Oblongata is a continuation of the medullary substance of the cerebrum and cerabellum, passing downwards, and a little backwards to the foramen magnum occipitale, where it assumes the name of medulla spinalis. It is rather of a depressed pyriform figure though called oblong; rising by two crura from the cerebrum, and two peduncles from the cerebellum: the enlargement formed from the union of these is called pons varolii., or tuberculum annulare, behind which is a stricture upon the medulla oblongata, followed by an enlargement, on each side, styled corpora pyramidalia. From the medulla oblongata arises the medulla spinalis, and all the nerves which pass from the head, except the first and second pairs. (See Nervi.) Death is the immediate consequence of an injury on the medullary part. See Cerebrum.
Medulla spinalis; cerebrum elongatum; AEon; is the continuation of the medulla oblongata, from the foramen magnum occipitale, through the vertebrae of the neck, back, and loins. It is of different sizes; in the neck flat and broad; in the back, small; in the loins, large; and at last it becomes a bundle of nerves, which have the name of cauda equina, because when taken out, and extended in water, they resemble a horse's tail, and as the medulla passes out of the foramen magnum, its external membrane the dura mater is united to the ligamentous lining of the bony cavity, but this connection does not extend beyond the first vertebra. The cineritious matter in the medulla spinalis is within the medullary.
A singular circumstance, of importance in explaining the phenomena of many diseases is, that the spinal marrow, though apparently one cord, is divided into two, easily separated, but united by a cineritious substance: it is therefore, double, but, from the mode of union, single only; and we thus see how, in the more important organs, an injury in one part of the marrow is attended with a diminution of the power only in the organ, and can explain the reason of the irreparable consequences of a distortion, or exostosis of the bony canal. The fasciculi of nervous fibrils are sent off from each portion of this double cord, anteriorly and posteriorly, passing through separate foramina of the sheath. The posterior nerves form a ganglion, and then unite with the anterior. Each fasciculus is attended by the external lamina of the dura mater, the internal is gradually lost, and the angle left by each in its first divarication is filled by a fine ligament, which appears to support the nerves on the front, and behind, in their passage. This denticulated ligament seems through the whole cord to distinguish the anterior and posterior bundles, and, near the cauda equina, has been considered as itself nervous, since it there loses its denser structure as having less to support. The tunica arachnoides is more distinct from the pia mater in the spinal marrow than in the brain, though it adheres more closely to this membrane at the bottom than above. It is apparently suspended by the denticulated ligament, and passes with the dura mater, along the nervous fibrils sent off.
The spinal arteries arise from the vertebral within the foramen magnum, which again passing through the occipital hole divide into two other branches, which run to the posterior portion of the medulla. Each runs through the respective grooves formed by the division of the cord. They divide, and again unite and anastomose occasionally, with the vertebral,the intercostal, the lumbar, and sacral arteries. We thus see that the effects of a slight change in the capacity of the bony canal, by interrupting the course of the blood, and, in part the nervous influence, may be gradually obviated by the efforts of nature alone. The medullary veins terminate in the vertebral, and in different sinuses, which have a free communication, apparently serving, as in the head, to keep the arteries full, and to prevent the fatal effects of temporary pressure. See Cerebrum.