390 SPECIAL PHYSIOLOGY. 



Peruvian Indians, who can distinguish by it. in the dark, persons of 

 different races. It is recorded of James Mitchell, who was born blind 

 and deaf, and was necessarily dumb, that he could distinguish persons, 

 and recognize strangers, by the sense of smell. 



The Organs and Sense of Smell in Animals. 



The sense of smell is undoubtedly very generally possessed by animals. Be- 

 sides being a source of enjoyment, and serving the important office of aiding 

 animals in the pursuit of living prey, and in their search after, and selection 

 of, other proper food, the sense of smell often assists them in the avoidance of 

 their natural enemies, likewise informs them of the presence of individuals of 

 their own species, and, as in Man, doubtless frequently warns them of the ex- 

 istence of noxious vapors, and other substances. 



This sense exists, in a more or less highly developed state of perfection, in 

 all the Yertebrata, whether they breathe air or respire in water, for it is pres- 

 ent even in all fishes. But, as we know and understand smell, it is, in its 

 highest degree, an atmospheric sense, and, in fishes and all lower aquatic ani- 

 mals which possess it, must exist in some modified, and probably less refined, 

 though acute, form. In a very few of the Cetacea, alone amongst the Yerte- 

 brata, is this sense entirely wanting. 



In all the air-breathing Yertebrata, i. e. , in Mammalia, Birds, Reptiles, and 

 the perfect Amphibia, the olfactory organs, however highly or simply developed, 

 are situated in the course of the respiratory passages, the nasal fossae being, 

 in all cases, pervious backwards, opening behind, usually into the upper part 

 of the pharynx, but, in the very lowest forms, into the mouth. In the Mam- 

 malia, the nose is, as a rule, highly developed ; the nasal fossee are capacious ; 

 the horizontal cribriform plate and the lateral cellular parts of the ethmoid 

 bone are large, and, as well as the turbinated bones, often highly complex ; 

 the various sinuses in the adjacent bones are well developed, though they are 

 not directly concerned in the sense of smell. The anterior part of the nose is 

 cartilaginous, and is provided with muscles ; it forms the so-called muzzle of 

 the dog, the snout of the pig and tapir, and the trunk of the elephant. In the 

 seals, walruses, beavers, and other diving mammals, the nostrils are slit-like, 

 very movable, and capable of being tightly closed at will ; a similar provision 

 is met with in many burrowing animals ; and in the camels, also, the large 

 movable nostrils can be closed against the tornado of the desert. In many 

 bats, the nose is developed into singularly formed folds or leaflets, which are 

 supposed to collect odors. But perhaps the most remarkable modification of 

 the anterior part of the nose, is the elephant's trunk, which is a double tube, 

 containing thousands of muscular bundles, and is not only a prehensile organ, 

 and a hydraulic pipe, but forms the usual respiratory passage. In this ani- 

 mal, besides the ordinary adjacent sinuses, which are very large in the frontal 

 bones, there are others in the temporal, parietal, and occipital bones, all of 

 which communicate with each other. 



In certain carnivorous animals, as in the dog and seal, and also, but not to 

 such an extent, in many Ruminants, as in the sheep and deer tribes, in all of 

 which the sense of smell is very acute, as witnessed in the quick recognition 

 of the presence of Man exhibited by the stag, and in the almost fabulous 

 power of certain dogs in following the scent of their prey upon the ground, 

 provision is made for a vast extension of the nasal mucous membrane, by a 

 most singularly complex lamination of the spongy bones, constituting the 

 structure named the labyrinth. The olfactory nerve is not, however, com- 

 monly distributed over this complicated portion of the nose, so that it only 

 indirectly subserves the olfactory sense. Possibly it delays, retains, and sub- 

 divides the atmosphere laden with odorous matters, in its intricate passages, 

 and so facilitates their oxidation ; or it may be intended to aid in warming 

 and moistening the air. In the Cetacea the nasal cavities are reduced to sim- 

 ple, long, narrow canals, destitute of turbinated bones, and having no adja- 

 cent sinuses. As in the rest of the Mammalia, the posterior nares open into 



