ORGANS OF PKKCia'TlOX- 199 



lose animals, they vary greatly in number, some having 

 only one, while others have two, four, eight, or more. 



The eyes of some animals are fixed ; while in others, 

 they are capable of a great extent of motion, resting on a 

 cushion of fat or gelatinous matter, or seated on the summit 

 of a moveable peduncle. 



The organs of vision are uniformly placed on the head; 

 or, in the absence of that part, on the anterior portion of 

 the body. They are usually situated on both sides of the 

 mesial line, and have always more or less of a dorsal aspect, 

 unless in Man, whose eyes are directed forwards. 



There is no proportion observed between the eye and the 

 size of the body. Insects present the greatest ocular sur- 

 face to the light, in proportion to the size of the body, 

 whales the least. 



IV. SENSE ^F HEARING. 



THE essential parts of the organ of hearing, consist of a 

 gelatinous pulp, inclosed in an elastic membrane, into which 

 the last branches of the auditory nerve penetrate. The 

 vibrations of sonorous bodies are supposed to excite move- 

 ments in this pulp, by which the nervous filaments are af- 

 fected, and the perception of sound produced. As it is 

 difficult to assign to each part of the ear its peculiar func- 

 tion, we shall confine our descriptions to the parts them- 

 selves, briefly stating as we proceed, the uses which they 

 are supposed to serve. 



1. Structure of the Organs of Hearing. The most com- 

 plicated part of the organ of hearing, is that in which the 

 auditory nerve terminates, and which is termed the La- 

 byrinth. In quadrupeds, it consists of several cavities, hol- 

 lowed out in the petrous substance of the temporal bone, 



