THE NERVOUS SYSTEM 421 



and masses of nerve tissue connected therewith, known as 

 * ganglia.' Whether the afferent nerves be those of special sense 

 — i.e., visual, olfactory, gustatory, and auditory — or those of 

 touch, pressure, heat, cold, and pain, the widely different results 

 obtained on stimulation depend upon the end-organ of the nerve, 

 and not on any difference in the character of the impulses. 

 There are afferent impulses constantly occurring which are 

 not within the knowledge of the animal ; they may be of an 

 excitatory nature, or of a controlling or inhibitory character. 

 We have studied this group characteristically in our consideration 

 of the vascular and respiratory system, pressor or depressor effects 

 occurring in the circulation in consequence of excitatory or inhibi- 

 tory impulses conveyed by afferent nerves, though not neces- 

 sarily by the same afferent nerve. The same occurs also in the 

 respiratory system. Nor are we in this question limited to the 

 two systems mentioned ; we have met with it in muscle-tone, 

 visceral movements, glandular secretion, and in other ways. 

 Efferent nerves are those in which the impulses are passing from 

 the centre to the periphery, and the largest system of efferent 

 nerves in the body is that passing to the muscles. The term 

 is used in its widest sense, for not only are the skeletal muscles 

 richly supplied, but also, as we have already seen, the unstriped 

 muscle of the viscera, bloodvessels, and body hairs, and the striped 

 muscle of the heart. Nor are efferent impulses solely motor ; they 

 may be secretory. We have studied this in connection with 

 saliva and gastric juice. Finally the impulses may be inhibitory, 

 and control or prevent muscular movement, and control or 

 prevent secretion. 



It must not be supposed that all the tissues of the body are 

 completely endowed with the nerve fibres described. Fibres 

 for the bloodvessels, excitatory and inhibitory, are not found in 

 all bloodvessels, and though excitatory efferent (motor) fibres 

 exist largely in skeletal muscle, there are no inhibitory fibres to 

 this tissue, a remarkable fact, which will be fully considered 

 later, in view of its extraordinary physiological importance. 



From the above it will be seen that a classification of nerves is 

 not a simple matter, and physiologists are not agreed as to the 

 best system to adopt. The afferent scheme given on p. 422 has 

 been suggested by Professor Sherrington. 



Personally we would add to the classification trophic nerves, or 

 those engaged in nutrition. Some physiologists hold that special 

 nerves governing nutrition do not exist, and that the process runs 

 concurrently with the nerves governing wear and repair. The 

 existence of nerves specially provided for nutritional purposes is 

 not perhaps generally accepted ; the evidence that exists in their 

 favour will be mentioned in due course. 



