External Morphology of the Spinal Cord
In position in the body, the spinal cord conforms to the curvatures of the canal in which it lies. In addition to the bony wall of the vertebral canal, it is enveloped and protected by its three membranes or meninges, which are continuous with the like membranes of the encephalon: first, the pia mater, which closely invests the cord and sends ingrowths into its substance, contributing to its support; second, the arachnoid, a, loosely constructed, thin membrane, separated from the pia mater by a considerable subarachnoid space ; third, the dura mater, the outermost and thickest of the membranes, separated from the arachnoid by merely a slit-like space, the subdural space.
The intimate association of the central system with all the peripheral organs is attained chiefly through the spinal cord, and this is accomplished by means of thirty-one pairs of spinal nerves, which are attached along its lateral aspects. The nerves of each pair are attached opposite each other at more or less equal intervals along its entire length, and in passing to the periphery they penetrate the meninges, which contribute to and are continuous with the connective-tissue sheaths investing them. Each nerve is attached by two roots, an afferent or dorsal root, which enters the cord along its postero-lateral sulcus, and an efferent or ventral root, which makes its exit along the ventro-lateral aspect.
With its inequalities in thickness and its conical termination the spinal cord is subdivided into four parts or regions: - (1) The cervical portion, with eight pairs of cervical nerves; (2) the thoracic portion, with twelve pairs of thoracic nerves; (3) the lumbar portion, with five pairs of lumbar nerves; and (4) the conus medullaris, or sacral portion, with five pairs of sacral and one pair of coccygeal nerves. From the termination of the conus medullaris, the pia mater continues below in the subarachnoid space into the portion of the vertebral canal not occupied by the spinal cord, and forms the non-nervous, slender, thread-like terminus, the filum terminale. This becomes continuous with the dura mater at its lower extremity.
In the early fetus the spinal nerves pass from their attachment to the spinal cord outward through the intervertebral foramina at right angles to the long axis of the cord, but, owing to the fact that the vertebral column increases consider- ably in length after the spinal cord has practically ceased growing, the nerve-roots become drawn caudal from their points of attachment, and, as is necessarily the case, their respective foramina are displaced progressively downward as the termination of the cord is approached, until finally the roots of the lumbar and sacral nerves extend downward as a brush of parallel bundles considerably below the levels at which they are attached. This brush of nerve-roots is the Cauda equina. The dura mater, being more closely related to the bony wall of the canal than to the spinal cord, extends with the vertebral column and thus envelops the Cauda equina, undergoing a slightly bulbous, conical dilation which decreases rapidly and terminates in the attenuated canal of the coccyx as the coccygeal ligament.
Wherever there is a greater mass of tissue to be innervated, the region of the nervous system supplying such must of necessity possess a greater number of neurons. Therefore, the regions of the spinal cord associated with the skin and musculature of the regions of the superior and inferior limbs are thicker than the regions from which the neck or trunk alone are innervated. Thus in the lower cervical region the spinal cord becomes broadened into the cervical enlargement, and likewise in the lumbar region occurs the lumbar enlargement. The spinal nerves attached to these regions are of greater size than in other regions.
The cervical enlargement [intumescentia cervicalis] begins with the third cervical vertebra, acquires its greatest breadth (12 to 14 mm.) opposite the lower part of the fifth cervical vertebra (origin of the sixth cervical nerves), and extends to opposite the second thoracic vertebra. Unlike the lumbar enlargement, its lateral is noticeably greater than its dorso-ventral diameter.
The lumbar enlargement [intumescentia lumbalis] begins gradually with the ninth or tenth thoracic vertebra, is most marked at the twelfth thoracic vertebra (origin of the fourth lumbar nerves), and rapidly diminishes into the conus medullaris.
Both the lumbar and thoracic regions are practically circular in transverse section. Neither diameter of the lumbar is ever so great as the lateral diameter of the cervical enlargement. The thoracic part attains its smallest diameter opposite the fifth and sixth thoracic vertebrae (attachment of the seventh and eighth thoracic nerves.)
The enlargements occur with the development of the upper and lower limbs. In the embyro they are not evident until the limbs are formed. In the orangutan and gorilla the cervical enlargement is greatly developed; the ostrich and emu have practically none at all.
Surface of the spinal cord
The cord is separated into nearly symmetrical right and left halves by the broad anterior median fissure into which the pia mater is duplicated, and opposite this, on the dorsal surface, by the posterior median sulcus. Along the lower two-thirds of the cord this sulcus is shallowed to little more than a line which marks the position of the posterior median septum; in the medulla oblongata it opens up and attains the character of a fissure. Each of the two lateral halves of the cord is marked off into a posterior, lateral, and anterior division by two other longitudinal sulci. Of these, the postero-lateral sulcus occurs as a slight groove 2 to 3.5 mm. lateral from the posterior median sulcus, and is the groove in which the root filaments of the dorsal roots enter the cord in regular linear series. The ventral division is separated from the lateral by the antero-lateral sulcus. This is rather an irregular, linear area than a sulcus. It is from 1 to 2 mm. broad, and represents the area along which the efferent fibers make their exit from the cord to be assembled into the respective ventral roots. This area varies in width according to the size of the nerve-roots, and, like the postero-lateral sulcus, its distance from the mid-line varies according to locality, being greatest on the enlargements of the cord. In the cervical region, and along a part of the thoracic, the posterior division is subdivided by a delicate longitudinal groove, the postero-intermediate sulcus, which becomes more evident toward the medulla oblongata and represents the line of demarcation between the fasciculus gracilis and the fasciculus cuneatus. Occasionally in the upper cervical region a similar line may be seen along the ventral aspect close to the anterior median fissure. This is the antero-intermediate sulcus, forming the lateral boundary of the ventral cerebro-spinal fasciculus.
Collectively, the entire space between the posterior median sulcus and the line of attachment of the dorsal roots is occupied by the posterior funiculus; the lateral space between the line of attachment of the dorsal and that of the ventral roots, by the lateral funiculus; and the space between the ventral roots and the anterior median fissure, by the anterior funiculus. Each of these funiculi is subdivided within into its component fasciculi.
The dorsal and ventral nerve-roots are not attached to the cord as such, but are first frayed out into numerous thread-like bundles of axons which are distributed along their lines of entrance and exit. These bundles are the root filaments [fila radicularia] of the respective roots. The fila of the larger spinal nerves are fanned out to the extent of forming almost continuous lines of attachment, while in the thoracic nerves there are appreciable intervals between those of adjacent roots. Throughout, the intervals are less between the fila of the ventral than between those of the dorsal roots.