ORGANS OF PERCEPTION. 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, im proportion to the size of the body,— 
whales the least. 
IV. SensE or HEARING. 
Tue 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 m the petrous substance of the temporal bone, 
