112 ZOOLOGY. 



panuin simplified; finally, in most fishes every vestige of 

 an external car and a middle ear disappears, and the apparatus 

 of hearing is composed of a membranous vestibule, surmounted 

 by three semicircular canals, furnished below with a small 

 sac, which seems to represent the cochlea, the whole being 

 suspended in the lateral part of the great cavity of the cra- 

 nium. 



In animals placed still lower in the scale, we find, also dis- 

 appearing, the cochlea and semicircular canals, the structures of 

 which we know not the use;* but the membranous vestibule 

 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 eye-ball 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, 



* Ac-.-, .riling t> the experiments of M. Flourens, it would seem that the 

 , lo it rn.-t i< >n of the semicircular canals does not destroy the hearing, but renders 

 it painful and confused. 



