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excrescences-the cerebral hemispheres-are budded off from it; and they become the dominant part of the nervous system (Fig. 458).
Each hemisphere is formed, however, from a relatively small part of the side wall of the prosencephalon, the rest of which goes to form the optic diverticula, the thalamus, and the hypothalamus, among other structures. The cerebral hemisphere is at first pre-eminently olfactory in function, the nerves of smell being inserted directly into it. But impressions of the associated sense of taste make their way into the cerebral hemisphere in the most primitive vertebrates: the gustatory nerves are inserted into the medulla oblongata, but fibre-paths are laid down to establish connexions with the hypothalamus, which in turn emits fibres to the cerebral hemisphere (Fig. 454). The thalamus is a greatly swollen part of the prosencephalic wall adjoining the mesencephalon. Its main part receives sensory impressions brought up from the spinal medulla and the terminal nuclei of the sensory cerebral nerves and transmits them to the cerebral hemisphere. Its caudal portion becomes specialised as a special receptive nucleus for visual and acoustic impressions for transmission to the cerebral hemisphere. It is called the metathalamus or corpora geniculata. Thus the cerebral hemisphere from being essentially a receptive organ for smell impressions ultimately becomes the terminus of all the sensory paths, and the structure that is concerned with the consciousness of all kinds of sensations. It also controls the voluntary movements of one-half of the body and emits a great strand of fibres-pedunculus cerebri to establish relations with the cerebellum and all the motor nuclei on the other side of the encephalon and spinal medulla (Fig. 454, p. 513).
The spinal medulla is that part of the central nervous system which occupies the upper two-thirds of the vertebral canal. It is an elongated cylindrical structure, slightly flattened in front and behind, which extends from the margin of the foramen magnum to the level of the inferior border of the body of the first lumbar vertebra or to the superior border of the body of the second lumbar vertebra. Its average length in the male is 45 cm. and in the female 43 cm.
A considerable amount of variation within certain limits (viz., the mid-point of the body of the last thoracic vertebra and the superior border of the body of the third lumbar vertebra) is observed in different individuals as to the precise level at which the spinal medulla ends inferiorly, and in the female there would appear to be a tendency for the medulla to reach a slightly lower point in the canal than in the male. Further, the relation presented by the spinal medulla to the vertebral column differs in a marked degree in the foetus and infant at different periods of development. Up to the third month of intra-uterine life the spinal medulla occupies the entire length of the vertebral canal; it extends downwards to the lowest limit of the vertebral canal. But from this time onwards, as growth proceeds, the vertebral column lengthens at a more rapid rate than the medulla. The spinal medulla, therefore, has the appearance of shrinking in an upward direction within its canal, and at
Fourth ventricle Medulla oblongata
Cervical swelling of the spinal medulla
Lumbar swelling of the spinal medulla
birth its inferior end is usually found to be opposite FIG. 459.-HUMAN FOETUS IN THE THIRD the body of the third lumbar vertebra.
The attitude assumed by the individual affects to
a small degree the position of the inferior end of the
MONTH OF DEVELOPMENT, WITH THE
spinal medulla. Thus, when the trunk is bent well forwards, the terminal part of the
spinal medulla rises slightly within its bony canal.
At the margin of the foramen magnum the spinal medulla becomes continuous with the medulla oblongata of the brain, whilst below, it tapers rapidly to a point and forms a conical extremity termed the conus medullaris. From the end of the conus medullaris a slender glistening thread is prolonged downwards within the vertebral canal, and finally anchors the spinal medulla to the back of the coccyx. This prolongation receives the name of the filum terminale.
The diameter of the spinal medulla is very much shorter than that of the vertebral canal within which it lies. A wide interval is left between its surface and the walls of its canal, and this excess of space is clearly a provision for allowing free movement of the vertebral column without producing any jarring contact between the delicate spinal medulla and the surrounding bones.
Three protective membranes are wrapped around the spinal medulla. From within outwards these are termed (1) the pia mater, (2) the arachnoid, and (3) the
dura mater. The pia mater is a fibrous membrane which forms the immediate investment. It is closely applied to the spinal medulla, and from its deep
First coccygeal vertebra
FIG. 460. THE CONUS MEDULLARIS AND THE
surface numerous fine septa penetrate into the substance of the spinal medulla. The arachnoid is an exceedingly delicate transparent membrane which is loosely wrapped around the spinal medulla so as to leave a considerable interval, between itself and the pia mater, termed the subarachnoid space, in which there is always a varying amount of cerebro-spinal fluid. Outside the arachnoid, the dura mater forms a wide, dense, fibrous, tubular sheath, which extends downwards within the vertebral canal for a considerable distance beyond the conical extremity of the spinal medulla. The spinal medulla is suspended within its sheath or theca of dura mater by two lateral wing-like ligaments, termed the ligamenta denticulata. These extend laterally from the sides of the spinal medulla and are attached by a series of pointed or tooth-like processes to the inner surface of the theca of dura mater. Between the wall of the vertebral canal and the dura mater there is a narrow interval, which is filled up by soft areolo-fatty tissue and numerous thin-walled veins arranged in a plexiform manner.
Thirty-one pairs of spinal nerves arise from the sides of the spinal medulla.
These are classified into eight cervical, twelve thoracic, five lumbar, five sacral, and one coccygeal; and according to the attachments of these groups of nerves the spinal medulla is arbitrarily subdivided into cervical, thoracic, lumbar, and sacral regions. In employing these terms, therefore, for different districts of the spinal medulla, it must be understood that the regions are determined by the nerve attachments and not by any direct relationship between these parts of the spinal medulla and the sections of the vertebral column which bear the same names.
Each spinal nerve is attached to the spinal medulla by an anterior or ventraland a posterior or dorsal root, and as these are traced to their central attachments they are seen to break up into a number of separate nerve fascicles or bundles, which spread out, in some cases very widely from each other, as they approach the side of the spinal medulla (Fig. 461). Each pair of nerves is therefore attached to a portion of spinal medulla of some length, and such a portion, with its pair of nerves, receives the name of a "segment of the spinal medulla." It must be clearly understood, however, that, in so far as the surface of the spinal medulla is concerned, there is no means of marking off one segment from another except by the nerve attachments.
In the cervical and lumbar regions of the spinal medulla the nerve-roots are somewhat crowded together, so that little or no interval is left between the adjoining root fila or fascicles of neighbouring nerves. In the thoracic region, however, distinct intervals may be observed, and the root fila are more loosely arranged. From this, it will be evident that the segments in different parts of the spinal medulla are not of equal length. In the cervical region the segments measure about 12 mm. in length, in the thoracic region from 20 to 24 mm., and in the lumbar region about 10 mm. The number of fila which attach the different nerve-roots to the spinal medulla is very different in different nerves, and is not necessarily the same in the same nerve-root in different individuals.
Roots of first
Owing to the great difference which exists between the length of the spinal medulla and the length of the vertebral column, the farther we pass down the greater the distance becomes between the attachment of the various nerve-roots to the spinal medulla and the intervertebral foramina through which the corresponding nerves leave the vertebral canal. The lower nerve-roots, therefore, have to traverse the vertebral canal for a considerable distance before they reach their apertures of emergence. It thus happens that the nerve-roots which spring from the lumbar and sacral regions of the spinal medulla attain a very great length and descend vertically in the lower part of the vertebral canal in a bunch or leash, in the midst of which lie the conus medullaris and the filum terminale. This great bundle of nerve-roots receives the appropriate name of the cauda equina.
FIG. 462.-SECTION THROUGH THE CONUS MEDULLARIS AND THE CAUDA EQUINA AS THEY LIE IN THE VERTEBRAL CANAL.
Enlargements of the Spinal Medulla.-Throughout the greater part of the thoracic region, the spinal medulla presents a uniform girth and a very nearly circular outline when seen in transverse section. In the cervical and lumbar regions, however, it shows marked swellings. The intumescentia cervicalis or cervical enlargement is the more evident of the two. It begins very gradually at the upper end of the spinal medulla, attains its greatest breadth (12 to 14 mm.) opposite the fifth or sixth cervical vertebra, and finally subsides opposite the second thoracic vertebra. To this portion of the spinal medulla are attached the great nerves which supply the upper limbs. The intumescentia lumbalis or lumbar enlargement begins at the level of the tenth thoracic vertebra, and acquires its maximum transverse diameter (11 to 13 mm.) opposite the last thoracic vertebra. Below, it rapidly tapers away into the conus medullaris. To the lumbar enlarge
ment are attached the great nerves of the lower limbs.
These enlargements of the spinal medulla are associated with the outgrowth of the limbs. In the earlier developmental stages of the spinal medulla they are not present, and they take form only as the limbs become developed. In different animals their size corresponds with the degree of development of the limbs. Thus, in the long-armed orang and gibbon the cervical swelling stands out with a remarkable degree of prominence.
Development of the Spinal Medulla.-The early stages of the process by which the originally simple epithelial neural tube becomes converted into the central nervous system have already been considered. It remains to be explained how the features specially distinctive of the spinal medulla are produced.
In the early stages of the development of the spinal medulla (Fig. 463), the neuroblasts are found to be scattered in the intermediate of the three bands of
FIG. 463.-DIAGRAM OF TRANSVERSE SECTION OF THE LEFT HALF OF EARLY NEURAL TUBE.
which the thick side wall of the neural tube is composed-the mantle layer. These primitive nerve-cells soon congregate in much larger numbers in the ventral part of the basal lamina (Fig. 464), so that the mantle layer expands there into a broad excrescence, which is the rudiment of the columna anterior or anterior cornu of gray matter. This anterior column contains the efferent or motor nervecells, the axons of which emerge as the anterior root of a spinal nerve. At this stage the rest of the mantle layer consists of a thin stratum of neuroblasts (Fig. 463), mainly intercalated cells, which receive the sensory impressions entering the spinal medulla through the radix posterior, and transmit impulses into axons passing (a) to the motor nuclei, (b) to the other side of the spinal medulla through the floor-plate (Fig. 463), or (c) into the superficial stratum (peripheral layer) of the spinal medulla where they bend upwards or downwards as constituent elements of the funiculi (or white columns). As development proceeds (Fig. 463) the substantia grisea (gray substance) formed of these intercalated cells becomes much more abundant and forms a broad blunt boss (Figs. 464, B and C), which is the rudiment of the columna posterior (O.T. posterior cornu).
The surfaces of these gray columns become coated with a layer of white sub
stance, composed at first mainly of the axons of cells in the root ganglia and intercalated cells in the spinal medulla; and as these funiculi increase in size they help to mould the form of the gray columns. This is displayed best in the case of the posterior column (O.T. posterior cornu). The major portion of the white substance, funiculus posterior, which accumulates behind (and afterwards lies on the medial side of) the posterior column, does not consist of fibres springing from intercalated cells, either of the spinal medulla or any other part of the central nervous system, but of the direct continuations of the central processes of the cells in the spinal ganglion on the posterior root (Figs. 463 and 464). A large proportion of the fibres of the posterior root do not enter the gray columns immediately after their insertion into the alar lamina, but bifurcate to form two vertical nerve-fibres, one passing upwards, and the other downwards, in the funiculus posterior before they end in the gray column, some distance above or below the place where they gained admission to the medulla spinalis. As the spinal medulla grows, the originally blunt posterior column becomes drawn backwards into an increasingly attenuated process, and the funiculus posterior, which was placed originally upon its lateral surface (Fig. 464, A), and then upon its posterior surface (Fig. 464, B), gradually assumes a wedge-shaped form (Figs. 464, C, and 466), upon the medial side of the gray matter.
Development of the Anterior Median Fissure, Posterior Median Septum, and of the Central Canal.-As the anterior columns
Posterior median septum
There has been considerable discussion as to the mode of formation of the posterior median septum; but there is now no doubt as to the essential facts. Early in the third month the walls of the posterior three-fourths (of the sagittal extent) of the central canal of the spinal medulla become approximated (Fig. 464), and later they fuse to obliterate that part of the canal. But the part of the septum thus formed is only an insignificant portion of the whole. For most of the septum is produced by the gradual elongation of the epithelial cells lining the remnant of the central canal as the fibremasses of the posterior funiculi expand and separate the posterior surface of the spinal medulla further and further from the situation FIG. 464.-THREE STAGES IN THE DEVELOP
of the canal (see Fig. 453, p. 511).
Furrows of the Spinal Medulla.-When cross-sections of the adult spinal