ESSENTIALS OF PHYSIOLOGY. 



this theory of continuity rests, however, is at present insufficient to 

 justify its adoption. 



The Function of the Neuron. The type of neuron which is 

 most easily studied in the mammal is the unipolar form found in the 

 ganglia on the posterior roots of the spinal nerves. As has already 

 been pointed out, these cells are functionally bipolar, the single pro- 

 cess resulting from the fusion of the two poles, and the two processes 

 separate at some little distance from the cell, one passing towards the 

 spinal cord and the other towards the periphery. Each becomes the 



axis cylinder of a medullated nerve 

 fibre, but that which has a centrifugal 

 course is functionally homologous with 

 the dendrons of the neurons of the 

 central nervous system. If either of 

 the two processes is cut off from the 

 cyton, it degenerates, while the por- 

 tion left in connection with the cell 

 body undergoes no obvious change. 

 It may therefore be assumed that the 

 cyton governs the nutrition of all parts 

 of the neuron. Moreover, when one 

 of its processes has been divided in 

 this way, so that the normal function 

 of the neuron is interfered with, 

 changes occur in the substance of the 

 cyton itself. The Nissl spindles under- 

 FIG. 6. -Two motor nerve-cells from 8 disintegration, so that the cell stains 

 (Photographed from diffusely with methylene blue (fig. 6). 

 This change is known as c/iromatolysis, 

 and it indicates that the Nissl bodies 

 a, normal ; b, after _a period of prolonged are concerned in some way with the func- 

 tional activity of the neuron. Further 



evidence in support of this conclusion is afforded by the fact that the 

 Nissl bodies of the cells of the central nervous system diminish in number 

 after an animal has been in active exercise. Chromatolysis also occurs 

 as a result of the action of certain poisons, in fevers, and in asphyxia. 



Conduction of an impulse in a neuron takes place in one direction 

 only. In the case of the fibres of the posterior spinal nerve roots, the 

 conduction is from the periphery to the central nervous system. The 

 anterior root fibres, on the other hand, conduct from centre to 

 periphery. They are the axons of multipolar nerve cells which lie in 

 the grey matter of the spinal cord. If the posterior root be divided 



the dog. 



preparations by Dr Gustav Mann. ) 

 From Schafer's Essentials 



Histology. 



of 



