This name is applied to the spinal portion of the nervous centre. It is a nervous stem, white, cylindrical, and symmetrical, and lies in, but does not completely fill, the vertebral canal; it is held in place by the denticulated ligament at each side. It is united with the encephalon by the medulla oblongata. Terminating in a point at its lower extremity, it rapidly increases in diameter, and forms the lumbar enlargement, so called from the region which it occupies; in the dorsal region it diminishes in size; augmenting anew as it approaches the neck, so as to form the cervical enlargement, it shrinks again about the middle of the cervical region, and then enlarges a third time at its superior extremity, and forms the medulla oblongata. The spinal marrow is marked in front and behind, throughout its entire length, by a fissure or median furrow, which divides it into two distinct halves, excepting a layer of white substance, which unites both the two fissures and the medullary fascicles to right and left This layer, sprinkled with holes, designed to give passage to vessels, is the perforated commissure.
The anterior median furrow is covered at the top by the interlacing of the nervous fascicles which run obliquely from one-half of the spinal marrow to the other, and of which we shall speak presently. The posterior furrow, like the anterior, disappears insensibly toward the inferior extremity of the spinal cord; at its upper extremity it opens at an acute angle where the medulla oblongata commences. Its form resembles the point of a pen—hence the name calamus scrip torius which has been given to this part of the medulla oblongata.
Each half of the spinal cord, separated from the other by the fissures indicated above, is composed of two cords or bundles; one, the posterior, giving origin to the posterior roots of the nerves, and the other, the anterior, to the anterior roots. These cords are attached to, and are continuations of, the pyramids of the medulla oblongata.
This latter is marked in front by the median furrow which extends beyond the interlacing of the fibres, of which mention has been made; on each side of this furrow there is an oblong elevation, these are the anterior pyramids; outside of which are two projections still more marked, called olives or the olivary bodies. Laterally there is a depression, grayish in colour, in which terminate the posterior roots of the spinal nerves; behind this we find a bundle of distinct fibres, the restiform or cord-shaped bodies; and lastly, outside of these bodies are the posterior pyramids, defining the calamus scrip-torius on either side. The cerebellum, as we have already seen, covers the posterior surface of the bulb, to which it is united by the restiform bodies or inferior peduncles of the cerebellum, and which contribute with the cavity of the calamus scriptorius to form the fourth ventricle.
The anterior pyramids terminate by the interlacing of their nervous fascicles, and this interlacement may be considered as the lower boundary of the medulla oblongata. These pyramids are contracted at the apex and at the base, and are inserted into the pons Varolii by a sort of neck or contraction.
The anterior and posterior roots of the spinal nerves form two parallel lines on the sides of the spinal marrow. The roots spring from the spinal cord, but it cannot be anatomically demonstrated that their fibres return to it beyond the point where they originate and where they constitute by their reunion the medullary fascicles. Opinions are not fixed on the mode by which union is effected between the nervous roots and the spinal cord, and we confine ourselves to the simple statement that the cord appears to consist of a greater number of nervous fibres than the nerves which spring from it.
The isthmus of the encephalon is formed by the expansion of the superior portion of the spinal cord; it is the central point of the greater and lesser brain, of which the hemispheres of the cerebrum and cerebellum are but the terminal developments.