Nervous System

The direct pyramidal tract is a small bundle of the motor fibres from the Rolandic area, which, instead of crossing to the other side at the decussation of the pyramids in the medulla, runs down by the side of the antero-median fissure. Its fibres, however, keep on gradually crossing to the opposite side through the anterior white commissure of the cord, and by the time the mid-thoracic region is reached it has usually disappeared.

The roots of the spinal nerves in the upper part of the canal rise from the cord nearly opposite the points at which they emerge between the vertebrae, but the farther one passes down the higher the origin of each root becomes above its point of emergence. Consequently the lumbar and sacral nerves run a long way down from the lumbar enlargement to their spinal foramina and are enclosed in the dural and arachnoid sheaths to form a mass like a horse's tail, which is therefore known as the cauda equina.

Embryology.

The early development of the neural tube from the ectoderm is outlined in the article on the BRAIN. When the neural groove becomes a tube it is oval in section with a very large laterally compressed central canal (see fig. 5). The original ecto dermal cells elongate and, radiating outward from the canal, are now known as spongioblasts, while the inner ends of some of them bear cilia and so the canal becomes ciliated. A number of round cells, known as germinal cells, now appear close to the central canal, except at the thin mid-dorsal and mid-ventral laminae (roof plate and floor-plate). From the division of these the primitive nerve cells or neuroblasts are formed and these later on migrate from the region of the canal and shoot out long processes—the axons. The permanent central canal of the cord was formerly said only to represent the ventral' end of the large embryonic canal, the dorsal part being converted into a slit by the gradual closing in of its lateral walls, thus forming the postero-median fissure. A. Robinson, however, does not believe that the posterior fissure is any remnant of the central canal (Studies in Anatomy, into the cord from the posterior root ganglia (see NERVE: Spinal), and, as they grow, form the columns of Goll and Burdach.

In the embryo up to the fifth month there is little difference in the appearance of the grey and white matter of the cord, but at that time the fibres in the columns of Burdach acquire their medullary sheaths or white substance of Schwann, the fatty mat ter of which is probably abstracted from the blood. Very soon

after these the basis bundles myelenate and then, in the sixth month, the columns of Goll. Next follow the direct cerebellar tracts and, in the latter half of the eighth month the tracts of Gowers, while the fibres of the pyramidal and Lissauer's tracts do not gain their medullary sheaths until just before or after birth. At first the spinal cord extends as far as the last mesodermal somite, but neuroblasts are only formed as far as the first coccy geal somite, so that behind that the cord is non-nervous and de generates later into the filum terminale. After the fourth month the nervous portion grows more slowly than the rest of the body and so the long cauda equina and filum terminate are produced. At birth the lower limit of the cord is opposite the third lumbar vertebra, but in post-natal development it recedes still farther to the lower level of the first.

For further details see standard text books, e.g., Quain's Anat omy and J. P. McMurrich, Development of the Human Body (1906).

Comparative Anatomy.

In the Amphioxus there is little dif ference between spinal cord and brain ; the former reaches the whole length of the body and is of uniform calibre. It encloses a central canal from which a dorsal fissure extends to the surface of the cord and it is composed of nerve fibres and nerve cells; most of the latter being grouped round the central canal or neurocoele, as they are in the human embryo. Some very large multipolar ganglion cells are present, and there are also large fibres known as giant fibres, the function of which is not clear.

When the reptiles are reached the cord shows slight enlarge ments in the regions of the limbs and these become more marked in birds and mammals.

In the lumbar region of birds the dorsal columns diverge and open up the central canal, converting it into a diamond-shaped space which is only roofed over by the membranes of the cord, and is known as the sinus rhomboidalis.