic Limb 105Skeleton of the Ox 125 Vertebral Column 125 Ribs 130 Sternum 131 Bones of the Skull 131 The Skull as a Whole 140 Bones of the Thoracic Limb 145 Bones of the Pelvic Limb 151Skeleton of the Sheep 156 Vertebral Column 156 Ribs 156 Sternum 157 Skull 157 Bones of the Thoracic Limb 160 Bones of the Pelvic Limb 160Skeleton of the Pig 161 Vertebral Column 161 Ribs 165 Sternum 166 Bones of the Skull 166 The Skull as a Whole 173 11
  9. 9. 12 CONTENTS PAGE Bones of the Thoracic Limb 176 Bones of the Pelvic Limb 180Skeleton of the Dog 184 Vertebral Column 184 Ribs 188 Sternum 188 Bones of the Skull 188 The Skull as a Whole 195 Bones of the Thoracic Limb 197 Bones of the Pelvic Limb 202 ARTHROLOGYSynarthroses 207DiARTHROSES 208Amphiarthroses 210Articulations of the Horse 211 Joints and Ligaments of the Vertebrae 211 Atlanto-occipital Articulation 214 Costo-vertebral Articulations 215 Costo-chondral Articulations 216 Chondro-sternal Ailiculations 216 Sternal Joints and Ligaments 217 Articulations of the Skull 217 Limb Articulations of the Thoracic 218 Limb Articulations of the Pelvic 229Articulations of the Ox, Pig, and Dog 244 MYOLOGYThe Muscles and Accessory Structures 252Fascl« and Muscles of the Horse 254 Fasciae and Muscles of the Head 255 Fascia? and Muscles of the Neck 266 Fascite and Muscles of the Back and Loins 276 Fasciae and Muscles of the Tail 279 Muscles of the Thorax 281 Abdominal Fasciae and Muscles 287 Muscles of the Thoracic Limb 293 Fasciae and Muscles of the Pelvic Limb 317Muscles of the Ox 343Muscles of the Pig 359Muscles of the Dog 368 SPLANCHNOLOGYDigestive System of the Horse 385 The Mouth 385 The Tongue 390 The Teeth 394 The SaHvary Glands 404 The Pharynx 405 The CEsophagus 409 The Abdominal Cavity 410 The Peritoneum 411 The Pelvic Cavity 412 The Stomach 415 The Small Intestine 419 The Large Intestine 422 The Pancreas 432 The Liver 434
  10. 10. CONTENTS 13 PAGE The Spleen 439 The Peritoneum 441Digestive System of the Ox 444Digestive System of the Sheep 470Digestive System of the Pig 477Digestive System of the Dog 491 THE RESPIRATORY SYSTEMRespiratory System of the Horse 508 The Nasal Cavity 508 The Larynx 514 The Trachea 523 The Bronchi 525 The Thoracic Cavity 525 The Pleuraj 526 The Lungs 530The Thyroid Gland of the IIorse 535The Thymus of the Horse 536Respiratory System of the Ox 537Respiratory System of the Pig 545Respiratory System of the Dog 548 THE UROGENITAL SYSTEMUrinary Organs of the Horse 554 The Kidneys 554 The Ureters 561 The Urinary Bladder 561The Adrenal Bodies 563LTrinary Organs of the Ox 564Urinary Organs of the Pig 567Urinary Organs of the Dog 569 THE MALE GENITAL ORGANSGenital Organs of the Stallion 571 The Testicles 571 The Scrotum 574 The Ductus Deferens 575 The Spermatic Cord 575 The Tunica VaginaUs 576 Descent of the Testicles 577 The Vesiculse Seminales 578 The Prostate 578 The Uterus MascuUnus 579 The Bulbo-urethral Glands 579 The Penis 580 The Prepuce 582Genital Organs of the Bull 586Genital Organs of the Boar 591Genital Organs of the Dog 593 THE FEMALE GENITAL ORGANSGenital Organs of the Mare 596 The Ovaries 596 The Uterine or Fallopian Tubes 599 The Uterus 599 The Vagina 602
  11. 11. 14 CONTENTS PAGB The Vulva 603 The Urethra 604 The Mammary Glands 604Genital Organs of the Cow 605Genital Organs of the Ewe 609Genital Organs of the Sow 610Genital Organs of the Bitch 612 ANGIOLOGYGeneral Considerations 614The Organs of Circulation 614Blood-vasctlar System of the Horse 617 The Pericardium 617 The Heart 617 The Pulmonary Artery 629 The Systemic Ai-teries 629 The Coronary Arteries 630 Common Brachiocephalic Trunk 631 Arteries of the Thoracic Limb 650 Branches of the Thoracic Aorta 660 Branches of the Abdominal Aorta 661 Arteries of the Pelvic Limb 673 The Veins 681 The Pulmonary Veins 681 Cardiac Veins 681 The Anterior Vena Cava and its Tributaries 682 The Posterior Vena Cava and its Tributaries 692The Lymphatic System 697 The Thoracic Duct 697 The Right Lymphatic Duct 698 The Lymph Glands and Vessels of the Head and Neck 698 The Lymph Glands and Vessels of the Thorax 700 The Lymph Glands and Vessels of the Abdomen and Pelvis 701 The Lymph Glands and Vessels of the Thoracic Limb 703 The Lymph Glands and Vessels of the Pelvic Limb 703The Pcetal Circulation 704Blood-vascular System op the Ox 705 The Pericardium and Heart 705 The Arteries 706 The Veins 719Lymphatic System of the Ox and Sheep 722Circulatory System of the Pig 734 The Pericardium and Heart 734 The Arteries 736 The Veins 740 Lymphatic System 740Circulatory System of the Dog 742 The Pericardium and Heart 742 The Arteries 744 The Veins 754 Lymphatic System 756 NEUROLOGY.—THE NERVOUS SYSTEMGeneral Considerations 760Nervous System of the Horse 764 The Spinal Cord 764 The Brain 768
  12. 12. CONTENTS 15 PAGE The Cranial Nerves 793 The Spinal Nerves 810Sympathetic Nervous System of the Horse 829Nervous System of the Ox 834Nervous System of the Pig 843Nervous System of the Dog 847 ^STHESIOLOGYThe Sense Organs and Common Integument of the Horse 857 The Eye 857 The Ear 870 The Common Integument 884 The Ergot and Chestnut 895 The OKactory and Gustatory Apparatus 895The Sense Organs and Common Integument of the Ox 896The Sense Organs and Common Integument of the Pig 900The Sense Organs and Common Integument of the Dog 902Index 907
  13. 13. THE ANATOMY OF THE DOMESTIC ANIMALS INTRODUCTION Anatomy the branch of biological science which deals with the form and isstructure of organisms, both animal and vegetal. It is therefore in close correlationwith physiology, which treats of the functions of the body. Etymologically the word "anatomy" signifies the cutting apart or disassociat-ing of parts of the body. In the earlier phases of its development anatomy wasnecessarily a purely descriptive science, based on such observations as were possible —with the unaided eye and simple dissecting instruments the scalpel, forceps, andthe like. At this time, therefore, the term adequately expressed the nature of thesubject. But as the scope of the science extended and the body of anatomicalknowledge grew, subdivisions became necessary and new terms were introduced todesignate special fields and methods of work. With the introduction of the mi-croscope and its accessories it became possible to study the finer details of structureand minute organisms hitherto unknown, and this field of inquiry rapidly developedinto the science of microscopic anatomy or histology as conventionally distinguishedfrom macroscopic or gross anatomy. In the same way the study of the changeswhich organisms undergo during their development soon attained sufficient im-portance to be regarded on practical grounds as a separate branch known asembryology.^ Comparative anatomy is the description and comparison of the structure ofanimals, and forms the basis for their classification. By this means including — —extinct forms in the scope of inquiry it has been possible to show the geneticrelationship of various groups of animals and to elucidate the significance of manyfacts of structure which are otherwise quite obscure. The deductions concerningthe general laws of form and structure derived from comparative anatomicalstudies constitute the science of morphology or philosophical anatomy. Themorphologist, however, deals only with such anatomical data as are necessary toform a basis for his generalizations. The anatomical knowledge required in thepractice of medicine and surgery is evidently of a different character and mustinclude many details which are of no particular interest to the morphologist. Special anatomy is the description of the structure of a single type or species,e. g., anthropotomy, hippotomy. Veterinary anatomy is the branch which deals with the form and structure ofthe principal domesticated animals. It is usually pursued with regard to pro-fessional requirements, and is therefore largely descriptive in character. As amatter of convenience, the horse is generally selected as the type to be studied indetail and to form a basis for comparison of the more essential differential charactersin the other animals. Two chief methods of study are employed — the systematic and the topo-graphic. In the former the body is regarded as consisting of systems of organs or 1 Tliis term is usually limited in its application to the earlier phases of development duringwhich the tissues and organs are formed. The term ontogeny is used to designate the entiredevelopment of the individual. The ancestral history or phylogeny of the species is constitutedby the evolutionary changes which it has undergone as disclosed by the geological record. 2 17
  14. 14. : :X8 INTRODUCTIONapparatus which are similar in origin and structure and are associated in the per-formance of certain functions. The divisions of systematic anatomy are 1. Osteology, the description of the Skeleton. 2. Arthrology, the description of the Joints. 3. Myology, the description of the Muscles and accessory structures. 4. Splanchnology, the description of the Viscera. This includes the followingsubdivisions (1) Digestive System (2) Respiratory System (3) Urogenital System (a) Urinary Organs Genital Organs (h) 5. Angiology, the description of the Organs of Circulation 6. Neurology, the description of the Nervous System 7. ^sthesiology, the description of the Sense Organs and Common Integu- ment The term topographic anatomy designates the methods by which the relativepositions of the various parts of the body are accurately determined. It presup-poses a fair working knowledge of systematic anatomy. — Descriptive Terms. In order to indicate precisely the position and directionof parts of the body, certain descriptive terms are employed, and must be under-stood at the outset. In the explanation of these terms it is assumed here thatthey apply to a quadruped such as the horse in the ordinary standing position.The surface directed toward the plane of support (the ground) is termed ventral(or inferior), and the opposite surface is dorsal (or superior) the relations of parts in ;this direction are named accordingly. The longitudinal median plane divides thebody into similar halves. A structure or surface which is nearer than another to themeclian plane is medial (or internal) to it, and an object or surface which is furtherthan another from the median plane is lateral (or external) to it. Planes parallelto the median plane are sagittal. Transverse or segmental planes cut the long axisof the body perpendicular to the median plane, or an organ or limb at right anglesto its long axis. A frontal plane is perpendicular to the median and transverseplanes. The term is also used with reference to parts of the limbs or various organsin a similar sense. The head end of the body is termed anterior or cranial; and thetail end posterior or caudal ; relations of structures with regard to the longitudinalaxis of the body are designated accordingly. With respect to parts of the head,the corresponding terms are oral and aboral. Certain terms are used in a specialsense as applied to the limbs. Proximal and distal express relative distances ofparts from the long axis of the body. The anterior face of the thoracic limb fromthe elbow downward is termed dorsal, and the opposite face volar. In the corre-sponding part of the pelvic limb the terms are dorsal and plantar respectively. Inthe same regions radial and ulnar (thoracic limb), tibial and fibular (pelvic limb),may be used to designate that side of the extremity on which the correspondingbone is situated; they are therefore equivalent respectively to medial and lateralin the animals with which we are concerned. The terras superficial (superficialis)and deep (profundus) are useful to indicate relative distances of i);irts from thesurface of the body. It is evidently advantageous to employ terms which are as far as possible independent ofthe position of the body in space and capable of general application, e. g., dorsal, ventral, proximal,etc. It is also desirable that the terms internal and external be reserved to indicate relations ofdepth in cavities or organs, and medial and lateral to designate relations to the median plane.Such terms are coming into more extensive use in human and veterinary anatomy, but the oldernomenclature is very firmly established and cannot well be discarded at once and entirely. Tofacilitate the transition, a table of the older and more recent terms is given below; the recentterms are in the first column and the older equivalents in the second.
  15. 15. INTRODUCTION 19A. Relating to Head, Neck, and Trunk: Superior Dorsalis Inferior Ventralis MediaUs I?tf™^ External LateraUs Cranialis 1 Anterior Oralis / Caudalis 1 Posterior Aboralis JB. Relating to Limbs: Superior Proximalis Distalis Mf"?^ Anterior DorsaUs Volaris , . Posterior Plantaris f Radialis ] Internal Tibialis f Ulnaris External Fibularis f
  16. 16. OSTEOLOGY THE SKELETON The term skeleton applied to the framework of hard structures which sup- isports and protects the soft tissues of animals. In the descriptive anatomy of thehigher animals it is usually restricted to the bones and cartilages, although theligaments Avhich bind these together might well be included. In zoology the term is used in a much more comprehensive sense, and includes all the hardersupporting and protecting structures. When the latter are situated externally, they form anexoskeleton, derived from the ectoderm. Examples of this are the shells and cliitinous coveringsof many invertebrates, the scales of fishes, the shields of turtles, and the feathers, hair, and hoofsof the higher vertebrates. The endoskeleton (with which we have to deal at present) is embeddedin the soft tissues. It is derived from the mesoderm, but includes the notochord or primitiveaxial skeleton, which is of entodermal origin. The skeleton may be divided primarily into three parts: (1) axial; (2) appen-dicular; (3) splanchnic. The axial skeleton comprises the vertebral column, ribs, sternum, and skull. The appendicular skeleton includes the bones of the limbs. The splanchnic or visceral skeleton consists of certain bones developed in thesubstance of some of the viscera or soft organs, e. g., the os penis of the dog and theOS cordis of the ox. The number of the bones of the skeleton of an animal varies with age, owingto the fusion during growth of skeletal elements which are separate in the fostusor the young subject. Even in adults of the same species numerical variationsoccur, e. g., the tarsus of the horse may consist of six or seven bones, and the carpusof seven or eight; in all the domestic mammals the number of coccygeal vertebraevaries considerably. The bones are commonly divided into four classes according to their shapeand function.^ Long bones (Ossa longa) are typically of elongated cylindrical form with (1)enlarged extremities. They occur in the limbs, where they act as supportingcolumns and as levers. The cylindrical part, termed the shaft or body (Corpus),is and incloses the medullary cavity, which contains the medulla or marrow. tubular, (2)Flat bones (Ossa plana) are expanded in two directions. They furnishsufficient area for the attachment of muscles and afford protection to the organswhich they cover. (3) Short bones (Ossa brevia), such as those of the carpus and tarsus, presentsomewhat similar dimensions in length, breadth, and thickness. Their chief func-tion appears to be that of diffusing concussion. Sesamoid bones, which are de-veloped in the capsules of some joints or in tendons, may be included in this group.They diminish friction or change the direction of tendons. (4) Irregular bones (Ossa irregularia) This group would include bones of .irregular shape, such as the vertebrae and the bones of the cranial base; they aremedian and unpaired. Their functions are various and not so clearly specializedas those of the preceding classes. ^ This classification is not entirely satisfactory; some bones, e. g., the ribs, are not clearlyprovided for, and others might be variously placed. 20
  17. 17. . STRUCTURE OF BONES 21 STRUCTURE OF BONES i Bones consist chiefly of bone tissue, but considered as organs they presentalso an enveloping membrane, termed the periosteum, the marrow, vessels, andnerves. The architecture of bone can be studied best by means of longitudinal andtransverse sections of specimens which have been macerated so as to remove mostof the organic matter. These show that the boneconsists of an external dense compact sub- shell ofstance, within which the more loosely arranged isspongy substance. In tyjDical long bones the shaftis hollowed to form the medullary cavity (Cavummedullare) The compact substance (Substantia compacta)differs greatly in thickness in various situations, inconformity with the stresses and strains to whichthe bone is subjected. In the long bones it isthickest in or near the middle part of the shaft andthins out toward the extremities. On the latter thelayer is very thin, and is especially dense and smoothon joint surfaces. The spongy substance (Substantia spongiosa)consists of delicatebony plates and spicules whichrun in various directions and intercross. These aredefinitely arranged with regard to mechanical re-quirements, so that systems of pressure and tensionplates can be recognized, in conformity with thelines of pressure and the pull of tendons and liga-ments respectively. The intervals between theplates are occupied by marrow, and are termedmarrow spaces (Cellulse meduUares). The spongysubstance forms the bulk of short bones and of theextremities of long bones; in the latter it is not con-fined to the ends, but extends a variable distancealong the shaft also. Some bones contain air-spaceswithin the compact substance instead of spongybone and marrow, and hence are called pneimiaticbones (Ossa pneumatica) These cavities are termed .sinuses, and are lined with mucous membrane;they communicate indirectly with the external air. Fig. 1. — Sagittal Section op LargeIn certain situations the two compact layers of flat Metatarsal Bone of Horsebones are not separated by spongy bone, but fuse (Right).with each other; in some cases of this kind the bone S.c, Compact substance; S.s.,is so thin as to be translucent, or may undergo spongy substance; Cm., medullary cavity; F.n., nutrient foramen. Noteabsorption, producing an actual deficiency. the greater thickness of the compact The flat bones of the cranial vault and sides substance of the anterior part of the shaft.are composed of an outer layer of orchnary compactsubstance, the lamina externa, an inner layer ofvery dense bone, the lamina interna or tabula vitrea, and between these a variableamount of spongy bone, here termed diploe. The periostetun is the membrane which invests the outer surface of bone,except where it is covered with cartilage. It consists of an outer protective fibrous 1 Only the gross structure is discussed here. For the microscopic structure reference is tobe made to histological works.
  18. 18. — ——22 OSTEOLOGYlayer, and an inner cellular osteogenic layer. During active gro^vth the osteogeniclayer is well developed, but later it becomes much reduced. The fibrous layervaries much in thickness, being in general thickest in exposed situations. Theadhesion of the periosteum to the bone also differs greatly in various places; itis usually very thin and easily detached where it is thickly covered with musculartissue which has little or no attachment. The degree of vascularity conforms tothe activit}^ of the periosteum. The endosteum is a thin fibrous membrane which lines the medullary cavityand the larger Haversian canals. The marrow (Medulla ossium) occupies the interstices of the spongj^ bone andthe medullary cavity of the long bones. There are two varieties in the adultred and yellow. In the young subject there is only red marrow (Medulla ossiummbra), but later this is replaced in the medullary cavity by yellow marrow (Medullaossium flava). The red marrow contains several types of characteristic cells andis a blood-forming suV)stance, Avhile the yellow is practically ordinary adipose tissue.^ — Vessels and Nerves. It is customary to recognize two sets of arteries theperiosteal and the medullary. The former ramify in the periosteum and give offFig. 2. Cross-section of Proximal Third of Shaft Fig. Cross-section of Distal Third of Shaft op OF Right Humerus of Horse. Left Humerus of Horse. Section passes through nutrient foramen and canal.innumerable small branches which enter minute openings (Volkmanns canals) onthe surface and reach the Haversian canals of the compact substance. Otherbranches enter the extremities of the long l^ones and supply the spongy bone and —marrow in them. In the case of the larger bones and especially the long bonesthe large nutrient or medullary artery (Arteria nutricia) enters at the so-called nu-trient foramen (Foramen nutricium), passes in a canal (Canalis nutricius) throughthe compact substance, and ramifies in the marrow; its branches anastomose -withthe central branches of the periosteal set. The larger veins of the spongy bone donot, as a rule, accompany the arteries, but emerge chiefly near the articular surfaces.Within the bone thej^ are destitute of valves. The l5anph-vessels exist as peri-vascular channels in the periosteum and the Haversian canals of the compact sub-stance. They also form a fine subperiosteal network, froxn which the larger vesselsproceed, usually in company with veins. Lymph-spaces exist at the periphery ofthe marrow. The nerves appear to be distributed chiefly to the blood-vessels. Special nerve- 1 Since yellow marrow is formed by regressive changes in red marrow, including fatty infiltra-tion and degeneration of the characteristic cells, wefind transitional forms or stages in the process.In aged or badly nourished subjects the marrow may undergo gelatinous degeneration, resultingin the formation of gelatinous marrow.
  19. 19. — DEVELOPMENT AND GROWTH OF BONE 23endings (Vater-Pacini corpuscles) in the periosteum are to be regarded as sensory,and probably are concerned in mediating the muscle sense (kinesthesia) DEVELOPMENT AND GROWTH OF BONEi The primitive embryonal skeleton consists of cartilage and fibrous tissue, inwhich the bones develop. The process is termed ossification or osteogenesis, andis by bone-producing cells, called osteoblasts. It is customary, effected essentiallytherefore, to designate as membrane bones those which are developed in fibroustissue, and as cartilage bones those which are preformed in cartilage. The princi-pal membrane bones are those of the roof and sides of the cranium and most of thebones of the face. The cartilage bones comprise,therefore, most of the skeleton. Correspondinglywe distinguish intramembranous and endochondralossification. In intramembranous ossification the processbegins at a definite center of ossification (Punctumossificationis) where the osteoblasts surround them- ,selves with a deposit of bone. The process extendsfrom this center to the periphery of the future bone,thus producing a network of bony trabeculae. Thetrabeculae rapidly thicken and coalesce, forming abony plate which is separated from the adjacentbones by persistent fibrous tissue. The superficialpart of the original tissue becomes periosteum, andon the deep face of this successive layers of perios-teal bone are formed by osteoblasts until the boneattains its definitive thickness. Increase in circum-ference takes place by ossification of the surround-ing fibrous tissue, which continues to grow untilthe bone has reached its definitive size. In endochondral ossification the process isfundamentally the same, but not quite so simple.Osteoblasts emigrate from the deep face of the peri- Fig. 4. Left Femur of Young Piq,chondrium or primitive periosteum into the cartilage Lateral View, to Show Di- VI8IOX OF A Long Bone intoand cause calcification of the matrix or ground- Shaft (s) and Extremities.substance of the latter. Vessels extend into the cal- Proximal extremity consists ofcifying area, the cartilage cells shrink and disappear, two parts, head (h) and trochanter major (t.m.), which have separateforming primary marrow cavities which are occupied centers of ossification. Distal extrem-by processes of the osteogenic tissue. There is thus and condyles ity consists of trochlea (0formed a sort of scaffolding of calcareous trabeculae (c); e.L, epiphyseal cartilages; s.f., supracondyloid fossa.on which the bone is constructed by the osteoblasts.At the same time perichondral bone is formed bythe osteoblasts of the primitive periosteum. The calcified cartilage is broken downand absorbed through the agency of large cells called osteoclasts, and is replacedby bone deposited by the osteoblasts. The osteoclasts also cause absorption of theprimitive bone, producing the marrow cavities; thus in the case of the long bonesthe primitive central spongy bone is largely absorbed to form the medullary cavityof the shaft, and persists chiefly in the extremities. Destruction of the centralpart and formation of subperiosteal bone continue until the shaft of the bone hascompleted its growth. A typical long bone is developed from three primary centers of ossification, Only a brief general statement of osteogenesis can be 1 made here; details must be soughtin embryological literature.
  20. 20. :24 OSTEOLOGYone, which appears first, for the diaphysis or shaft and one for each epiphysis oiextremity. Many bones have secondary centers from which processes or apophysesdevelop. The foregoing outhne accounts for the gro^Hh of bones except in regard tolength. Increase in length may be explained briefly as follows Provision for con- :tinued ossification at either end of the diaphysis is made by a layer of actively — —groAving cartilage the epiphyseal cartilage which intervenes between the diaph-ysis and the epiphysis. It is evident that so long as this cartilage persists andgrows, new bone may continue to be formed at its expense, and increase of lengthis possible. When the epiphyseal cartilage ceases to grow, it undergoes ossification,the bone is consolidated, and no further increase in length is possible. This fusiontakes place at fairly definite periods in the various bones, and it is of value to knowthe usual times at which it occurs in the larger bones of the limbs at least. In thecase of membrane bones, increase in circumference is provided for by the ossificationand new formation of the surrounding fibrous tissue. After the bones have reached their full size, the periosteum becomes relatively reduced andinactive so far as its osteogenic layer is concerned; the bone-forming function may be stimulatedby various causes, as is well seen in the heaUng of fractures and the occurrence of bony enlarge-ments. Profound changes occur in the skeleton after birth, and during the period of growth the bonesare much more plastic than might be supposed. In the new-])orn foal, for example, it is evidentthat the rnetacarpal and metatarsal bones are relatively long and the scapula and humerus short;also that in general the shafts of the long bones are slender in comparison with the extremities.The various prominences are much less pronounced than in the adult, and most of the minorsurface markings are absent, so that the bones have a relatively smooth appearance. The periodof gro^v-th may be regarded as terminating with the union of the extremities and shafts of the longbones and the fusion of the parts of other bones. During adult life the skeletal changes proceedmore slowly; they comprise accentuation of the larger prominences and depressions and the ap-pearance of smaller ones. These secondary markings are chiefly correlated with the attachmentsof muscles, tendons, and hgaments, or are produced by pressure exerted by various structures onthe bones. Later in hfe ossification invades more or less extensively the cartilages and the at-tachments of tendons and ligaments. Senile changes in the bones, consisting of decrease of theorganic matter and rarefaction of the bone tissue, render them brittle and liable to fracture. CHEMICAL COMPOSITION OF BONE Dried bone consists of organic and inorganic matter in the ratio of 1 2 ap- :proximately. The animal matter gives toughness and elasticity, the mineralmatter hardness, to the bone tissue. Removal of the organic matter by heat doesnot change the general form of a bone, but reduces the weight by about one-third,and makes it very fragile. Conversely, decalcification, while not affecting the formand size of the bone, renders it soft and pliable. The organic matter (ossein) whenboiled yields gelatin. The following table represents the composition in 100 partsof ox bone of average quality Gelatin 33.30 Phosphate of Ume 57.35 Carbonate of lime 3.85 Phosphate of magnesia 2.05 Carbonate and chlorid of sodium 3.45 100.00 PHYSICAL PROPERTIES OF BONE Fresh dead bone has a yellowish-white color; when macerated or boiled andbleached, it is white. The specific gravity of fresh compact bone is a little over1.93. It is very hard and resistant to pressure; a 5-millimeter cube of compactbone of the ox will resist pressure up to 852 pounds, if the pressure be applied inthe line of the lamellae (Rauber). Its tensile strength is estimated to be nearlytwice that of oak.
  21. 21. THE VERTEBRAL COLUMN 25 DESCRIPTIVE TERMS Thesurfaces of the bones present a great variety of eminences and depressions,as well as perforations. The prominences and cavities may be articular, or non-articular, furnishing attachment to muscles, tendons, ligaments, or fascia. Anumber of descriptive terms are used to designate these features, and the followingare some of those in general use: Process (Processus)is a general term for a prominence. Atuberosity (Tuber, Tuberositas) is a large, rounded projection; a tubercle(Tuberculum) is a smaller one. The term trochanter is applied to a few prominences, e. g., the trochanters ofthe femur. A spine (Spina) or spinous process (Processus spinosus) is a pointed projection. A crest (Crista) is a sharp ridge. A line (Linea) is a very small ridge. Ahead (Caput) is a rounded articular enlargement at the end of a bone; itmay be joined to the shaft by a constricted part, the neck (Collum). A condyle (Condylus) is an articular eminence which is somewhat cylindrical;a non-articular projection in connection with a condyle may be termed an epi-condyle (Epicondylus). A trochlea is a pulley-like articular mass. A glenoid cavity (Cavitas glenoidalis) is a shallow articular depression, and acotyloid cavity or acetabulum is a deeper one. The term facet is commonly applied to articular surfaces of small extent,especially when they are not strongly concave or convex. The terms fossa, fovea, groove or sulcus, and impression are applied to variousforms of depressions. A foramen is a perforation for the transmission of vessels, nerves, etc. A sinus is an air-cavity within a bone or bones; it is lined with mucous mem-brane and communicates with the exterior. Other terms, such as canal, fissure, notch, etc., require no explanation.^ THE VERTEBRAL COLUMN The (Columna vertebralis) is the fundamental part of the vertebral coliramskeleton. It a chain of median, unpaired, irregular bones which consists ofextends from the skull to the end of the tail. In the adult certain vertebrae havebecome fused to form a single bony mass with which the pelvic girdle articulates.Vertebrae so fused are termed fixed (or false") vertebrae (Vertebrae immobiles), asdistinguished from the movable (or Hrue") vertebrae (Vertebrae mobiles). The column is subdivided for description into five regions, which are namedaccording to the part of the body in which the vertebrae are situated. Thus thevertebrae are designated as cervical, thoracic, lumbar, sacral, coccygeal (Vertebraecervicales, thoracales, lumbales, sacrales, coccygeae). The number of vertebrae ina given species is fairly constant in each region except the last, so that the verte-bral formula may be expressed (for the horse, for example) as follows: C7Ti8L6S5Cyi5.21. The vertebrae in a given region have characters by which they may be dis-tinguished from those of other regions, and individual vertebrae have specialcharacters which are more or less clearly recognizable. All typical vertebrae have 1 As might be expected from the history of anatomy, a good many of these terms are more orless interchangeable; furthermore, a given skeletal feature may differ greatly in various species.
  22. 22. —26 OSTEOLOGYa common plan of structure, which must first be understood. The parts of whicha vertebra consists are the body, the arch, and the processes. The body (Corpus vertebrae) is the more or less cylindrical mass on which theother parts are constructed. The anterior and posterior extremities of the bodyare attached to the adjacent vertebrse by intervertebral fibro-cartilages, and areusually convex and concave respectively. The dorsal surface is flattened and entersinto the formation of the vertebral canal, while the ventral aspect is rounded later-ally, and is in relation to various muscles and viscera. In the thoracic region thebody presents two pairs of facets (Foveae costales) at the extremities for articula-tion with part of the heads of two pairs of ribs. The arch (Arcus vertebrae) is constructed on the dorsal aspect of the body.It consists originally of two lateral halves, each of which is considered to consistof a pedicle and a lamina. The pedicle (Radix arcus vertebrge) forms the lateralpart of the arch, and is cut into in front and behind by the vertebral notches (In-cisura vertebralis cranialis, caudalis). The notches of two adjacent vertebrae formintervertebral foramina (Foramina intervertebralia) for the passage of the spinal nerves and vessels; in some vertebrae, how- ever, there are complete foramina instead of notches. The laminae are plates which complete the arch dorsally, uniting with each other medially at the root of the spinous process. The body and the arch form a bony ring which incloses the vertebral foramen (Foramen vertebrale); the series of verte- bral rings, together with the ligaments which unite them, inclose the vertebral canal (Canalis vertebralis), Avhich contains the spinal cord and its coverings and vessels. The articular processes, two anterior and two posterior (Processus articulares Cosraijkcef craniales, caudales), project from the bor- ders of the arch. They present articular surfaces adapted to those of adjacent verte-Fig. 5. First Thoracic Vertebra of Horse. brae, and the remaining surface is roughened To illustrate plan of structure of vertebrae. for muscular and ligamentous attachment. The spinous process or spine (Processusspinosus) is and projects dorsally from the middle of the arch. It varies single,greatly in form, size, and direction in different vertebrae. It furnishes attachmentto muscles and ligaments. The transverse processes (Processus transversi) are two in number and projectlaterally from the sides of the arch or from the junction of the arch and body.In the thoracic region each has a facet for articulation with the tubercle of a rib(Fovea costalis transversalis) They also give attachment to muscles and liga- .ments. Some vertebrae have also a ventral spine or a haemal arch. Mammillary processes (Processus mamillares) are found in most animals onthe last thoracic and anterior lumbar vertebrse, between the transverse and an- on the latter.terior articular processes or Accessory processes (Processus accessorii), when present, are situated betweenthe transverse and posterior articular processes. Development. —The vertebrae are developed by ossification in the cartilageAvhich surrounds the notochord and forms the sides of the neural canal. There arethree primary centers of ossification, one for the body and one for each side of the
  23. 23. . THE COSTAL CARTILAGES 27arch. Secondary centers appear later for the summit of the spinous process(except in the cervical region), the extremities of the transverse processes, and thethin epiphyseal plates at the extremities of the body. Sometimes there are at first two centers for the body, which soon fuse. The process of ossifi-cation extends from the lateral centers to form, not only the corresponding part of the arch, butalso the processes and a part of the body next to the root of the arch (Radix arcus) In the horse .and ox the body and arch are fused at birth or unite very soon after, but the epiphyses do notfuse till growth is complete. In the pig, sheep, and dog the body and arch are united at l)irthby cartilage (neurocentral synchondrosis), but fuse in the first few months. THE RIBS The ribs (Costae) are elongated curved bones which form the skeleton of thelateral thoracic walls. They are arranged serially in pairs which correspond innumber to the thoracic vertebrae. Each articulates dorsally with two vertebraeand is continued ventrally by a costal cartilage. Those which articulate with thesternum by means of their cartilages are termed sternal ribs (Costae sternales);the remainder are asternal ribs (Costae asternales). Ribs at the end of the serieswhich have their ventral ends free in the abdominal wall are named floating ribs(Costae fiuctuantes). The intervals between the ribs are termed intercostal spaces(Spatia intercostalia) A typical rib^ consists of a shaft and two extremities. The shaft (Corpuscostae) is band-like and varies much in length, breadth, and curvature in differentribs. In the case of some ribs the curvature is not uniform, but is accentuated at acertain point, termed the angle of the rib (Angulus costae) this occurs at a variable ;distance from the vertebral end, and is usually marked by a rough ridge. Thedirection also varies; the first rib is usually almost vertical, while the remainderslope backward in increasing degree. The lateral surface (Facies lateralis) is con-vex, and the medial surface (Facies medialis) flattened from edge to edge; on thelatter, close to the posterior border, is the costal groove (Sulcus costae), which fadesout ventrally. It contains the intercostal vein. The anterior and posteriorborders (Margo cranialis, caudalis) are thin and sharp on some ribs, rounded onothers. The vertebral extremity (Extremitas vertebralis) consists of the head, neck,and tubercle. The head (Capitulum costae) is the actual end of the rib, and isrounded and somewhat enlarged. It presents two facets (Facies articularis capitulicostae) for articulation with the bodies of two adjacent thoracic vertebrae; thesesurfaces are separated by a groove in which the conjugal ligament is attached.The neck (CoUum costae) joins the head to the shaft. It varies in length anddiameter. Its lateral surface is rough, its medial smooth. The tubercle (Tuber-culum costae) projects backward at the junction of the neck and shaft. It has afacet (Facies articularis tuberculi costae) for articulation with the transverse processof the posterior vertebra of the two with which the head articulates. The tuberclegradually approaches the head in the posterior ribs, and eventually fuses with it. The sternal extremity (Extremitas sternalis) is commonly slightly enlarged,and is roughened at the junction with the costal cartilage. — Development. The ribs are ossified in cartilage from three centers one each —for the shaft (and sternal end), head, and tubercle; the third center is absent insome ribs at the terminal part of the series. THE COSTAL CARTILAGES These (Cartilagines costales) are bars of hyaline cartilage which continue theribs. Those of the sternal ribs articulate with the sternum, while those of the 1 The term is employed here, as is usual in descriptive anatomy, to designate only the bonypart of the rib (Os costale) ; morphologically it includes the cartilaginous part also.
  24. 24. 28 OSTEOLOGYasternal ribs overlap and are attached to each other to form the costal arch (Arcuscostalis). The cartilages of floating ribs are not attached to those adjacent. THE STERNUM The stemxun (or breast-bone) is a median segmental bone -which completes theskeleton of the thorax ventrally, and articulates Avith the cartilages of the sternalribs laterally. It consists of six to eight bony segments (Sternebra^) connected byintervening cartilage in the yoimg subject. Its form varies with that of the thoraxin general anilwith the development of the clavicles in animals in which these bonesare present. Its anterior extremity, the manubrium stemi or presternum, isspecially affected by the latter factor, being broad and strong when the claviclesare well ileveloped and articulate with it (as in man), relatively small and laterallycompressed Avhen they are absent (as in the horse) or rudimentary (as in the dog).The cartilages of the first pair of ribs articulate with it. The body or mesostemimi(Corpus sterni) presents laterally, at the junction of the segments, concave facets(Incisura^ costales) for articulation Avith the cartilages of the sternal ribs. Theposterior extremity or metastemiun presents the xiphoid cartilage (Processusxiphoideus) ; this is thin and wide, as in the horse and ox, or narroAv and short, asin the dog. — Development. The cartilaginous sternmn is formed by the fusion mediallyof two lateral bars which unite the ventral ends of the first eight or nine costalcartilages, and is primitively imsegmented. The manubrium ossifies from a singlecenter, but the centers for the other segments appear to be primitively paired.The sternum never becomes completely ossified; details in regard to persistingcartilage will be given in the special descriptions. The layer of compact tissue isfor the greater part very thin and the spongy substance is open-meshed and ver^vascular. THE THORAX The skeleton of the thorax comprises the thoracic vertebnp dorsally. the ribsand costal cartilages laterally, and the sternmn ventrally. The thoracic cavity(CaAinn thoracis) resembles in shape an irregular truncated cone; it is compressedlaterally, especially in front, and the dorsal wall or roof is much longer than theventral wall or floor. The anterior apertvire or inlet (Apertura thoracis cranialis) isbc^unded by the first thoracic vertebra dorsally, the first pair of ribs and costalcartilages laterally, and the manubrium sterni ventrally. The posterior aperture(Apertura thoracis caudalis) is bounded by the last thoracic vertebra, the lastpair of ribs, the costal arches, and the anterior part of the xiphoid cartilage. It may be noted here that the diaphragm (which forms the partition hetween the thoracicand abdominal caA-ities) does not follow the costal arches in its posterior attachment, so that theposterior ribs enter also into the formation of the abdominal wall. THE SKULL The term usually imderstood to include skull is all of the bones of the head.The head consists of the cranium and the face, and it is therefore convenient todiAnde the bones into cranial antl facial groups. The cranial bones (Ossa cranii) inclose the brain AA-ith its membranes andvessels and the essential organs of hearing. They concur with the facial bones informing the orbital and nasal caAities, in AA-hich the peripheral organs of sight amiof smell are situated. The facial bones (Ossa faciei) form the skeleton of the oral and nasal caAities,and also support the pharAnx, the larynx, and the root of the tongue.
  25. 25. THE BONES OF THE THORACIC LIMB 29 Most membrane; those of the bones of the skull are flat bones, developed inof the cranial base may be classed as irregular, and are developed in cartilage. Onlytwo form permanent movable joints with other parts of the skull. The mandible(or lower jaw-Vjone) forms diarthrodial joints with the temporal bones, and thehyoid bone is attached to the latter Vjy bars of cartilage. The other bones formimmovable joints, most of which disappear with age. In order to study the separate bones, skulls of young subjects are necessary, since later mostof the hnes of demarcation become effaced. The relations of each bone to its surroundings shouldbe specially noted, since the final object is to understand the skull as a whole. In the descriptionswhich follow the skull is considered with its long axis horizontal, and that of the horse will serveas a type. THE BONES OF THE THORACIC LIMB The thoracic limb (Extremitas thoracalis) consists of four chief segments, viz.,the shoulder girdle, the arm, the forearm, and the manus. The shoulder girdle (Cingulum extremitatis thoracalis) when fully developed, ,consists of three bones — the scapula (or shoulder-blade), the coracoid, and theclavicle (or collar-bone). In the domesticated mammals only the scapula, a large,flat bone, is well developed,and the small coracoid element has fused with it, whilethe clavicle is either absent or is a small rudiment embedded in the brachiocephali-cus muscle. There is therefore no articulation of the shoulder with the axialskeleton. The shoulder girdle is fully developed in birds and the lower mammals (monotremata) . Inthe higher manmials the coracoid is reduced to the coracoid process of the scapula, and the develop-ment of the clavicle is in conformity with the function of the hmb. Thus in typical quadrupeds,such as the horse and ox, in which the foreUmbs are used only for support and locomotion, theclavicle is absent. Other animals which use these hmbs for grasping, burrowing, chmbing, etc.(e. g., man, apes, moles), have well-developed claicles which connect the scapula with the stemiun. The arm (Brachium) contains a single long bone, the humerus (or arm bone). In the forearm (Antibrachium) are two bones, the radius and ulna. Thesevary in relative size and mobility. In the horse and ox the two bones are fu.sed,and the distal part of the limb is fixed in the position of pronation. The radius isplaced in front and supports the weight. The ulna is well developed only in itsproximal part, which forms a lever for the extensor muscles of the elbow. In thepig the ulna is the larger and longer of the two bones, but is closely attached tothe back of the radius. In the dog the ulna is also well developed and a smallamount of movement is possible between the two bones. The manus,! the homologue of the hand in man, consists of three subdivi-sions, viz., the carpus, metacarpus, and digit or digits. The carpus, popularly termed the "knee" in animals, and homologous withthe wrist of man, contains a group of short bones, the ossa carpi.^ These are typic-ally eight in number and are arranged in two transverse rows — a proximal or anti-brachial, and a distal or metacarpal. The bones of the proximal row, named fromthe radial to the ulnar side {i. e., from within outward), are the radial, intermediate,ulnar, and accessory carpal bones. The bones of the distal row are designatednumerically, in the same direction, as first, second, third, and fourth carpal bones. This nomenclature, introduced by Gegenbaur, and now used largely by comparative anat-omists, seems decidedly preferable to the ^ariety of terms borrowed from human anatomy andbased on the form of the bones in man. The follo^vang table of synonyms in cornmon use is ap-pended for comparison. The Latin terms and abbreviated notations are given in parentheses. 1 It is unfortunate that there is no popular name for this part of the limb. The term "fore-foot" is sometimes appUed to it, but this leads to confusion, since the word "foot" has long beenused in a different sense. 2 The term "knee" as applied to this region is unfortunate, but the usage is very firmlyestablished and there is no other popular name.
  26. 26. 30 OSTEOLOGYThe central carpal bone (Os carpi centrale) is omitted, since it is not a separate element in theanimals under consideration here. Radial (Os carpi radiale, Cr) Scaphoid Intermediate (Os carpi intermedium, Ci) Semilunar Ulnar (Os carpi ulnare, Cu) Cuneiform Accessory (Os carpi accessorium, Ca) Pisiform First carpal (Os carpale primum, CI) Trapezium Second carpal (Os carpale secundum, C2j Trapezoid Third carpal (Os carpale tertium, C3) Os magnum Fourth carpal (Os carpale quartum, C4) Unciform The metacarpus contains typically five metacarpal bones (Ossa metacarpaliaI-V), one for each digit; they are long bones and are designated numerically fromthe radial to the ulnar side (i. e., from within outward). This arrangement occursin the dog, although the first metacarpal is much smaller than the others, and thesecond and fifth are somewhat reduced. Further reduction has taken place in theother animals, resulting in the perissodactyl and artiodactyl forms. In the horsethe first and fifth metacarpals are absent, the third is the large supporting meta-carpal bone and carries the single digit, while the second and fourth are much re-duced. In artiodactyls (e. g., ox, sheep, pig) the third and fourth are the chiefmetacarpals and carry the well developed digits; they are fused in the ox and sheep.The others are variously reduced or absent, as noted in the special descriptions tofollow. The fossil remains of the ancestors of the existing Equidse illustrate in a most complete man-ner the reduction which has occurred in this respect. The earliest known ancestor of the horse,Eohippus of the Lower Eocene, had four well developed metacarpal bones, each of which carrieda digit; the first metacarpal bone was small. Intermediate forms show the gradual evolution ofthe race from this primitive animal, which was about the size of the domestic cat. There is reasonto believe that earlier forms had five digits. The digits (Digiti manus) are homologous with the fingers of man, and aretypically five in number. They are designated numerically from the radial tothe ulnar side, in correspondence with the metacarpus. The full number is presentin the dog. In the ox and pig the third and fourth are well developed and supportthe weight, while the second and fifth are reduced. The existing horse has a singledigit, the third of his pentadactyl ancestors. The skeleton of each fully developeddigit consists of three phalanges and certain sesamoid bones. The first phalanx(Phalanx prima) articulates with the corresponding metacarpal bone above andwith the second phalanx (Phalanx secunda) below. The third phalanx (Phalanxtertia) is inclosed in the hoof or claw, and is modified to conform to the latter. Thesesamoid bones (Ossa sesamoidea) are developed along the course of tendons or inthe joint capsules at points where there is increased pressure. Two proximalsesamoids (Ossa sesamoidea phalangis primse) occur at the flexor side of the meta-carpo-phalangeal joint and form a pulley for the flexor tendon. The distal sesa-moid (Os sesamoideum phalangis tertise) is similarly placed between the deepflexor tendon and the joint between the second and third phalanx; it is absent inthe dog, which has a small sesamoid on the extensor side of the metacarpo-phalan-geal joint, and often at the proximal interphalangeal joint also. Numerous cases are recorded of the occurrence of supernumerary digits (hyperdactylism)in the horseand other animals. In some pigs, on the other hand, the two chief digits are fused,,and the condition (syndactylism) appears to be inherited. THE BONES OF THE PELVIC LIMB The pelvic limb (Extremitas pelvina), like the thoracic, consists of four seg-ments, viz., the pelvic girdle, thigh, leg, and the pes; the last is subdivided intotarsus, metatarsus, and digits. The pelvic girdle (Cingulum extremitatis pelvinse) consists of the os coxae (or
  27. 27. THE BONES OF THE PELVIC LIMB 31hip bone), which joins its fellow of the opposite side ventrally at the symphysispelvis, and articulates very finnlj^ with the sacrum dorsally. The two coxal bones,together with the sacrum and the first three or more coccygeal vertebrae, constitutethe bony pelvis. The os coxae consists originally of three flat bones, the ilium,ischium, and pubis, which meet at the acetabulum, a large cotyloid cavity withwhich the head of the femur articulates. These three parts are fused before growthis complete, but are considered separately for convenience of description. Theiliimi (Os ilium) is situated in the lateral wall of the pelvis, the pubis (Os pubis) inthe anterior part, and the ischium (Os ischii) in the posterior part of the ventralwall. The thigh (Femur), like the arm, contains a single long bone, the femur (orthigh bone) (Os femoris). This articulates with the acetabulum above and thetibia and patella below. The skeleton of the leg (Crus) comprises three bones (Ossa cruris), viz., thetibia, fibula, and patella. The tibia is a large, prismatic long bone which supportsthe weight, and articulates distally with the tibial tarsal bone. The fibula issituated along the lateral border of the tibia, from which it is separated by theinterosseous space of the leg. It is much more slender than the tibia and does notarticulate with the femur. In the pig and dog it has a complete shaft and twoextremities, but in the horse and ox it is much reduced and otherwise modified.The patella (or "knee-cap") is a short bone which articulates with the trochlea ofthe distal end of the femur; it is to be regarded as a large sesamoid bone intercalatedin the tendon of the quadriceps femoris muscle. The tarsus (or "hock") consists of a group of short bones, the ossa tarsi,numbering five to seven in the different animals. The proximal or crural row con-sists of two bones, the tibial and fibular tarsals ; the former is situated at the tibialside, and has a trochlea for articulation with the distal end of the tibia; the latter,situated at the fibular side, has a process, the tuber calcis, which projects upwardand backward and constitutes a lever for the muscles which extend the hock joint.The distal or metatarsal row consists of four bones when seven tarsal elements arepresent, as in the pig and dog. They are best designated numerically as first tarsal,second tarsal, etc. The central tarsal is interposed between the rows. The preceding terms are anglicized abbreviations of those introduced by Gegenbaur intocomparative anatomy. The Latin names and synonyms are given in the following table: Tibial (Os tarsi tibiale, Tt.) Astragalus or Talus Fibular (Os tarsi fibulare, Tf.) Calcaneus or Os calcis Central (Os tarsi centrale, Tc.) Scaphoid or Navicular First Tarsal (Os tarsale primum, Tl) First or internal cuneiform Second Tarsal (Os tarsale secundum, T2) Second or middle cuneiform Third Tarsal (Os tarsale tertium, T3) Third or external cuneiform Fourth Tarsal (Os tarsale quartum, T4) Cuboid The metatarsal and digitalbones resemble in general those of the correspondingregions of the thoracic limb; the differential features will be noted in the specialdescriptions.
  28. 28. —32 THE SKELETON OF THE HORSE THE SKELETON OF THE HORSE The skeleton of the horse consists of 205 bones, as shown in the following table: Vertebral column 54 Ribs 36 Sternum 1 Skull (including auditory ossicles) 34 Thoracic limbs 40 Pelvic limbs 40 205 In this enumeration the average number of coccygeal vertebrae is taken to be 18, the tem-poral and OS coxae are not divided into parts, the usual number of carpal and tarsal elements istaken, and the sesamoids are included. t Fig. 6. Skeleton of Horse, with Outune of Contour of Body. I.H., Atlas; 7.H., seventh cervical vertebra; I.R., first thoracic vertebra; 17. R., seventeenth thoracic vertebra;I.L., first lumbar vertebra; O.L., sixth lumbar vertebra; K, sacrum; I.S., first coccygeal vertebra; 16.S., sixteenthcoccygeal vertebra; ^.i?., sixth rib; e. A., costal cartilage; 18. R., lust rih; 1, scapula; 1, cartilage of scapula; 2, spineof scapula; 4, humerus; 4 lateral epicondyle of humerus; 5, lateral tuberosity of humerus; 6, deltoid tuberosity;7, shaft of ulna; 8, olecranon; 9, radius; 10, carpus; accessory carpal bone; 12, metacarpus; 13, digit; 14, sternum, 11,14", xiphoid cartilage; 15, ilium; 16, 16, angles of ilium; 17, ischium; 18, femur (shaft) 19, trochanter major; 20, ;patella; 21, tibia (shaft); 21, lateral condyle of tibia; 2.3, fibula; 22, tarsus; 24, tuber calcis; 25, metatarsus;26, digit; 27, trochanter minor of femur; 28, trochanter tertius of femur. (After EUenberger-Baum, Anat. fiirKiinstler.)
  29. 29. — — THE VERTEBRAL COLUMN 33 The Vertebral Column The vertebral formula of the horse is C7Ti8L6S5Cyi5-2i.Anterior artic- ular process of axisSpinous proc- ess of axis Transverseprocess of axis Fig. 7. Cervical Vertebrae of Horse; Dorsal View. Fig. 8. Cervical Vertebr.e of Horse; Ventral View. a, Articular processes; b, transverse processes; 1, dorsal arch of atlas; 2, w-ing of atlas; 3, intervertebral a. ventral tubercle of atlas; Transverse processes; 1, foramen of atlas; 6, foramen 3, S, fossa atlantis; anterior articular cavities of atlas; foramen of atlas; 4, alar transversarium of atlas; 6, dens of axis; 7, intervertebral 4, alar foramen; 6, foramen transversarium; 6, ventral foramen of axis; 8, foramen transversarium of axis; 9, spinous processes. 3
  30. 30. —34 THE SKELETON OF THE HORSE THE CERVICAL VERTEBRA The cervical vertebrae (Vertebrae cervicales) are seven in number. The first and second cervical vertebrae are highly modified in conformity with the special functions of support and movements of the head. The sixth and seventh present special characters, but do not differgreatly from the tjpe. With the exception of the first, they are quadrangular, mass- ive, and longer than the ver- tebrae of other regions; they decrease in length from the second to the last. The third, fourth, and fifth have the fol- lowing characters: 1. The body is long as compared with those of other vertebrae. The ventral surface median ventral presents a which becomes more spine, Head prominent as it is traced back- Fig. 9. Sixth Cervical Vertebra of Horse; Anterior View. ward, and is tuberculate at its posterior end it separates two ; concave areas. The dorsal sur-face has a flat central area which is narrow in the middle of the vertebrae, and ^^deat either end it gives attachment to the dorsal longitudinal ligament. ; On eitherside of this area there is a groove which lodges the longitudinal spinal vein. Theselateral grooves are connected about the middle of the surface by a transverse furrow,in which there are several for-amina through which veinsemerge from the spongy sub-stance of the body. The an- — Spinous process Vertebralterior extremity or head (Ca- foramen Articularput vertebrae) has an oval process Iarticular surface which facesforward and downward; it isstrongly convex, and widerabove than below. The pos-terior extremity is larger andhas a nearly circular cotyloid PlBff^ — fcavity (Fossa vertebrae). 2. The arch is large andstrong. It is perforated on by a foramen whicheither sidecommunicates with the for-amen trans versarium. Thevertebral notches are large. 3. The articular processesare large. Their articular sur-faces are extensive, oval in outline, and concave; the anterior ones are slightl}^ The remaining surface isdirected dorso-medially; the posterior, ventro-laterally.mainly roughened for ligamentous and muscular attachment. A crest connects
  31. 31. THE ATLAS 35the articular processes of the same side on the fourth and fifth; on the third it doesnot reach the anterior process. 4. The transverse processes are large and plate-like. Each arises by tworoots, one from the arch and one from the body; between these is the foramentransversariiun, through which the vertebral vessels and a nerve pass. The aggre-gate of these foramina constitutes the canalis transversarius. The process divideslaterally into anterior and posterior branches, which are thickened and rough formuscular attachment. 5. The spinous process has the form of a low crest (Crista spinosa), whichwidens behind, and is connected by ridges with the posterior articular processes. The sixth cervical vertebra has the following distinctive features: It is shorterand wider than the fifth. The arch is large, especially posteriorly. The posteriorarticular processes are shorter, thicker, and further apart; each is connected withthe corresponding anterior one by a thick ridge. The spinous process is less rudi-mentary; it is half an inch or more (ca. 1.5 cm.) in height. The transverse proc-esses have three branches; the third part is a thick, almost sagittal plate, whichforms with its fellow and the body a wide ventral groove; the other branchescorrespond to those of the typical vertebrae, but are short and thicker.^ The fora-men transversarium is large; below its posterior end there is a fossa. Theventral spine is small and is less prominent posteriorly. The seventh cervical vertebra is readily distinguished by the following charac-ters It is shorter and wider than the others. : The body is flattened dorso-ventrallyand wide, especially behind; here it has a facet on each side for articulation withpart of the head of the first rib. The arch and its notches are large. The anteriorarticular processes are wider and longer than the posterior pair. The spinousprocess is an inch or more (ca. 3 cm.)in height. The transverse process isundivided, and has no foramen trans-versarium.2 The ventral crest is re-placed by a pair of tubercles. The Atlas This vertebra is decidedly atypi-cal in The body form and structure.and spinous process are absent. Ithas the form of a strong ring, fromwhich two curved plates, the wings,project laterally. The ring incloses avery large vertebral foramen, and con- Fig. Atlas of Horse, Dorsal, View after Removal OF Dorsal Arch.sists of two lateral masses connected 1, Anterior articular cavities; 2, 2, posterior articularby dorsal and ventral arches. surfaces; 3, articular surface of ventral arch for dens of The lateral masses (Massse later- axis; 4, transverse ridge; 5, 5, alar foramina; 6, 6, foram-ales) present two deep oval anterior ina transversaria.articular cavities (Foveas articularescraniales) which receive the occipital condyles; they are separated by a wide notchabove and a narrow one below. The lateral margin is also notched, and a trian-gular non-articular depression cuts into the medial part of each cavity. The pos- somewhat saddle-shaped;terior articular surfaces (Facies articulares caudales) arethey are confluent on the ventral arch, but are widely separated dorsally, and donot conform in shape to the corresponding surfaces of the axis. 1 The third branch of the transverse process and the fossa are sometimes absent or reducedon one side. 2 In some specimens a large foramen transversarium is present on one side or (rarely) onboth sides.
  32. 32. 36 THE SKELETON OF THE HORSE dorsal arch (Arcus dorsalis) presents a median dorsal tubercle (Tuber- Theculum and is concave ventrally. It is perforated on either side near its dorsale)anterior margin by the intervertebral foramen (Foramen invertebrate). The an-terior border is deeply notched, and the posterior is thin and concave. The ventral arch (Arcus ventrahs) is thicker, narrower, and less curved thanthe dorsal. On its lower surface is the ventral tubercle (Tuberculum ventrale),into which the terminal tendon of the longus colli muscle is inserted. The upperface has posteriorly a transversely concave articular surface, the fovea dentis, onwhich the dens or odontoid process of the axis rests. In front of this is a transverserough excavation and a ridge for the attachment of the ligamentum dentis. The wings or alae are modified transverse processes. They are extensivecurved plates which project ventro-laterally and backward from the lateral masses.The dorsal surface is concave. Between the ventral aspect of the wing and thelateral mass is a cavity, the fossa atlantis; in this there is a foramen which opensinto the vertebral canal. The border is thick and rough; its position can be recog-nized in the living animal. Two foramina perforate each Aving. The anterior one,the foramen alare, is connected with the intervertebral foramen by a short groove.The posterior one is the foramen transversariimi. — Development. The atlas ossifies from four centers, two for the ventral arch,and one on either side for each lateral mass, wing, and half of the dorsal arch. At —birth the bone consists of three pieces the ventral arch and two lateral parts,which are separated by a layer of cartilage in the dorsal median line and by twoventro-lateral layers. These parts are usually fused at about six months. The Axis The axis (Axis s. Epistropheus) is the longest of the vertebrae, and is character-ized by the presence of the dens or odontoid process, which projects from the an-terior part of the body. The anterior extremity of the body presents centrally the dens or odontoid Spinous process Posterior articular process Dens Trajisverse process Anterior articular process Body Fig. 12. — Axis of Horse, Left View. 1, Arch; 2, intervertebral foramen; 3, notch; 4. foramen transversarium.process (Dens axis) this has a convex articular surface ventrally for articulation ;with the ventral arch of the atlas, and two rough depressions for the attachment ofthe ligamentum dentis dorsally. Flanking this on either side are the modifiedanterioi articular processes, which have saddle-shaped articular surfaces confluent

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