NERVOUS SYSTEM). The nerve cells are arranged in three main columns, ventral, intermedio-lateral and posterior vesicular.
The ventral cell column has the longest cells, and these are again subdivided into antero-mesial, antero-lateral, postero-lateral and central groups. The intermedio-lateral cell column is found in the lateral horn of the thoracic region.
The posterior vesicular or Clarke's column is also largely confined to the thoracic region, and lies in the mesial part of the posterior cornu. It is the place to which the sensory fibres of the sympathetic system (visceral af ferents) run. The white matter, as has been shown, surrounds the grey and passes across the mid dle line to form the white com missure, which lies in front of the grey. It is composed of neuroglia and medullated nerve fibres, which are arranged in definite tracts, although in a section of a healthy cord these tracts cannot be distinguished even with the microscope. They have been and are still being gradually mapped out by pathologists, physiologists and embryologists.
A sensory nerve on passing into the cord (fig. 3) through the posterior nerve root lies close to the mesial side of the posterior horn of grey matter, where most of it runs upward. The next root higher up takes the same po sition and pushes the former one toward the middle line, so that the lower nerve fibres occupy an area close to the postero-median fissure known as the tract of Goll, while the higher lie more externally in the tract of Burdach.
The greater part of each nerve sooner or later enters the grey mat ter and comes into close relation with the cells of Clarke's column, but some fibres run right up to the nucleus gracilis and cur.eatus in the medulla (see BRAIN), while a few turn down and form a descending tract, which, in the upper part of the cord, is situated in the inner part of the tract of Burdach and is known as the comma tract, but lower down gradually shifts quite close to the postero-median fissure and forms the oval area of Flechsig. It will be obvious that both these tracts could not be seen in the same sec tion, and that fig. 3 is only a diagrammatic outline of their
position.
A few fibres of each sensory nerve ascend in a small area known as Lissauer's tract on the outer side of the posterior nerve roots, and eventually enter the substantia gelatinosa.
To the outer side of Lissauer's tract and lying close to the lateral surface of the cord is the direct cerebellar tract, the fibres of which ascend from the cells of Clarke's column to the cerebellum. As Clarke's column is only well developed in the thoracic region this tract obviously cannot go much lower.
In front of the last and also close to the lateral surface of the cord is another ascending tract, the tract of Gowers, or, as it is sometimes called, the lateral sensory fasciculus. It probably begins in the cells of the posterior horn, and runs up to join the fillet and also to reach the cerebellum through the superior cerebellar peduncle. The crossed pyramidal tract lies internal to the direct cerebellar tract, between it and the posterior cornu. It is the great motor tract by which the fibres coming from the Rolandic area of the cerebral cortex are brought into touch with the motor cells in Owens College, 1891). Probably the fissure is formed partly by an infolding and partly from the original central canal. The antero-median fissure is caused by the ventral part of the cord growing on each side, but not in the mid-line where no germinal cells are.
The anterior nerve roots are formed by the axons of the neuro blasts in the developing anterior cornea, but the posterior grow the anterior cornu of the opposite side. This tract extends right down to the fourth sacral nerve.
In front of the crossed pyramidal tract is the lateral basis bundle, which probably consists of association fibres linking up different segments of the cord.
The anterior basis bundle lies in front and on the mesial side of the anterior cornu, and through it pass the anterior nerve roots. Like the lateral bundle it consists chiefly of association fibres, but it is continued up into the medulla as the posterior longitudinal bundle to the optic nuclei.