386 COMPARATIVE ZOOLOGY 



diseased, the left side of the body loses the power of 

 voluntary motion.' 



The sympathetic nervous system is a double chain of 

 ganglia, lying along the sides of the vertebral column in 

 the ventral cavity. From these ganglia nerves are 

 given off, which, instead of going to the skin and mus- 

 cles, like the spinal nerves, form networks about those 

 internal organs over which the will has no control, as 

 the heart, stomach, and intestines. Apparently their 

 office is to stimulate these organs to constant activity, 

 but is little understood. 



I. The Senses 



Sensation is the consciousness of impressions on the 

 sensory organs. These impressions produce some 

 change in the brain ; but what that change is, is a dark- 

 ness on which no hypothesis throws light. Obviously, 

 we feel only the condition of our nervous system, not 

 the objects which excite that condition. 141 



All animals possess a general sensibility diffused over 

 the greater part of the body. 142 This sensibility, like as- 

 similation and contractility, is one of the primary phys- 

 iological properties of protoplasm. But, besides this 

 (save in the very lowest forms), they are endowed with 

 special nerves for receiving the impressions of light, 

 sound, etc. These nerves of sense, as they are called, 

 although structurally alike, transmit different sensations: 

 thus, the ear can not recognize light, and the eye can not 

 distinguish sounds. In the vertebrates, the organs of 

 sight, hearing, and smell are situated in pairs on each 

 side of tire head ; that of taste, in the mucous membrane 

 covering the tongue ; while the sense of touch and that 

 of temperature are diffused over the skin, including the 

 mucous membrane of the mouth, throat, and nose. 



