THE NERVOUS SYSTEM 433 



excites no apparent consumption of material, no heat, and, so 

 far as can be determined, no fatigue, places nerve in a special 

 class, for there is no other tissue in the body which does not show- 

 signs of fatigue when constantly stimulated. Yet there is some 

 evidence that a nerve does not entirely escape from the effects of 

 work. There is a ' refractory period ' found to exist in nerves 

 which have been stimulated for some time, and a nerve kept 

 in an atmosphere of nitrogen loses its irritability, and rapidly so, 

 if constantly stimulated. It is therefore suggested that ordinary 

 fatigue products may be rapidly oxidised in nerve, and so no 

 accumulation and consequent fatigue is possible. The view 

 which at the present time receives the most favour is that nerves 

 are capable of being fatigued, but so rapidly is the process of 

 repair carried out that fatigue properties are not exhibited. 



It is believed that the nature of the impulses in all nerves is the 

 same, but that the differences in the results obtained depend 

 upon the termination of the nerves in the tissues (see p. 423). 



The Neurone Doctrine. — It is only within the last twenty years 

 that it has been possible to visualise the elements concerned in 

 the working of the nervous system, not only with regard to their 

 structure, but also their function. That the nervous system 

 consisted of fibres and nerve-cells had long been known, that the 

 cells were capable of nourishing the fibre was shown years ago by 

 the experiments of Waller on the dorsal roots of the cord, but 

 there was something lacking. The origin of the nerve-fibres was 

 unknown, while the distribution of the cells in the grey substance 

 did not appear to be arranged on any system connected with a 

 physiological basis. The doctrine of the neurone supplies what 

 was wanting. A neurone is a nerve-cell, with its various pro- 

 cesses and nerve-fibres. The cell-body,* the fibre, and the pro- 

 cesses are a physiological unit, a nervous system in miniature ; 

 myriads of such microscopic systems constitute the nervous 

 system. 



A nerve-cell consists of a mass of protoplasm intersected by 

 fine fibrils known as neuro-fibrils. Towards the centre of the cell 

 is a refractile nucleus, and lying between the neuro-fibrils are 

 certain peculiar bodies, which, though granular in nature, 

 stain with methylene blue, an exception to the general staining 

 reactions of cell granules (Fig. 127). These bodies are known as 

 Nissl's granules, or chromophile substance ; they exist in 

 the form of angular-shaped masses or rods, and extend into 

 all the processes of the cell, excepting that from which the 



* The ' nerve-cell body ' and the ' nerve-cell ' are not the same ; the latter 

 includes the dendrites and axons — it is, in fact, the neurone. The former 

 is the cell which gives birth to the neurone, and it is best distinguished as 

 the perikaryon. 



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