NERVOUS SYSTEM AND GENERAL SENSATION. 



47 



Fijr. 12. 



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

 duced to a single ring, encircling 



the mouth, and giving off threads 

 towards the circumference. It dif- 

 fers essentially from that of the 

 Mollusks, by being disposed in a 

 horizontal position, 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 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, &c. 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 insensible. 



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

 tion is limited to this general sense ; 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 acted upon by certain kinds of stimuli, 

 as light, sound and odor, and which are called the SENSES. 

 These are five in number, namely : sight, hearing, smell, 

 taste, and touch. 



