116 PHYSIOLOGY 



and the olfactory nerves by volatile substances. The 

 one is distributed over the tongue in higher animals 

 and the other over the upper part of the nose cavity. 

 To what extent the lower animals possess these senses is 

 a question. They can certainly distinguish substances 

 that please them for food and they frequently make 

 their way unerringly to food from a distance, but it is 

 not probable that they smell and taste consciously as 

 we do. 



The nerves stimulated by vibrations are perhaps the 

 most sensitive of all. The optic nerve which is respon- 

 sible for sight is stimulated by ether vibrations; the 

 auditory nerve by means of which we hear, by air vibra- 

 tions. Some of the lower animals have rudimentary 

 organs sensitive to sound and light but they have no 

 eyes that can distinguish form as do ours and no ears 

 that hear as do ours. The human eye is marvelously 

 developed. Like a photographer's camera it reproduces 

 an image on the sensitive surface of the retina and the 

 impression of form and color is conveyed to the brain 

 by the optic nerve. 



Sympathetic System. — Many of the spinal nerves are 

 connected with a chain of ganglia which lie in two rows 

 on the ventral side of the spinal column. The ganglia 

 and the nerves that are given off from them form what is 

 called the sympathetic nervous system (Fig. 71). It is 

 so called because it is extremely sensitive to the condition 

 of the body. Through it the organs are adjusted auto- 

 matically to their needs and to the demands made upon 

 them. If food is present in the stomach its glands are 

 stimulated to activity. If the blood pressure falls the 



