62 YERTEBRATA: MAMMALIA. 



and the source of voluntary motion. The nerves bring to 

 the brain the impressions which they receive, and they 

 carry the influences which the brain sends to all parts of the 

 body. And it is an interesting fact that there are nerves 

 which carry only sensation to the brain, and others which 

 transmit the will of the brain to the different parts of the 

 body that is, there are nerves of sensation, and nerves of 

 motion. And experiment has shown that the posterior 

 roots, above described, are nerves of sensation, and the an- 

 terior are nerves of motion. 



The Mammals are richly endowed with all the special 

 senses as sight, hearing, smell, taste, and touch ; and they 

 have special organs appropriate to the exercise of those 

 senses. 



In the Mammals, and in all other vertebrates, the eyes 

 are two in number, and situated in cavities of the skull, 

 called orbits. The globe of the eye is a hollow sphere 

 formed by three membranes, one within another, and filled 

 with humors more or less fluid. The outside membrane is 

 thick and tough, and is called the sclerotic. In front, and 

 continuous with it is the translucent cornea, through which 

 the rays of light pass into the interior of the eye. The 

 membrane next to the sclerotic is the choroid, which in 

 front is separated from the sclerotic, and which is there 

 called the iris, and is the part which gives the color to the 

 eye. The iris readily contracts and dilates, and thus the 

 aperture at its center, called the pupil, is diminished or en- 

 larged according to the amount of light to which the eye 

 is exposed. The next, and inside membrane is the retina, 

 which is an expansion of the optic nerve. It is upon this 

 that the images of objects are received and make im- 

 pressions which are carried to the brain by the optic 

 nerve. 



