294 ORGANS OF THE SENSES. 



These convoluted parts for the highly vascular petuitary 

 membrane, and corresponding in magnitude with that of 

 the olfactory nerves of birds, are for the most part still 

 cartilaginous and very limited in their extent, and the 

 imperfect development of this organ is often compensated 

 for by the extensive distribution of the fifth pair of nerves 

 on the upper and lower jaws, as in many aquatic species 

 which seek their food in mud. 



All the internal parts of the organs of smell become more 

 complex and elaborate in the mammiferous animals, new 

 and enlarged cavities open into their interior, as the frontal, 

 maxillary, and sphenoidal sinuses, the large nasal glands of 

 birds are here reduced to follicles, and the exterior nares 

 assume a more lengthened and expanded form than in the 

 oviparous classes. The large olfactory nerves here pene- 

 trate the numerous openings of a broad cribriform ethmoidal 

 plate, excepting in the cetacea, arid spread over the very 

 extensive surface of the convoluted cellular part of the 

 ethmoid, and the two turbinated bones on each side. The 

 turbinated bones are most lengthened and simple in form 

 in the long-muzzled ruminantia, pachyderma, and other 

 herbivorous quadrupeds, where the various communicating 

 sinuses are largest, arxl these bones form the most com- 

 plicated labyrinths in the short-muzzled carnivora, where 

 the sense of smell is most acute, and by which they pursue 

 their prey through all their windings and concealments, or 

 seek them by night. The exterior openings of the nostrils 

 are valvular in the beavers, seals, and other diving quadru- 

 peds, to protect them during their rapid movements through 

 that dense element ; they are almost as valvular in the camels 

 and dromedaries for the sands of the desert, and in many 

 scraping and burrowing quadrupeds, and they are prolonged 

 in many cetacea into wide valvular and contractile sacs, 

 which enable them, like the crocodiles, to breathe freely 

 with every other part of their body concealed under the 

 surface of the water. The nostrils terminate in vertical 

 foliated membranous expansions in the phyllostoma and 

 many other bats, and in fimbriated radiating margins in the 

 condylura ; they form the digging instruments in the hog 

 tribe ; they are long and flexible in the nasua, more muscular, 

 prolonged, and mobile in the tapir, expanded into a bottle- 



