The central nervous system consists of the encephalon or brain, contained within the cranium, and the medulla spinalis or spinal cord, lodged in the vertebral canal; the two portions are continuous with one another at the level of the upper border of the atlas vertebra.
The peripheral nervous system consists of a series of nerves by which the central nervous system is connected with the various tissues of the body. For descriptive purposes these nerves may be arranged in two groups, cerebrospinal and sympathetic, the arrangement, however, being an arbitrary one, since the two groups are intimately connected and closely intermingled.
The enteric nervous system , a subsystem of the peripheral nervous system, has the capacity, even when severed from the rest of the nervous system through its primary connection by the Vagus nerve , to function independently in controlling the gastrointestinal system
The nervous tissues are composed of nerve cells and their various processes, together with a supporting tissue called neuroglia, which, however, is found only in the brain and medulla spinalis. Certain long processes of the nerve cells are of special importance, and it is convenient to consider them apart from the cells; they are known as nerve fibers.
axis-cylinder is the essential part of the nerve fiber, and is always present; the medullary sheath and the neurolemma are occasionally absent, expecially at the origin and termination of the nerve fiber
The neurolemma or primitive sheath presents the appearance of a delicate, structureless membrane. Here and there beneath it, and situated in depressions in the white matter of Schwann, are nuclei surrounded by a small amount of protoplasm
When nerve fibers are cut across, the central ends of the fibers degenerate as far as the first node of Ranvier; but the peripheral ends degenerate simultaneously throughout their whole length. The axons break up into fragments and become surrounded by drops of fatty substance which are formed from the breaking down of the medullary sheath. The nuclei of the primitive sheath proliferate, and finally absorption of the axons and fatty substance occurs
Wallerian degeneration occurs after axonal injury in both the peripheral nervous system (PNS) and central Nervous System (CNS). It occurs in the axon stump distal to a site of injury and usually begins within 24-36 hours of a lesion. Prior to degeneration distal axon stumps tend to remain electrically excitable. After injury, the axonal skeleton disintegrates and the axonal membrane breaks apart.
forms the elongated, nearly cylindrical, part of the central nervous system which occupies the upper two-thirds of the vertebral canal.
Its average length in the male is about 45 cm., in the female from 42 to 43 cm., while its weight amounts to about 30 gms.
It extends from the level of the upper border of the atlas to that of the lower border of the first, or upper border of the second, lumbar vertebra.
Above, it is continuous with the brain; below, it ends in a conical extremity, the conus medullaris, from the apex of which a delicate filament, the filum terminale, descends as far as the first segment of the coccyx
closely invests the spinal cord and sends delicate septa into its substance; a narrow band, the ligamentum denticulatum, extends along each of its lateral surfaces and is attached by a series of pointed processes to the inner surface of the dura mater.
the wall of the hind-brain - form the medulla oblongata, the pons, and cerebellum, while its cavity is expanded to form the fourth ventricle
The mid-brain - forms the cerebral aqueduct ( aqueduct of Sylvius ), which serves as a tubular communication between the third and fourth ventricles; its walls are thickened to form the corpora quadrigemina and cerebral peduncles
The fore-brain undergoes great modification:
anterior part or telencephalon expands laterally in the form of two hollow vesicles, the cavities of which become the lateral ventricles, while the surrounding walls form the cerebral hemispheres and their commissures;
the cavity of the posterior part or diencephalon forms the greater part of the third ventricle, and from its walls are developed most of the structures which bound that cavity
extends from the lower margin of the pons to a plane passing transversely below the pyramidal decussation and above the first pair of cervical nerves; this plane corresponds with the upper border of the atlas behind, and the middle of the odontoid process of the axis in front; at this level the medulla oblongata is continuous with the spinal cord
The anterior district is named the pyramid ( pyramis medullæ oblongatæ ) and lies between the anterior median fissure and the antero-lateral sulcus. Its upper end is slightly constricted, and between it and the pons the fibers of the abducent nerve emerge;
The two pyramids contain the motor fibers which pass from the brain to the medulla oblongata and spinal cord: corticobulbar and corticospinal fibers
When these pyramidal fibers are traced downward it is found that some two-thirds or more of them leave the pyramids in successive bundles, and decussate in the anterior median fissure, forming what is termed the pyramidal decussation
constitutes the largest part of the hindbrain. It lies behind the pons and medulla oblongata; between its central portion and these structures is the cavity of the fourth ventricle. It rests on the inferior occipital fossæ, while above it is the tentorium cerebelli, a fold of dura mater which separates it from the tentorial surface of the cerebrum
The cerebellum consists of three parts, a median and two lateral, which are continuous with each other, and are substantially the same in structure. The median portion is constricted, and is called the vermis, from its annulated appearance which it owes to the transverse ridges and furrows upon it; the lateral expanded portions are named the hemispheres.
two cylindrical masses situated at the base of the brain, and largely hidden by the temporal lobes of the cerebrum
The depressed area between the crura is termed the interpeduncular fossa, and consists of a layer of grayish substance, the posterior perforated substance, which is pierced by small apertures for the transmission of bloodvessels
two large ovoid masses, situated one on either side of the third ventricle and reaching for some distance behind that cavity. Each measures about 4 cm. in length, and presents two extremities, an anterior and a posterior, and four surfaces, superior, inferior, medial, and lateral.
is a flattened, somewhat quadrilateral band of fibers, situated at the junction of the floor and anterior wall of the third ventricle. Most of its fibers have their origins in the retina, and reach the chiasma through the optic nerves, which are continuous with its antero-lateral angles. In the chiasma, they undergo a partial decussation
is a median cleft between the two thalami. Behind, it communicates with the fourth ventricle through the cerebral aqueduct, and in front with the lateral ventricles through the interventricular foramen. Somewhat triangular in shape, with the apex directed backward, it has a roof, a floor, an anterior and a posterior boundary and a pair of lateral walls
constitute the largest part of the brain, and, when viewed together from above, assume the form of an ovoid mass broader behind than in front, the greatest transverse diameter corresponding with a line connecting the two parietal eminences.
contains a sickle-shaped process of dura mater, the falx cerebri. In front and behind, the fissure extends from the upper to the under surfaces of the hemispheres and completely separates them, but its middle portion separates them for only about one-half of their vertical extent; for at this part they are connected across the middle line by a great central white commissure, the corpus callosum
is situated about the middle of the lateral surface of the hemisphere, and begins in or near the longitudinal cerebral fissure, a little behind its mid-point. It runs sinuously downward and forward, and ends a little above the posterior ramus of the lateral fissure, and about 2.5 cm. behind the anterior ascending ramus of the same fissure
on the medial surface of the hemisphere. It begins near the occipital pole in two converging rami, and runs forward to a point a little below the splenium of the corpus callosum, where it is joined at an acute angle by the medial part of the parietoöccipital fissure
on the medial surface of the hemisphere; it begins below the anterior end of the corpus callosum and runs upward and forward nearly parallel to the rostrum of this body and, curving in front of the genu, is continued backward above the corpus callosum, and finally ascends to the supero-medial border of the hemisphere a short distance behind the upper end of the central sulcus
are irregular cavities situated in the lower and medial parts of the cerebral hemispheres, one on either side of the middle line. They are separated from each other by a median vertical partition, the septum pellucidum, but communicate with the third ventricle and indirectly with each other through the interventricular foramen