108 HUMAN PHYSIOLOGY 



traced outwards into the body, we discover that 

 their endings are of diverse character. Thus in the 

 skin we find nerve endings devoted to the exercise 

 of the sense of touch. In the nose, the ends of the 

 olfactory nerves, or those of smell, terminate in 

 special cells called olfactory cells, which may be pre- 

 sumed to be the organs whereby the smell sense is 

 excited through contact with odoriferous particles 

 which come in contact with them. More complex are 

 the endings of the nerves of sight and hearing, these 

 being connected with a complex apparatus in the 

 shape of eye and ear respectively. An important 

 point to be noted is that each ordinary nerve of the 

 body, apart from the nerves of sensory organs (eye, 

 ear, etc.), will be found to include two kind of fibres 

 indistinguishable from each other, but performing 

 different duties. Thus, as will afterwards be shown, 

 each nerve is like a double telegraph wire, in the 

 sense that one set of its fibres conveys messages 

 from brain to body, the other set carrying messages 

 in the opposite direction, from body to brain. 



ABOUT THOUGHT. In the case of the lowest 

 animals no question arises regarding the possession 

 of will, intellect or mind. They may be considered 

 to be in the position of living machines, which 

 respond automatically to the impressions made by 

 the outer world on their sensitive parts. As, how- 

 ever, we advance in the animal scale, we begin to 

 find, with the rise and development of the nervous 

 system, new phases of nervous work. These fresh 

 aspects are associated with a higher development of 

 the nervous apparatus, this development mostly 

 taking the form of the massing together, especially 

 in the head region, of nerve centres, which we have 

 seen to represent collections of nerve cells. The 



