714 



THE NERVOUS SYSTEM. 



is only found in the upper part of the cord ; it gradually diminishes as it is traced 

 downward, and disappears about the middle of the dorsal region. It consists of cen- 

 trifugal or descending fibres which can be traced downward from the pyramid of the 

 medulla of the same side, and are derived from the motor area of the cerebral cortex. 

 The fibres of this tract decussate in their course down the cord, passing across the 

 middle line through the anterior white commissure ; this explains the gradual dimi- 

 nution and eventual disappearance of the tract. (2) In the hinder part of the antero- 

 lateral column is a somewhat triangular area, larger than the preceding, which is 

 named the crossed pyramidal tract. This also consists of descending fibres, which 

 are derived from the pyramid of the medulla of the opposite side, and which have 

 crossed in the decussation of the pyramids. The fibres are derived from the motor 

 area of the cerebral cortex of the opposite side. Thus it will be seen that all the 

 fibres from the motor area, which descend through the internal capsule, the crus 

 cerebri, and the pons Varolii to the pyramidal body of the medulla, decussate ; 

 some at the upper part of the cord, and these descend through it as the crossed 

 pyramidal tract ; and others, which descend as the direct pyramidal tract and cross 

 through the anterior commissure of the cord to reach the crossed pyramidal tract 

 of the opposite side. Although this is the usual method of describing the crossing 

 of the direct pyramidal tract in the cord, it seems probable that its fibres cross in 

 the anterior commissure and pass directly to the anterior horn of gray matter, to 

 end by forming synapses around its cells. (3) The antero-lateral ascending tract 

 (Gower's tract) is an extensive crescent-shaped strand which skirts the circumfer- 

 ence of the anterior three-quarters of the antero-lateral column of the cord. Behind, 

 where it is thickest, it lies in the angle formed by the direct cerebellar and crossed 

 pyramidal tracts, becoming narrower as it passes forward toward the direct pyrami- 

 dal tract. It consists of centripetal or ascending fibres, which arise from cells situ- 

 ated at the base of the posterior horn and which cross to the opposite side of the 



cord in the anterior gray commis- 

 sure. They can be traced upward 

 through the medulla and pons to 

 the cerebellum, reaching the latter 

 through its superior peduncles. If 

 the spinal cord is divided in the cer- 

 vical region, some scattered fibres in 

 this column degenerate in a down- 

 ward direction. This would seem 

 to prove therefore that it contains 

 some descending fibres, which are 

 believed to be derived from the same 

 side of the cerebellum. (4) The 

 direct cerebellar tract is situated at 

 the circumference of the cord behind 

 the preceding and external to the 

 crossed pyramidal tract, occupying 

 a narrow area which extends back- 

 ward as far as the postero-lateral 

 fissure or nearly so. It commences at the level of the upper lumbar region, and 

 increases in size as it ascends and passes through the restiform body of the medulla 

 to the cerebellum. Its fibres are derived from the cells of the posterior vesicular 

 column of Clarke in the gray matter of the cord. (5) Close to the point where 

 the posterior roots enter the cord, in the antero-lateral column, is a small collection 

 of fibres, which is known as the tract of Lissauer ; it is formed by some of the 

 fibres of the posterior roots which run upward in the tract for a short distance, and 

 then enter the posterior horn of the gray matter. (6) The rest of the antero- 

 lateral column of the spinal cord is occupied by the antero-lateral ground bundle. 

 It surrounds the anterior cornu and separates the antero-lateral tract and the crossed 

 pyramidal tract from the gray matter of the cord. It consists of (a) longitudinal 



FIG. 384. Transverse section of the gray substance of the 

 spinal cord, near the middle of the dorsal region. Magni- 

 fied 13 diameters. 



