300 THE SENSORIAL FUNCTIONS. 



substance, is surrounded by two delicate bands o** 

 muscular fibres, apparently provided for the pur- 

 pose of first bringing the gland into a proper posi- 

 tion, and then, by compressing it, discharging its 

 contents. Mr. Bell conceives that the use of this 

 secretion is to act as a bait for attracting fish 

 towards the sides of the mouth, where they can be 

 readily seized in the mode usual to the alligator, 

 which is that of snapping sideways at the objects 

 he aims at devouring. 



The organs of smell in Fishes are situated in 

 cavities, placed one on each side, in front of the 

 head : they are merely blind sacs, having no com- 

 munication with the mouth or throat, and indeed no 

 other outlet but the external openings, which are 

 generally two to each sac. The principal entrance 

 is furnished with a valve, formed by a moveable 

 membrane, appearing like a partition dividing each 

 nostril into two cavities, and serving the purpose of 

 preventing the introduction of any foreign body. 

 The organ itself is situated behind this valve, and 

 consists either of a membrane, curiously plaited 

 into numerous semicircidar folds, or of tufted or 

 arborescent filaments. Cilia have been discovered 

 on the surface of these cavities by Purkinje and 

 Valentin. Fig. 388 shows the nasal cavities (s, s,) 

 with their plaited membrane in the Perch ; and Fig. 

 389, in the Skate ; the laminae in the former being 

 radiated, and in the latter, foliated, or parallel to 

 each other. On the surface of these organs, what- 

 ever be their shape, the olfactory nerves (n), arising 

 from the anterior lobes (o) of the brain are distri- 

 buted ; and the great size of these nerves would 

 lead us to infer considerable acuteness in the sense 



