NERVOUS SYSTEM AND GENERAL SENSATION. 



23 



65. In the Radiata (Fig. 12), the nervous system is re- 

 duced to a single ring, encircling 

 the mouth. It differs essentially 

 from that of the Mollusks, by be- 

 ing disposed in a horizontal posi- 

 tion, and by its star-like form. 



66. The nerves branch off and 

 diffuse sensibility to every portion 

 of the body, and thereby men and 

 the higher animals are enabled to 

 Fig. 12. gain a knowledge of the general 



properties of the objects which surround them ; every point 

 of the body being made capable of determining whether 

 an object is hot or cold, dry or moist, hard or soft. There 

 are some parts, however, the ends of the fingers, for exam- 

 ple, in which this sensibility is especially acute, and these 

 also receive a larger supply of nerves. 



67. On the contrary, those parts which are destitute of 

 sensibility, such as the feathers of birds, the wool of ani- 

 mals, or the hair of man, are likewise destitute of nerves. 

 But the conclusive proof that sensibility resides in the 

 nerves is, that when the nerve which supplies any member 

 of the body is severed, that member at once becomes insen- 

 sible. 



68. There are animals in which the faculty of percep- 

 tion is limited to this general sensation ; but their number is 

 small, and in general, they occupy the lowest place in the 

 series. Most animals, in addition to the general sensibility, 

 are endowed with peculiar organs for certain kinds of per- 

 ceptions, which are called the SENSES. These are five in 

 number, namely : sight, hearing, smell, taste, and touch. 



