NERVOUS SYSTEM— NOSE— EYE. 287 



which are carried on in the animal as in the vegetable without the 

 necessity for any direct stimulus from a nervous centre, such as 

 the growth of each separate tissue throughout the body, which 

 takes place in the former, just as it does in the latter, by a speciea 

 of cell-development and metamorphosis independent of nervous 

 energy ; but though this growth is thus accomplished, yet it would 

 soon be starved out for want of pabulum, were it not for the supply 

 of food to the stomach, which requires the mandate of the nervous 

 system for its performance, and so on with every corresponding 

 action of the body. 



The nervous system is made up of two distinct substances, 

 one grey in color, and granular in structure, which is the seat of 

 all nervous power ; the other white and fibrous, which is the tele- 

 graph wire by which this power is communicated. Sometimes the 

 grey matter envelops the white, and at others it is enclosed within 

 it, but in every case each has its peculiar office, as above men- 

 tioned. Each collection of grey matter is called a ganglion, what- 

 ever its shape may be ; but the white fibres may be either in the 

 form of commissures for connecting the ganglia together, or they 

 may be agents for communicating with other organs, and are then 

 called nerves. 



THE OKGAN OF SMELL. 



The nose of the horse, like all the solipedes, is endowed with a 

 sensibility far greater than that of man ; but in this respect he is 

 not equal to many other animals, such as the dog and cat kinds, 

 and the sole use which he makes of this sense is in the selection 

 of his food. 



THE EYE. 



The organ of sight may be considered as consisting first of 

 all of an optical instrument very similar to the camera obscura, 

 now so commonly used in photography, and secondly of the parts 

 which are employed to move, adjust, and protect it from injury. 



The eye itself consists of three transparent humors, which 

 answer the purpose of the lens of the camera, by collecting the rays 

 of light upon the back of the eye. These are the aqueous in front, 

 the crystalline lens in the middle, and the vitreous humor behind. 

 The first is a perfectly transparent and limpid fluid, secreted by 

 the Hning of the chamber in which it lies, and capable of being 

 rapidly renewed in case of a puncture letting it out. The lens, on 

 the contrary, has the consistence of very hard jelly, and is 

 arranged in concentric layers, like the coats of an onion. It is 

 merely a double convex lens, precisely like that of the camera in 

 its action, and is the chief agent in producing the impression of an 

 object upon the sensitive part of the eye. Behind it is the vitre- 

 ous humor, composed, like the aqueous, of a limpid fluid, but in- 



