412 ORGANS OF THE SENSES. 



young dogs, in four of which he made an mtra-cranial section 

 of the first pair, while he cut only the posterior roots of the 

 olfactory branch of the fifth ; he found that the latter retained 

 its sense of smell, while the four others lost it entirely. 



The sense of smell is much more acute in animals than in 

 man ; it serves them as a valuable guide, and is the moving 

 cause in many of their instinctive and deliberate actions : 

 thus .connected with the sense of taste it enables them to 

 distinguish the different kinds of food suited to them, arid it 

 is the agent of numerous impressions connected with the 

 reproductive functions, etc. 1 



IV. THE SENSE OF HEARING. 



Hearing is that sense by means of which we are conscious 

 of the waves of sound produced by the vibration of bodies 

 in the ambient medium (air or water). 



The organ of hearing is extremely complicated ; in order 

 to understand it we will first examine it in those animals 

 which inhabit the water; in these it is most simple. The 

 essential and fundamental part of the organ of hearing, as 

 found in the inferior fishes, consists of a small bag full of 

 fluid, in which nerve fibres terminate in connection with a 

 special epithelium, furnished with prolongations resembling 

 great cilia, or small rods, that vibrate with every movement 

 of the fluid. The waves of the surrounding medium (fluid) 

 are thus transmitted almost directly to the nerve termina- 

 tions, and excite these hitter. This organ is found in all the 

 higher animals : it consists of the saccule or smaller vestibu- 

 lar vesicle and the utricule or common sinus. With these 

 are connected similar diverticuli, consisting of pouches of 

 different forms, but always full of (had (endolymph) : these, 

 in the higher classes of fish, are the membranous semicircu- 

 lar canals ; in reptiles, and especially in birds, there is also 

 a peculiar long and exceedingly complicated canal or tube, 

 which is wound and twisted around itself like a spiral stair- 

 case, and is called the cochlea. The tube of this cochlea is 

 also divided by a partition culled the spiral plate (lamina 

 spiralis ossea) into two secondary tubes, which communicate 

 with each other at the summit of the organ, while, at the 

 base, one communicates with the rest of the internal ear or 



1 See G. Colin, " Physiologie comparee des Animaux." Vol. 

 I. p. ;)10. 



