434 . THE NERVOUS SYSTEM. 
cerebellar tract, and, like it, next the surface of the lateral column. It is also an 
ascending tract, and it likewise (in part at least) ultimately reaches the cerebellum, 
although after leaving the cord it takes a different route to gain its destination. 
In transverse sections of the cord it presents a comma-shaped appearance, the 
thick part abutting against the direct cerebellar tract, and the narrower portion 
tapering forwards into the region of the emerging anterior nerve-roots. The tract 
of Gowers begins at a lower level in the cord than the direct cerebellar tract 
and it increases in volume as it is traced upwards. The fibres of this tract have 
probably their origin in the cells of the posterior horn of gray matter, but on this 
point there is at present no precise information. 
The crossed pyramidal tract (fasciculus cerebro-spinalis lateralis) is a large well- 
defined descending tract, which hes immediately in front of the posterior horn of 
eray matter and subjacent to the direct cerebellar tract, which shuts it out from 
the surface of the cord. Below the point where the direct cerebellar tract begins 
the crossed pyramidal tract becomes superficial, and in this position it can be 
traced as low as the fourth sacral nerve, at which level it ceases to exist as a 
distinct strand. The crossed pyramidal tract is composed of an admixture of both 
large and small fibres. These arise in the brain from the large pyramidal cells 
of the motor or Rolandic area of the cerebral cortex, and pass downwards through 
various subdivisions of the brain to gain the spinal cord. As they enter the cord 
they cross the mesial plane from one side to the other, and it thus happens that 
the crossed pyramidal tract in the right lateral column of- the cord has its origin 
in the cortex of the left cerebral hemisphere, and vice versa. As the tract descends | 
in the cord it gradually diminishes in size; and this is due to the fact that, as it 
traverses each spinal segment, numerous fibres leave it to enter the anterior horn 
of gray matter, and end in connexion with the ventral motor cells from which the 
fibres of the anterior nerve-roots arise. The entire strand is ultimately exhausted 
in this way. Numerous collateral fibrils sprmg from the pyramidal fibres, and, 
entering the gray matter, end in a similar manner, and in this way a single pyra- 
midal fibre may be connected with several spinal segments before it finally ends. 
The crossed pyramidal tract must be regarded as a great motor strand which 
brings the spinal motor apparatus under the control of the will. 
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I 
Schaffer believes that many of the pyramidal fibres end in connexion with the cells 
of Clarke’s column. 
The lateral basis-bundle (fasciculus lateralis proprius) represents the remainder 
of the lateral column. Our information regarding it is still imperfect; but it 
would appear that its fibres are largely derived from the cells situated in all parts 
of the gray matter, and also from the nerve-cells of the opposite side of the cord. 
After a course of very varying length in the basis-bundle, these fibres turn inwards — 
and re-enter the gray matter. Such fibres may thus be regarded as inter-segmental 
association fibres binding two or more segments of the cord together. 
Anterior Column of the Cord.—One well-defined tract is situated in the 
anterior column. This is termed the direct pyramidal tract. The remainder of 
the column receives the name of the anterior basis-bundle. 
The direct pyramidal tract (fasciculus cerebro-spinalis anterior) is usually a 
nerve-strand of small size which lies next the antero-median fissure. As a rule, 
it cannot be traced lower than the middle of the dorsal region of the cord. It is 
a descending tract and must be associated with the crossed pyramidal tract of the — 
opposite side, seeing that both of these strands arise from the motor area of the 
cortex of the same cerebral hemisphere. From this, it must be clear that the 
direct pyramidal tract does not cross the mesial plane as it enters the cord, but 
descends on the side of the cord corresponding to the cerebral hemisphere in 
which it arises. All the same its fibres do not end in the same side of the 
cord, but at every step along the path of the strand they make use of the anterior 
commissure and cross to the opposite side of the cord, to terminate in relation 
to the opposite ventral motor cells in the same manner as the crossed pyramidal 
fibres. 
