166 HEARING. 



Dr. B. The same facts are established by many 

 other similar cases which have been observed by physi- 

 cians and surgeons, but it would be unnecessary to men- 

 tion them. Among the inferior animals, the eye becomes 

 more simple in its structure, until in some of the very 

 lowest orders, it ceases to exist entirely. In some classes 

 we see, the structure of the eye is modified according to 

 the medium in which the animal lives. Thus, in Birds 

 the anterior portion is remarkably convex, because the 

 rays of light passing through the rarer portions of the at- 

 mosphere, suffer greater divergency, and consequently 

 the eye must possess a corresponding power of conver- 

 gency. In Fishes, on the contrary, the eye is very flat, 

 because the rays before they reach the eye, pass through 

 the water which is a much denser medium than the air. 

 There is a curious form of the eye existing in the Insect 

 tribe, called the compound eye. These animals possess 

 two, apparently single, eyes in their head ; but when ex- 

 amined by a microscope, they are found to be composed 

 of an aggregate of a vast number of single eyes ; or 

 rather the external coat which corresponds to the cornea, 

 is made up of these numerous six-sided figures. Every 

 one of these is undoubtedly furnished with a nerve, and 

 is in fact, a perfect eye. In the eye of a common house- 

 fly there have been counted 8000. In Spiders, the 

 eyes are single and are six or eight in number, placed in 

 different parts of their body. 



Emily. They then literally share with man the priv- 

 ilege of " looking before and after." Their enemies are 

 so numerous however, that they must really need it. 



Dr. B. The next sense to which we shall turn our 

 attention, is that of hearing. This sense, briefly defined, 

 is the one which makes us acquainted with the impres- 

 sions of sound; and sound, you know, is the result of 

 motions communicated to elastic fluids by the vibrations 

 of solid bodies. The organ of hearing is an apparatus 

 most admirably contrived, for collecting sounds, and 

 producing the requisite impression on the nerves, which 



