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TEXT-BOOK OF PHYSIOLOGY 



and functions. Some of the more important tracts are shown in Fig. 229. 

 They may be divided, however, into efferent, afferent, and associative fibers 



1. The anterior column, comprising that portion between the anterioi 

 longitudinal fissure and the anterior roots, has been subdivided into: 



(a) The direct pyramidal tract, or column of Tiirck. This tract borders 

 the longitudinal fissure and extends from the upper extremity of the cord as 

 far down as the mid- thoracic region. From above downward this tract 

 diminishes in size, for the reason that its fibers or their collaterals cross at 

 successive levels to the opposite side of the cord by way of the anterior com- 

 missure to enter the gray matter of the anterior horn. These fibers are the 

 continuations of fibers which take their origin in cells which are located in 

 the cortex of the cerebral hemisphere of the same side. The terminal 

 filaments of these fibers or axons are in physiologic relation either directly 

 or indirectly through intercalated neuron cells with the dendrites of the 

 cornual cells. When divided in any part of their course, these fibers undergo 

 descending degeneration. 



(b) The antero-lateral ground bundle or root zone. This tract lies external 

 to the pyramidal tract, surrounds the anterior horn of the gray matter, and 

 extends throughout the length of the cord. It is composed of short com- 

 missural or associative fibers which come from nerve-cells in the gray matter 

 from the same and opposite sides of the cord. After entering the white 

 matter they divide into two branches, pursue opposite directions, then re- 

 enter the gray matter at higher and lower levels and come into relation with 

 other nerve-cells : 



2. The lateral column, comprising that portion between the ventral 

 and dorsal roots, has been divided into: 



(a) The antero-lateral tract of Gowers. This tract is somewhat crescentic 

 in shape and situated on the lateral aspect of the cord external to the antero- 

 lateral root zone. It extends throughout the entire length of the cord. 

 When divided it undergoes ascending degeneration, which would indicate 

 that the axons originate in nerve-cells in the gray matter.. This tract is 

 therefore probably afferent in function. 



The majority of the fibers composing this tract on reaching the pons 

 turn backward, pass through the superior medullary velum to terminate in 

 the dorsal vermis of the cerebellum. 



(b) The lateral limiting tract. This tract, which is quite narrow, lies 

 close to the external border of the gray matter. It is composed of fibers 

 which do not degenerate to any considerable extent after transverse section 

 and are in all probability associative fibers which come from nerve- cells in 

 the gray matter to reenter at lower and higher levels. It is also believed by 

 some investigators that the anterior portion contains efferent and the pos- 

 terior portion afferent fibers; for this reason it is frequently termed the 

 mixed lateral tract. 



(c) The crossed pyramidal tract. This tract occupies the posterior por- 

 tion of the lateral column, though its exact position varies somewhat in 

 different regions of the cord. In the cervical and thoracic regions it is 

 covered by a layer of fibers. In the lumbar region, however, it comes to the 

 surface. From above downward this tract gradually diminishes in size, 

 for the reason that its fibers and their collaterals enter the gray matter at 

 successive levels. The terminal branches of these fibers are in close physio- 



