Mbbs syllabus 3rd semester Georgia Kutaisi Atsu

1. Angiology

2. The heart and lungs are situated in the
thorax, the walls of which afford them
protection. The heart lies between the two
lungs, and is enclosed within a fibrous
bag, the pericardium, The capacity of the
cavity of the thorax
does not correspond
with its apparent
size externally,
because

3. - the space enclosed by the lower ribs is
occupied by some of the
abdominal viscera; and
- the cavity extends above
the anterior parts of the 1st
ribs into the neck.
The size of the thoracic
cavity is constantly varying
during life with the movements of the ribs
and diaphragm, and with the degree of
distention of the abdominal viscera.

4. From the collapsed state of the lungs as
seen when the thorax is opened in the
dead body, it would appear as if the viscera
only partly filled the cavity,
but during life there is no
vacant space, that which is
seen after death being filled
up by the expanded lungs.

5. The Upper Opening of the Thorax.
The parts which pass through the upper
opening of the thorax are, from before
backward, in or near the middle line, the
Sternohyoideus and Sternothyreoideus
muscles, the remains of the thymus, the
inferior thyroid veins, the
trachea, esophagus thoracic
duct, and the Longus colli
muscles;

6. The Lower Opening of the Thorax.
The lower opening of the thorax is wider
transversely than from before backward. It
slopes obliquely downward and backward,
so that the thoracic cavity
is much deeper behind
than in front.
The diaphragm closes
the opening and forms
the floor of the thorax.

7. The floor is flatter at the center than at the
sides, and higher on the right side than on
the left; in the dead body the right side
reaches the level of the upper border of the
5th costal cartilage, while
the left extends only to
the corresponding part
of the 6th costal cartilage.

8. The pericardium is a conical fibro-serous
sac, in which the heart and the roots of the
great vessels are contained. It is placed
behind the sternum and the cartilages of the
3rd, 4th , 5th , 6th , and 7th ribs of the left side,
in the mediastinal cavity.

9. In front, it is separated from the anterior wall
of the thorax, in the greater part of its
extent, by the lungs and pleuræ; but a small
area, somewhat variable in size, and usually
corresponding with the left half of the lower
portion of the body of the
sternum and the medial
ends of the cartilages of
the 4th and 5th ribs of the
left side, comes into
direct relationship with
the chest wall.

10. The lower extremity of the thymus, in the
child, is in contact with the front of the
upper part of the pericardium. Behind, it
rests upon the bronchi, the esophagus, the
descending thoracic aorta,
and the posterior part of
the mediastinal surface of
each lung.

11. Laterally, it is covered by the pleuræ, and
is in relation with the mediastinal surfaces
of the lungs; the phrenic nerve, with its
accompanying vessels, descends between
the pericadium and pleura on
either side. Posterior wall of
the pericardial sac, showing
the lines of reflection of the
serous pericardium on the
great vessels.

12. Pericardium.
Although the pericardium
is usually described as a
single sac, an examination
of its structure shows that it
consists essentially of two
sacs intimately connected
with one another, but
totally different in structure. The outer sac,
known as the fibrous pericardium, consists
of fibrous tissue.

13. The inner sac, or serous pericardium, is a
delicate membrane which
lies within the fibrous sac
and lines its walls; it is
composed of a single layer
of flattened cells resting on
loose connective tissue.The
heart invaginates the wall of
the serous sac from above
and behind, and practically obliterates its
cavity, the space being merely a potential
one.

14. The fibrous pericardium forms a flask-
shaped bag, the neck of which is closed by
its fusion with the external coats of the great
vessels, while its base is attached to the
central tendon and to the
muscular fibers of the left
side of the diaphragm.

15. Over a small area the central tendon of
the diaphragm and the pericardium are
completely fused.
Above, the fibrous pericardium not only
blends with the external
coats of the great vessels,
but is continuous with the
pretracheal layer of the
deep cervical fascia.

16. By means of these upper and lower
connections it is securely anchored within
the thoracic cavity. It is also attached to the
posterior surface of the sternum by the
superior and inferior sterno
pericardiac ligaments;
The upper passing to the
manubrium, and the lower
to the xiphoid process.

17. The vessels receiving fibrous prolongations
from this membrane are: the aorta, the
superior vena cava, the right and left
pulmonary arteries, and the four pulmonary
veins. The inferior vena cava
enters the pericardium through
the central tendon of the
diaphragm, and receives no
covering from the fibrous
layer.

18. The serous pericardium is, as already stated,
a closed sac which lines the fibrous pericarp-
dium and is invaginated by the heart; it
therefore consists of a visceral and a parietal
portion. The visceral portion,
or epicardium, covers the
heart and the great vessels,
from the latter is continuous
with the parietal layer which
lines the fibrous pericardium.

19. The portion which covers the vessels is
arranged in the form of two tubes. The aorta
and pulmonary artery are enclosed in one
tube, the arterial mesocardium. The superior
and inferior venæ cavæ and
the four pulmonary veins
are enclosed in a second
tube, the venous
mesocardium,

20. between the aorta and
pulmonary artery in
front and the atria
behind —
is the transverse sinus.

21. Cor-The heart is a hollow muscular organ
of a somewhat conical form; it lies
between the lungs in the middle
mediastinum and is enclosed in the
pericardium. It is placed obliquely in the
chest behind the body of the sternum and
adjoining parts of
the rib cartilages

22. The heart projects farther into the left
than into the right half of the thoracic
cavity, so that about one-third of it is
situated on the right and two-thirds on the
left of the median plane.

23. The heart, in the adult, measures
about 12 cm. in length,
8 to 9 cm. in breadth at the broadest part,
and 6 cm. in thickness.
Its weight, in the male, varies from 280 to
340 grams;
In the female, from 230 to 280 grams.
The heart continues to increase
in weight and size up to an advanced
period of life; this increase is more
marked in men than in women.

24. Component Parts. As has already been
stated, the heart is subdivided by septa into
right and left halves, and a constriction
subdivides each half of the organ into two
cavities, the upper cavity
being called the atrium,
the lower the ventricle. The
heart therefore consists of
four chambers, viz., right
and left atria, and right and
left ventricles.

25. The division of the heart into four cavities is
indicated on its surface by grooves. The
atria are separated from the ventricles by
the coronary sulcus (auriculoventricular
groove); this contains the
trunks of the nutrient
vessels of the heart, and is
deficient in front, where it
is crossed by the root of
the pulmonary artery.

26. The interatrial groove, separating the two
atria, is scarcely marked on the posterior
surface, while anteriorly it is hidden by the
pulmonary artery and
aorta.
The ventricles are
separated by two grooves,
one of which, the
anterior longitudinal
sulcus, the other posterior
longitudinal sulcus,

27. The anterior longitudinal sulcus, is situated
on the sternocostal surface of the heart,
close to its left margin, the other posterior
longitudinal sulcus, on the diaphragmatic
surface near the
right margin;

28. these grooves extend from the base of
the ventricular portion to a notch, the
incisura apicis cordis, on the acute
margin of the heart just to the right of
the apex.

29. Base (basis cordis), directed upward,
backward, and to the right, is separated
from the 5th, 6th, 7th , and 8th thoracic
vertebræ by the esophagus, aorta, and
thoracic duct. It is formed mainly by the left
atrium, and, to a small extent, by the back
part of the right atrium.

30. Somewhat quadrilateral in form, it is in
relation above with the bifurcation of the
pulmonary artery, and is bounded below by
the posterior part of the
coronary sulcus,
containing the coronary
sinus.

31. The four pulmonary veins, two on either
side, open into the left atrium, while the
superior vena cava
opens into the upper,
and the inferior vena
cava into the lower,
part of the right.

32. Apex cordis is directed downward,
forward, and to the left, and is overlapped
by the left lung and pleura:

33. Apex lies behind the 5th left intercostal
space, 8 to 9 cm. from the mid-sternal
line, or about 4 cm. below and 2 mm. to
the medial side of the left mammary
papilla.

34. The superior vena cava returns the blood
from the upper half of the body, and opens
into the upper and back part of the right
atrium, the direction of its orifice being
downward and forward.
Its opening has no valve.

35. The inferior vena cava, larger than the
superior, returns the blood from the lower
half of the body, and opens into the lowest
part of the atrium, near the atrial septum,
its orifice being directed
upward and backward,
and guarded by a rudiment-
tary valve, the valve of the
inferior vena cava
(Eustachian valve).

36. The coronary sinus opens into the atrium,
between the orifice of the inferior vena cava
and the atrioventricular opening. It returns
blood from the substance of the heart and is
protected by a semicircular valve, the valve
of the coronary sinus.
The foramina venarum
minimarum are the orifices
of minute veins, which
return blood directly from
the muscular substance
of the heart.

37. The right atrioventricular opening (tricuspid
orifice) is the large oval aperture of
communication between the atrium and
the ventricle; it will be described with the
right ventricle.

38. The valve of the inferior vena cava, Eustachian
valve is situated in front of the orifice of the
inferior vena cava. It is semilunar in form, its
convex margin being attached to the anterior
margin of the orifice; its concave margin, which
is free, ends in two cornua, of
which the left is continuous with
the anterior edge of the limbus
fossæ ovalis while the right
is lost on the wall of the atrium.

39. The valve of the coronary sinus (valvula
sinus coronarii ,The besian valve) is a
semicircular fold of the lining membrane of
the atrium, at the orifice of the coronary
sinus.
It prevents the regurgitation of blood into
the sinus during the contraction of the
atrium. This valve may be double or it may
be cribriform.

40. The fossa ovalis is an oval depression on
the septal wall of the atrium,
and corresponds to the
situation of the foramen
ovale in the fetus.
It is situated at the
lower part of the septum,
above and to the left of the
orifice of the inferior vena
cava.

41. The limbus fossæ ovalis (annulus ovalis) is
the prominent oval margin of the fossa
ovalis. It is most distinct above and at the
sides of the fossa; below, it is deficient. A
small slit-like valvular opening is
occasionally found, at the upper margin of
the fossa, leading upward beneath the
limbus, into the left atrium; it is the remains
of the fetal aperture between the two atria.

42. The intervenous tubercle (tuberculum
intervenosum; tubercle of Lower) is a small
projection on the posterior wall of the
atrium, above the fossa ovalis. It is in man
is scarcely visible. It was supposed by
Lower to direct the blood
from the superior vena
cava toward the
atrioventricular opening.

43. Right Ventricle - ventriculus dexter is
triangular in form, and extends from the
right atrium to near the apex of the heart. Its
anterosuperior surface is
rounded and convex, and
forms the larger part of the
sternocostal surface of the
heart. Its under surface is
flattened, rests upon the
diaphragm, and forms a
small part of the diaphragmatic surface of
the heart.

44. Its posterior wall is formed by the
ventricular septum, which bulges into the
right ventricle, so that a transverse section
of the cavity presents a semilunar outline.
Right
ventricle
Left
ventricle

45. Its upper and left angle forms a conical
pouch, the conus arteriosus, from which the
pulmonary artery arises. A tendinous band,
which may be named the tendon of the
conus arteriosus, extends upward from the
right atrioventricular fibrous
ring and connects the poste-
rior surface of the conus
arteriosus to the aorta.

46. The right atrioventricular orifice is the large
oval aperture of communication between the
right atrium and ventricle. Situated at the
base of the ventricle, it measures about 4
cm. in diameter and is surrounded by a
fibrous ring, covered by the lining
membrane of the
Heart.

47. It is considerably larger than the
corresponding aperture on the left side,
being sufficient to admit the ends of four
fingers. The right atrioventricular orifice
is guarded by the
tricuspid valve.

48. The opening of the pulmonary artery is
circular in form, and situated at the summit
of the conus arteriosus, close
to the ventricular septum.
It is placed above and to the
left of the atrioventricular
opening, and is guarded by the
pulmonary semilunar valves.

49. The tricuspid valve (valvula tricuspidalis)
consists of three somewhat triangular
cusps or segments. The largest cusp is the
anterior cusp. A second, the posterior cusp,
and a third, the medial or septal cusp.

50. They are formed by duplicatures of the lining
membrane of the heart, strengthened by
intervening layers of fibrous tissue: their
central parts are thick and strong, their
marginal portions thin and
translucent, and in the
angles between the latter
small intermediate
segments are sometimes
seen.

51. Their bases are attached to a fibrous ring
surrounding the atrioventricular orifice and
are also joined to each other so as to form a
continuous annular membrane, while their
apices project into the
ventricular cavity.

53. The pulmonary semilunar valves are three
in number, two in front and one behind,
formed by duplicatures of the lining
membrane, strengthened by fibrous tissue.
They are attached, by their
convex margins, to the wall
of the artery, at its junction
with the ventricle, their free
borders being directed
upward into the lumen of
the vessel.

54. The free and attached margins of each are
strengthened by tendinous fibers, and the
former presents, at its middle, a thickened
nodule (corpus Arantii). From this nodule
tendinous fibers radiate through the
segment to its attached margin, but are
absent from two narrow crescentic portions,
the lunulæ, placed one on either side of the
nodule immediately adjoining the free
margin.

55. Left Atrium, atrium sinistum - is rather
smaller than the right, but its walls are
thicker, measuring about 3 mm.; it
consists, like the right, of
two parts, a principal
cavity and an auricula.

56. The principal cavity is cuboidal in form,
and concealed, in front, by the pulmonary
artery and aorta; in front and to the right it
is separated from the right
atrium by the atrial septum;
opening into it on either
side are the two pulmonary
veins.

57. Auricula (auricula sinistra) is somewhat
constricted at its junction with the
principal cavity; it is longer, narrower, and
more curved than that of the right side, and
its margins are more deeply
indented. It is directed
forward and toward the
right and overlaps the
root of the pulmonary
artery.

58. The interior of the left atrium presents the
following parts for examination:
- Openings of the four pulmonary veins.
- Left atrioventricular opening.
- Musculi pectinati.

59. The pulmonary veins, four in number, open
into the upper part of the posterior surface
of the left atrium —two on either side of its
middle line: they are not provided with
valves. The two left veins frequently end
by a common opening.

60. The left atrioventricular opening is the
aperture between the left atrium and
ventricle, and is rather smaller than the
corresponding opening on the right side.
The musculi pectinati, fewer and smaller
than in the right
auricula, are confined
to the inner surface
of the auricula.

61. Left Ventricle, ventriculus sinister- is longer
and more conical in shape than the right,
and on transverse section its concavity
presents an oval or nearly circular
outline.

62. The left atrioventricular opening (mitral
orifice) is placed below and to the left of the
aortic orifice. It is a little smaller than the
corresponding aperture of the opposite side,
admitting only two fingers. It is guarded by
the bicuspid or mitral valve.

63. Aorta laid open to show the semilunar
valves. The aortic opening is a circular
aperture, in front and to the right of the
atrioventricular, from which it is separated
by the anterior cusp of the bicuspid valve.
Its orifice is guarded by the
aortic semilunar.

64. The bicuspid or mitral valve is
(valvula bicuspidalis) attached
to the circumference of the
left atrioventricular orifice in
the same way that the tricuspid
valve is on the opposite side.

65. The cusps are of unequal size, and are
larger, thicker, and stronger than those of
the tricuspid valve. The larger cusp is placed
in front and to the right between the
atrioventricular and aortic orifices, and is
known as the anterior or aortic cusp; the
smaller or posterior cusp is placed behind
and to the left of
the opening.

66. The aortic semilunar valves are three in
number, and surround the orifice of the
aorta; two are anterior (right and left) and
one posterior. They are similar in structure,
and in their mode of attachment, to the
pulmonary semilunar valves, but are larger,
thicker, and stronger.

67. Opposite the valves the aorta presents
slight dilatations, the aortic sinuses
(sinuses of Valsalva), which are larger than
those at the origin of the pulmonary artery.

68. The musculi papillares are two in number,
one being connected to the anterior, the
other to the posterior wall; they are of large
size, and end in rounded extremities from
which the chordæ tendineæ arise. The
chordæ tendineæ from
each papillary muscle
are connected to both
cusps of the bicuspid
valve.

69. Ventricular Septum (interventricular septum)
is directed obliquely backward and to the
right, and is curved with the convexity
toward the right ventricle: its margins
correspond with the anterior
and posterior longitudinal
sulci. The greater portion of
it is thick and muscular and
constitutes the muscular
ventricular septum.

70. its upper and posterior part, which
separates the aortic vestibule from the lower
part of the right atrium and upper part of the
right ventricle, is thin and fibrous, and is
termed the membranous ventricular septum.
An abnormal communication
may exist between the
ventricles at this part owing
to defective development of
the membranous septum.

71. Strucutre.
The heart consists of muscular fibers, and
of fibrous rings which serve for their
attachment. It is covered by the visceral
layer of the serous
pericardium-epicardium,
and lined by the
endocardium. Between
these two membranes is
the muscular wall or -
myocardium.
Epicard
ium
Myocardium
Endocardium
Parietal
pericardium

72. endocardium is a thin, smooth membrane
which lines and gives the glistening
appearance to the inner surface of the
heart; it assists in forming the valves by
its reduplications, and is continuous with
the lining membrane of the large
bloodvessels.
It consists of connective tissue and elastic
fibers, and is attached to the muscular
structure by loose elastic tissue which
contains bloodvessels and nerves; its free
surface is covered by endothelial cells.

73. The fibrous rings surround the
atrioventricular and arterial orifices,
and are stronger upon the left than on
the right side of the heart.
The atrioventricular rings serve for
the attachment of the muscular fibers
of the atria and ventricles, and for the
attachment of the bicuspid and
tricuspid valves.

74. The fibers of the heart differ very remarkably
from those of other striped muscles. They
are smaller by one-third, and their
transverse striæ are by no means so well-
marked. They show faint longitudinal
striation. The fibers are made up of distinct
quadrangular cells, joined
end to end so as to form
a syncytium.

75. Each cell contains a clear oval nucleus,
situated near its center. The extremities
of the cells have a tendency to branch or
divide, the subdivisions
uniting with offsets from
other cells, and thus
producing an anastomosis
of the fibers.

76. The connective tissue between the
bundles of fibers is much less than in
ordinary striped muscle, and no
sarcolemma has been proved to exist.

77. Purkinje Fibers
Between the endocardium and the
ordinary cardiac muscle are found,
imbedded in a small amount of connective
tissue, peculiar fibers known as Purkinje
fibers. The fibers are very much larger in
size than the cardiac cells and differ from
them in several ways. In longitudinal
section they are quadrilateral in shape,
being about twice as long as they are
broad.

78. The muscular structure of the heart
consists of bands of fibers, which present
an exceedingly intricate interlacement.
They comprise the fibers
- of the atria,
- of the ventricles, and
- the atrioventricular
bundle of His.

79. The fibers of the atria are arranged in two
layers: a superficial, common to both
cavities, and a deep, proper to each. The
superficial fibers are most distinct on the
front of the atria, across the bases of which
they run in a transverse direction, forming a
thin and incomplete layer.
Some of these fibers run into
the atrial septum. The deep
fibers consist of looped and
annular fibers.

80. The fibers of the ventricles are arranged in a
complex manner, and various accounts
have been given of their course and
connections.
They consist of superficial
and deep layers, all of which,
with the exception of two, are
inserted into the papillary
muscles of the ventricles.

81. The superficial layers consist of the
following: (a) Fibers which spring from the
tendon of the conus arteriosus and sweep
downward and toward the left across the
anterior longitudinal sulcus and around the
apex of the heart, where they
pass upward and inward to
terminate in the papillary
muscles of the left ventricle;

82. The deep layers are three in number; they
arise in the papillary muscles of one
ventricle and turn in at the longitudinal
sulcus and end in the papillary muscles of
the other ventricle.
The layer which is most
superficial in the right
ventricle lies next
the lumen of the left.

83. Those of the first layer almost encircle the
right ventricle and, crossing in the septum
to the left, unite with the
superficial fibers from
the right atrioventricular
ring to form the
posterior papillary
muscle.

84. Those of the second layer have a less
extensive course in the wall of the right
ventricle, and a correspondingly greater
course in the left, where they join with the
superficial fibers from the
anterior half of the tendon
of the conus arteriosus to
form the papillary muscles
of the septum.

85. The atrioventricular bundle of His, is the
only direct muscular connection known to
exist between the atria and the ventricles.
Its cells differ from ordinary cardiac muscle
cells in being more spindle-shaped. They
are, moreover, more loosely arranged and
have a richer vascular supply than the rest
of the heart muscle.

86. Sinoatrial
(SA) node
Purkinje
fibers
Atrioventricular
(AV) node

87. It arises in connection with two small
collections of spindle-shaped cells, the
sinoatrial and atrioventricular nodes.
The sinoatrial node is situated on the
anterior border of the opening of the
superior vena cava; from its
strands of fusiform fibers
run under the endocardium
of the wall of the atrium to
the atrioventricular node.

88. The atrioventricular node lies near the orifice
of the coronary sinus in the annular and
septal fibers of the right atrium; from it the
atrioventricular bundle passes forward in the
lower part of the membranous septum, and
divides into right and left fasciculi. These run
down in the right and left
ventricles, one on either side
of the ventricular septum,
covered by endocardium.

89. In the lower parts of the ventricles they
break up into numerous strands which end
in the papillary muscles and in the
ventricular muscle generally. The greater
portion of the atrioventricular bundle
consists of narrow, somewhat fusiform
fibers, but its terminal strands are
composed of Purkinje fibers.

90. Vessels and Nerves.
The arteries supplying the heart are the right
and left coronary from the aorta; the veins
end in the right atrium.
The lymphatics end in the thoracic and right
lymphatic ducts.The nerves are derived from
the cardiac plexus, which are formed partly
from the vagi, and partly from the
sympathetic trunks. They are freely
distributed both on the surface and in the
substance of the heart, the separate nerve
filaments being furnished with small ganglia.

91. The Cardiac Cycle and the Actions of
the Valves.
By the contractions of the heart the
blood is pumped through the arteries
to all parts of the body. These
contractions occur regularly and at the
rate of about seventy per minute. Each
wave of contraction or period of
activity is followed by a period of rest,
the two periods constituting what is
known as a cardiac cycle.

92. Each cardiac cycle consists of three phases,
which succeed each other as follows: (1) a
short simultaneous contraction of both
atria, termed the atrial systole, followed,
after a slight pause, by (2) a simultaneous,
but more prolonged, contraction of both
ventricles, named the ventricular systole,
and (3) a period of rest, during which the
whole heart is relaxed.

93. The atrial contraction commences around
the venous openings, and sweeping over
the atria forces their contents through the
atrioventricular openings into the ventricles,
regurgitation into the veins being prevented
by the contraction of their muscular coats.

95. Peculiarities in the Vascular System in the
Fetus
The chief peculiarities of the fetal heart are
the direct communication between the atria
through the foramen ovale, and the large
size of the valve of the inferior vena cava.
Among other peculiarities the following
may be noted. In early fetal life the heart is
relatively large in size. As development
proceeds it is gradually drawn within the
thorax, but at first it lies in the middle line;
toward the end of pregnancy it gradually
becomes oblique in direction.

96. (2) For a time the atrial portion exceeds the
ventricular in size, and the walls of the
ventricles are of equal thickness: toward the
end of fetal life the ventricular portion
becomes the larger and the wall of the left
ventricle exceeds that of the right in
thickness.
(3) Its size is large as compared with that of
the rest of the body, the proportion at the
second month being 1 to 50, and at birth, 1
to 120, while in the adult the average is
about 1 to 160.

97. The foramen ovale, situated at the lower
part of the atrial septum, forms a free
communication between the atria until the
end of fetal life.
After birth the foramen
ovale is obliterated.
The valve of the inferior
vena cava serves to
direct the blood from that
vessel through the foramen
ovale into the left atrium.

98. The peculiarities in the arterial system of the
fetus are the communication between the
pulmonary artery and the aorta by means of
the ductus arteriosus, and the continuation
of the hypogastric arteries as the umbilical
arteries to the placenta.

99. The ductus arteriosus is a short tube, about
1.25 cm. in length at birth, and of the
diameter of a goose-quill. In the early
condition it forms the continuation of the
pulmonary artery, and opens into the aorta,
just beyond the origin of the left subclavian
artery; and so conducts the greater amount
of the blood from the right ventricle into the
aorta. When the branches of the pulmonary
artery have become larger relatively to the
ductus arteriosus, the latter is chiefly
connected to the left pulmonary artery.

100. The hypogastric arteries run along the sides
of the bladder and thence upward on the
back of the anterior abdominal wall to the
umbilicus; here they pass out of the
abdomen and are continued as the umbilical
arteries in the umbilical cord to the placenta.
They convey the fetal blood to the placenta.

101. Fetal Circulation The fetal blood is
returned from the placenta to the fetus by
the umbilical vein. This vein enters the
abdomen at the umbilicus, and passes
upward along the free margin of the
falciform ligament of the liver to the under
surface of that organ, where it gives off
two or three branches, one of large size to
the left lobe, and others to the lobus
quadratus and lobus caudatus.

102. At the porta hepatis (transverse fissure of
the liver) it divides into two branches: of
these, the larger is joined by the portal vein,
and enters the right lobe; the smaller is
continued upward, under the name of the
ductus venosus, and joins the inferior vena
cava. The blood, therefore, which traverses
the umbilical vein, passes to the inferior
vena cava in three different ways.

103. The lungs of the fetus being inactive, only
a small quantity of the blood of the
pulmonary artery is distributed to them by
the right and left pulmonary arteries, and
returned by the pulmonary veins to the left
atrium: the greater part passes through
the ductus arteriosus into the aorta,
where it mixes with a small quantity of the
blood transmitted by the left ventricle into
the aorta.

104. Through this vessel it descends, and is in
part distributed to the lower extremities
and the viscera of the abdomen and
pelvis, but the greater amount is
conveyed by the umbilical arteries to the
placenta.

105. The placenta serves the purposes of
nutrition and excretion, receiving the
impure blood from the fetus, and
returning it purified and charged with
additional nutritive material.
Nearly the whole of the blood of the
umbilical vein traverses the liver before
entering the inferior vena cava; hence the
large size of the liver, especially at an
early period of fetal life.

106. The right atrium is the point of meeting of a
double current, the blood in the inferior
vena cava being guided by the valve of this
vessel into the left atrium, while that in the
superior vena cava descends into the right
ventricle.
At an early period of fetal life it is highly
probable that the two streams are quite
distinct; for the inferior vena cava opens
almost directly into the left atrium, and the
valve of the inferior vena cava would
exclude the current from the right ventricle.

107. At a later period, as the separation
between the two atria becomes more
distinct, it seems probable that some
mixture of the two streams must take
place.
The pure blood carried from the placenta to
the fetus by the umbilical vein, mixed with
the blood from the portal vein and inferior
vena cava, passes almost directly to the
arch of the aorta, and is distributed by the
branches of that vessel to the head and
upper extremities.

108. The blood contained in the descending
aorta, chiefly derived from that which has
already circulated through the head and
limbs, together with a small quantity from
the left ventricle, is distributed to the
abdomen and lower extremities.

109.  Changes in the Vascular System at Birth.
At birth, when respiration is established, an
increased amount of blood from the
pulmonary artery passes through the lungs,
and the placental circulation is cut off. The
foramen ovale is closed by about the 10th
day after birth: the valvular fold above
mentioned adheres to the margin of the
foramen for the greater part of its
circumference, but a slit-like opening is left
between the two atria above, and this
sometimes persists.

110.  The ductus arteriosus begins to contract
immediately after respiration is
established, and is completely closed
from the 4th to the 10th day; it ultimately
degenerates into an impervious cord, the
ligamentum arteriosum, which connects
the left pulmonary artery to the arch of the
aorta.

111.  Of the hypogastric arteries, the parts
extending from the sides of the bladder to
the umbilicus become obliterated
between the second and fifth days after
birth, and project as fibrous cords, the
lateral umbilical ligaments, toward the
abdominal cavity, carrying on them folds
of peritoneum.

112.  The umbilical vein and ductus venosus
are completely obliterated between the
second and fifth days after birth; the
former becomes the ligamentum teres, the
latter the ligamentum venosum, of the
liver.

113. The blood entering the atrium through the
superior vena cava is directed downward
and forward, i.e., toward
the atrioventricular orifice,
while that entering through
the inferior vena cava is
directed upward and back-
ward, toward the atrial
septum. This is the normal
direction of the two
currents in fetal life.

114.  The pulmonary veins are not provided
with valves.
 The superior vena cava, Its opening has
no valve.