656 



PISCES. 



Fig. 320. 



(1803.) The olfactory nerves of Fishes, derived from the lobes alluded 

 to, vary greatly in composition and proportionate size : sometimes they 

 are quite capillary ; sometimes thick, though still simple ; occasionally 

 they are double or triple, and in some cases are composed of numerous 

 fibres bound up in fasciculi. 



(1804.) The organs of smell to which these nerves are destined are 

 of very simple structure : Two 

 excavations are found near 

 the anterior part of the snout, 

 lined with a delicate pituitary 

 membrane, which is variously 

 folded, in order to increase the 

 extent of the sentient surface 

 (fig. 320) ; and it may be pre- 

 sumed that, from the number of 

 plicae, which varies amazingly, 

 some estimate maybe 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 ol- 

 factory nerve swells out into a 

 ganglion (fig. 322,1), from which 

 nervous fibrils radiate, to be dis- 

 tributed over the plicated lining of the nose (k) . 



(1805.) The second pair of ganglia met with in the brain of a Fish 

 (fig. 322, 6) 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 mem- 

 brane, so that when unfolded it presents the appearance delineated at 

 fig. 321, A. 



(1806.) The eye itself differs in many points of structure from that of 

 terrestrial Vertebrata, its organization being of course adapted 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 crystalline lens is therefore 

 increased to the utmost extent, and the antero-posterior diameter of the 

 eyeball necessarily contracted in the same ratio, in order that the retina 

 may be placed exactly in the extremely short focus of the powerful lens. 



(1807.) The eyes of all the Vertebrata are constructed upon princi- 

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



Organ of smell in the Skate. 



