34 ESSENTIALS OF PHYSIOLOGY. 



hearing, around gland cells and in other situations. Terminations 

 in special end-organs are found in the touch corpuscles of the skin 

 and in the end-plates in striped muscle fibres. 



The chief chemical constituents of nerve fibres are protein, nucleo- 

 protein, lecithin, and cholesterol. The myelin sheath of niedullated 

 fibres is mainly formed of lecithin. 



THE FUNCTION OF NERVE FIBRES. 



Nerve fibres have one function only, that of the conduction of 

 impulses. Generally speaking, any one nerve fibre conducts in one 

 direction only, and the fibres are classified as efferent and afferent, 

 according as they conduct impulses away from or towards the central 

 nervous system. The direction in which an impulse normally passes 

 along a nerve depends upon the connections of the fibres and not upon 

 a property of the fibres themselves, these being in reality capable of 

 conducting in either direction. Thus in a motor nerve fibre the 

 excitatory process originates in the central nervous system and is 

 propagated towards the muscle, and in a sensory nerve the excitatory 

 process is set up in the nerve-ending, for example in the skin, and is 

 propagated towards the nerve centres. 



The function of nerve fibres has been studied chiefly by means of 

 artificial stimuli, mechanical, thermal, electrical, and chemical, motor 

 nerve fibres being generally used for experiment because the propaga- 

 tion of an impulse in a motor nerve is easily demonstrated by the 

 resulting muscular contraction. Two other phenomena accompany or 

 follow the propagation of a disturbance along a nerve: (1) an electric 

 response, and (2) the occurrence of a refractory period. (1) The 

 electric response is detected by the use of a galvanometer, and consists 

 in a brief wave of negativity which accompanies the disturbance, the 

 excited part of the nerve being negative to the resting part. (2) The 

 refractory period follows immediately the passage of the disturbance, 

 and lasts about one-thousandth of a second. During that period a 

 second stimulus applied to a motor nerve fails to excite a muscular 

 contraction. 



In the study of the function of nerve fibres two points have to 

 be considered, the excitatory process and the propagation of the 

 resulting disturbance. 



The Excitatory Process. Nothing is known as to the physico- 

 chemical nature of the normal excitatory process. A similar condition 

 may, however, be set up artificially in various ways. For example, 

 cutting or pinching a motor nerve, or the sudden application to it 

 of heat, or dipping it in strong salt solution or in glycerol, or the 



