118 ZOOLOGY. 



is an organ which is never wanting in any ear. A membranous 

 sac filled with a fluid, into which penetrate the nerves of 

 hearing, is the essential of the organ. In this fluid are 

 suspended some solid corpuscles which oscillate incessantly, 

 and which may be compared to the otolites of the internal 

 ear in fishes. 



In most insects there is no vestige to be found of the organ 

 of hearing, and yet those animals do not seem insensible to 

 sound. Finally, in zoophytes, and in animals still lower, the 

 faculty of hearing itself appears to be wanting. 



OF THE SENSE OF SIGHT. 



230. The sense of sight enables us to discover the pre- 

 sence of objects by their colours. It takes cognizance also of 

 their form, size, and relative position to us. The optic nerve, 

 the eyeball and its appendages, constitute the apparatus. 



231. Structure of the Eye. The globe of the eye is a 

 hollow sphere, composed of membranes, and humours more 

 or less fluid. When the globe of the eye is dissected from 

 without inwards, it is found to be composed of the tunic, 

 called sclerotic, white and fibrous, and of great strength. 

 In front, and continuous with this, is the translucent cornea, 

 by which the rays of light pass into the interior of the 

 eye. If the cornea be opened, the aqueous humour escapes. 

 By removing the cornea, or a large portion of it, we 

 expose the iris, a coloured circular membrane, placed like 

 a partition between the anterior and posterior chambers of 

 the aqueous humours. The iris floats in this humour, is con- 

 tractile, and by its contractility the aperture in its centre, 

 called the pupil, and which in man is circular, varies 

 perpetually with the amount of light to which the eye is 

 exposed. This circular membrane is muscular, or at least 

 fibrous and contractile, and is abundantly supplied with 

 nerves. 



Continuous with the iris, and extending backwards beneath 

 the sclerotic, is the dark vascular membrane, called choroid, 

 connected with the pigment of the eye ; and within it the 

 retina, or membrane in which the optic nerve terminates. 



The humours which these membranes contain and circum- 

 scribe are in succession, proceeding from before backwards, 

 the aqueous humour, the lens, and the vitreous humour. 



The optic nerve enters the eyeball towards the back part, 



