76 INTRODUCTION TO NEUROLOGY 



possesses. Not only is there a considerable number of sense 

 organs not represented at all in our traditional list of five senses, 

 but several of these five are complex. Thus, the internal ear 

 includes two quite distinct organs the cochlea, which serves as 

 a receptor for sounds, and the labyrinth, whose semicircular 

 canals serve as the chief sense organs concerned in the regula- 

 tion of bodily position and the maintenance of equilibrium, func- 

 tions which are quite distinct from hearing. The skin, too, 

 serves not only as the chief organ of touch, but also the addi- 

 tional functions of response to warm, cold, and painful impres- 

 sions, besides some other more obscure sensory activities, such 

 as tickle. 



An acceptable classification of the sense organs or receptors 

 of the body must take account of their anatomical relations, of 

 the nature of the physical or chemical forces which serve as the 

 adequate stimuli, of the subjective qualities which we experience 

 upon their excitation, and of the character of the physiological 

 responses which commonly follow their stimulation. The last 

 point has been too much neglected. 



In fact, the most fundamental division of the nervous sys- 

 tem which we have, cutting down through the entire bodily 

 organization, is based upon this physiological criterion. From 

 this standpoint we divide the nervous organs into two great 

 groups: (1) a somatic group pertaining to the body in general 

 and its relations with the outer environment, and (2) a visceral, 

 splanchnic, or interoceptive group. The latter group comprises 

 the nerves and nerve-centers concerned chiefly with digestion, 

 respiration, circulation, excretion, and reproduction. These are 

 intimately related with the sympathetic nervous system and 

 those parts of the central nervous system directly connected 

 therewith, though the more highly specialized members of this 

 group are independent of the sympathetic system. The somatic 

 group comprises the greater part of the brain and spinal cord and 

 the cranial and spinal nerves, or, briefly, the cerebro-spinal ner- 

 vous system as distinguished from the sympathetic system (see 

 p. 225). This is the mechanism by which the body is able to ad- 

 just its own activities directly in relation to those of the outside 

 world to procure food, avoid enemies, and engage in the 

 pursuit of happiness. 



