MAMMALIA. 701 



into various sinuosities, so disposed, no doubt, as to concentrate 

 sonorous impressions. In Man, as the anatomist is aware, nume- 

 rous small muscles act upon the auricular cartilages ; but in quad- 

 rupeds possessed of moveable ears the number and size of these 

 muscles are prodigiously increased, and the ears are thus directed 

 with facility in any required direction. 



(820.) More minutely to describe the structure of the auditory 

 apparatus in the Mammiferous class would be foreign to our pre- 

 sent purpose : nevertheless, we must not omit to notice one most 

 remarkable provision whereby the Whales, strangely circumstanced 

 as those creatures are, are permitted to hear either through the me- 

 dium of the air they breathe, or of the water in which they pass 

 their lives. The reader will at once appreciate the difficulties of 

 the case : the ear of a fish, without any external communication, 

 although best adapted to receive the stunning concussions convey- 

 ed through the denser element, could never appreciate the more 

 delicate vibrations of the air, and the ordinary Mammiferous ear 

 would be perpetually deafened by the thundering of the water. 

 How is the Whale to hear what is going on in either the sea or 

 the atmosphere ? 



The plan adopted is simple and efficacious: The external 

 meatus of the ear is reduced to the smallest possible diameter, the 

 canal being barely wide enough to admit a small probe ; this is 

 the hydrophonic apparatus, and is all that is exposed for the recep- 

 tion of aquatic sounds. The Eustachian tube, on the contrary, is 

 very large, and opens into the blow-hole through which the Whale 

 respires atmospheric air : if, therefore, the Cetacean comes to the 

 top of the water to breathe, it is the Eustachian tube that conveys 

 aerial sounds to the ear, and thus it hears sufficiently under both 

 conditions. 



(821.) So far, as the student will have perceived, the different 

 portions of the encephalon to which we have adverted correspond 

 most exactly to similar parts met with even in the brain of a 

 reptile : where then are we to look for those grand differences 

 whereby the Mammiferous brain is peculiarly characterized ? The 

 peculiarities of the brain of a Mammal are entirely due, first, to 

 the increased proportional developement of the cerebral hemi- 

 spheres ; and, secondly, to the existence of lateral cerebellic lobes, 

 in connection with both of which additional structures become re- 

 quisite. 



In those Marsupial tribes that form the connecting links 



