ORGANS OF THE SENSES. 28i> 



meatus is now increased in length, bounded by ossious 

 walls, defended internally by short hairs and by an acrid 

 secretion from its parietes, and generally terminated ex- 

 ternally by an expanded cartilaginous concha variously 

 formed and developed, which is adapted by its mobility 

 and its conical shape to collect the sonorous vibrations from 

 different directions and to convey them to the internal ear. 

 The concha is generally very small or wanting in the aquatic 

 mammalia. In the cetacea, where the cochlea is often 

 very large and the semi-circular canals comparatively small, 

 we observe only a narrow winding perforation leading out- 

 wards from the membrana tympani to the surface of the head, 

 there is no external concha, and the minute entrance to the 

 meatus is scarcely perceptible on the surface of the skin. 

 The concha is still wanting in the seals and walruses, and 

 is very small in otarise, beavers, otters, and other diving 

 mammalia. It is deficient in the ornithorhyncus, and in 

 several digging animals, as the mole and the manis. The 

 monotrema are also distinguished by several other marks 

 of inferiority in their organs of hearing by which they are 

 allied to birds and reptiles, as the shortness of the external 

 meatus, the anchylosis of the ossicula auditus, the rudi- 

 mentary state of the cochlea, and the free projection of the 

 osseous semi-circular canals into the cavity of the cranium. 

 As we ascend through the tribes of mammalia that live more 

 exclusively upon the land, we find the exterior cartilaginous 

 concha acquiring greater magnitude and symmetry, and, by 

 the articulation of the cartilage and the great development 

 of its muscular apparatus, it acquires the means of more 

 varied and extensive motion. It is largest, most moveable, 

 and, commonly directed backwards, in the timid and feeble 

 rodentia, ruminantia, pachyderma and other herbivorous 

 quadrupeds where the cerebral hemispheres are proportionally 

 small, and it is least and directed forwards in the carni- 

 verous tribes where the brain is large. It is large, however, 

 in the bats and most nocturnal quadrupeds, and we observe 

 it in the quadrumana, especially in gibbons and orangs, 

 gradually acquiring the short, flat, round form and motion- 

 less character of the human concha. The internal ear of 

 man, like that of inferior mammalia, has its membranous 

 labyrinth filled with an ento- lymph and separated by a thin 



PART III. U 



