228 HUMAN PHYSIOLOGY 



anatomical changes (disappearance of the nerve tissue and 

 the replacement of it by connective tissue). The results 

 of anatomic investigations of the course of these fibres 

 agree with the pathological evidences. 



After transverse severing of the cord (by injury or disease) 

 a secondary degeneration of the pyramidal column below 

 the injury takes place. Since a nerve fibre, severed from 

 its nerve cell, degenerates (see page 223), this leads to the 

 conclusion that the cells of the pyramid fibres lie in the brain. 



Of the nerves derived from the centrifugal tracts of the cord must 

 be mentioned the nerves for respiration and perspiration and the 

 vaso-motor nerves (see pages 75, 83 and 109). A few fibres 

 (vasodilators and motor fibres for the intestine) are supposed to 

 leave the cord by posterior roots. 



2. THE SENSORY TRACTS 



These are paths which, emanating from the fibres of the 

 posterior roots, pass, either directly or by the interposition of 

 cells throughout Coil's column through the direct cerebcllar 

 tract and, as scattered fibres, through the lateral bundle. 



The peripheral sensory fibres end directly in the cells of 

 the spinal ganglia. They are really elongated dendrites of 

 these cells (see page 215). From these cells the axis- 

 cylinders proceed through the posterior roots into the spinal 

 cord and there separate into two large groups : 



1. Fibres which cross the Burdach column diagonally, 

 reach Coil's column and through this proceed upward to the 

 brain. 



2. Fibres the end-tufts of which come into contact with 



the cells of the gray substance. The neurites of these cells 



i 

 pass 



(a) To the direct cerebellar tract on the same side, and in 

 this tract proceed upward to the brain. The cells of these 

 neurites lie in the columns of Clark, which are masses of 

 cells on the median side of the basis of the posterior horns. 



(/;) Through the gray or white commissure to the other 

 side, and pass upward as scattered fibres in the lateral 

 bundles, or perhaps also in Gower's column. 



