522 PISCES FISHES. 



a delicate pituitary membrane, which is variously folded, in order 

 to increase the extent of the sentient surface (Jig- 230) ; and it 

 may be presumed, that from the number of plicse, which varies 

 amazingly, some estimate may be formed of the relative perfection 

 of the sense of smell in different genera. Into each olfactory 

 chamber the water is freely admitted by two distinct orifices, while 

 behind the pituitary membrane the olfactory nerve swells out into 

 a ganglion (Jig. 232, 1), from which nervous fibrils radiate, to be 

 distributed over the plicated lining of the nose (). 



(560.) The second pair of ganglia met with in the brain of a 

 fish (Jig. 232, b) give origin to the optic nerves (2), and may 

 therefore very properly be regarded as representing the tubercula 

 quadrigemina of the mammiferous brain. The nerves of vision 

 derived therefrom have no commissure, and present in many species 

 a peculiar structure which is not a little remarkable ; each nerve 

 being composed of a broad band of nervous substance, folded up 

 like a fan, and enclosed in a dense membrane, so that when un- 

 folded it presents the appearance delineated at fig. 231, A. 



(561.) The eye itself differs in many points of structure from 

 that of terrestrial Vertebrata, its organization being of course ad- 

 apted to bring the rays of light to a focus upon the retina in the 

 denser element in which the fish resides ; the power of the crystal- 

 line lens is therefore increased to the utmost extent, and the 

 antero-posterior diameter of the eye-ball necessarily contracted in 

 the same ratio, in order that the retina may be placed exactly in 

 the extremely short focus of the powerful lens. 



The eyes of all the Vertebrata are constructed upon principles 

 essentially similar, and present the same tunics and lenses as are 

 met with in the human eye, and, generally speaking, arranged in 

 the same manner as in man. It is not our intention, therefore, in 

 the following pages minutely to describe the anatomy of the eye in 

 every class which will come under our notice ; but taking the human 

 eye, with the construction of which we presume our readers to be 

 intimately acquainted, as a standard of comparison, point out those 

 modifications of the general type of structure common to this divi- 

 sion of animated nature. 



The first thing which strikes the attention of the anatomist, 

 when examining the eye of a fish, is the size of the crystalline lens, 

 and its spherical form. This shape, and the extreme density of 

 texture which the lens exhibits, are, indeed, perfectly indispens- 

 able. The aqueous humour, being nearly of the same density 



