THE NERVOUS SYSTEM 



47i 



perhaps, also, some touch impulses are conveyed, but little 

 is known regarding this. The pain and temperature fibres 

 end in the grey matter after entering the cord, and frcm 

 these fresh axons are formed, which run up both sides of the 

 cord. In the medulla the fibres terminate, and fresh axons 

 arise from its cells, which pass to the optic thalamus of the 

 opposite side. This is the chief station for all varieties of 

 afferent stimuli. In both the ascending and descending paths 

 there are fibres terminating in the 

 cord — i.e., not running the full 

 length of the tract. This is es- 

 pecially the case with the afferent 

 fibres, the larger number of which 

 enter the grey matter, where many 

 terminate by arborising around 

 tract cells. Many tract cells fur- 

 nish further fibres, which continue 

 onwards to maintain the path. 



All sensory impulses pass to the 

 brain on the side opposite to their 

 origin, and all motor impulses 

 leave the brain on the opposite 

 side to that to which they are 

 distributed, so that injury to a 

 left motor area leads to a right 

 body paralysis (see Fig. 141). 



The termination of the sensory 

 fibres in the cord has been in 

 part referred to. Having passed 

 through their cell-station in the 

 ganglion on the dorsal spinal 

 nerve, they enter the cord, but 

 do not directly pass along it as 

 an afferent tract, but penetrate 

 the grey matter, where many 

 terminate (Fig. 139, A) ; others are 



provided with a fresh axon, and then continue the headwards 

 passage as a spinal tract. As a matter of fact, the entry of the 

 fibres into the cord is by no means so simple as the above would 

 suggest, and it is further complicated by the fact that the fibres 

 on entering the cord divide into a Y or T, one passing backwards 

 for a short distance, the other forwards, both, perhaps, giving 

 off collaterals (see Fig. 140). Some of these collaterals establish 

 cell-communication with the grey matter, and so to the motor 

 system, a reflex arc being thus formed (Fig. 139, A and B). The 

 reflex arc may be complicated by the introduction of an inter- 



Fig. 140. — Branching of Dor- 

 sal Root Fibres in Cord 

 (Donaldson, after Ramon y 

 Cajal). 



DR, dorsal root fibres entering the 

 cord, and dividing Y- or T-wise 

 into fibres running forwards and 

 backwards, which give off colla- 

 terals, Col. Cells, CC, can be 

 seen in the grey matter of the 

 cord with which the collaterals 

 establish communication. 



