DEVELOPMENT OF THE SPINAL CORD.
The part of the neural tube that becomes the spinal cord appears of oval form on transverse section. The central canal forms a dorso-ventrally directed cleft, which is bounded laterally by the thickened walls of the medullary tube, but dorsally and ven trally by thinner parts of the same ; therefore, a separation into a right and left half is easily recognizable. The thinner dorsal and ventral walls appear as commissures behind and before, the dorsal or posterior commissure being called the roof-plate and the ventral or anterior commissure the floor-plate. During the further development, these plates grow relatively little, while both lateral halves continue to thicken, their growth being especially marked ventrally. In this locality on each side appears a ventral projection. Consequently, the floor-plate is pushed farther from the surface and, finally, a median longitudinal cleft, the fissura median anterior, is formed in front. A similar change occurs in the dorsal region, the roof-plate being likewise pushed in and disappearing at the bottom of the sulcus medianus posterior. The spinal cord now consists of two robust lateral halves, separated from each other by an anterior fissure and a posterior sulcus. During this further development also the central canal has changed its form, since the dorsal part of the original dorso-ventrally directed cleft becomes closed in consequence of the apposition of the lateral walls.
At first the spinal cord extends the entire length of the vertebral canal with a fairly constant volume. The lower end of the cord becomes rudimentary and defined from the preceding part, assumes a conical form and becomes the conus medullaris. A fur ther alteration in the extension of the spinal cord is brought about by the inequality between its growth and that of the surrounding vertebral canal. The latter constantly increases
in length, the lower segment of the spine developing with especial vigor. Since the growth of the cord fails to keep pace with that of the spine, the cord apparently shortens and no longer extends the entire length of the vertebral canal. The conus medullaris is drawn up from the sacral canal and enters the lumbar region, until, finally, it is found opposite the first or second lumbar vertebra. During this ascensus medullae spinalis the end of the conus medullaris is drawn out into a thin thread, which extends as far as the coccygeal region and is known as the filum terminale. A further consequence of this ascensus is a change in the course of the nerves emerging from the spinal cord. In the cervical region the course of the nerves is still horizontal ; in the thoracic region it is more and more oblique ; while in the lumbar region, and, still more in the sacral, the nerves are directed downward. The nerve-trunks emerging from the last part of the cord lie, therefore, for a long distance within the vertebral canal before they leave the latter. They surround the conus medullaris and the filum terminale and in this manner lead to the formation of the so-called hdrse-tail or cauda equina. In completion, the spinal cord undergoes some further changes in its form. Gradually two segments acquire greater development, the one in the cervical portion and the other in the upper part of the lumbar region. They are known as the cervical enlargement (intumescentia cervicalis) and the lumbar enlargement (intumescentia lumbalis) respectively.