Mammalia: Nervous System 673 



dissolved (or suspended?) in the external water which, in one way or 

 another, is introduced into the sensory sacs. Physiologically, there 

 would seem to be little difference between these organs and the cu- 

 taneous " taste "-organs of fishes, since both are stimulated chemically 

 by substances in the external water. In lung-breathers the olfactory 

 cavities serve as respiratory passages, and the olfactory sensory cells 

 are stimulated by air-borne substances emanating from more or less 

 distant external sources. 



It is important to any animal that it be aware of external condi- 

 tions to as great a radius of distance as possible. Sight and hearing 

 have limitations. An object may look as if it were good to eat, but not 

 taste good. Chemical information concerning external objects is of 

 great advantage, whether it makes the animal aware of the proximity 

 of food or of enemies. That the sensations produced by the so-called 

 "olfactory organs" are highly important to fishes is guaranteed by 

 the fact that the correlating centers of the brain are dominated by the 

 olfactory mechanism. But organs which serve to test the chemical 

 nature of the external medium are even more important to the land 

 animal. Most fishes live in a far-reaching expanse of transparent water 

 affording unobstructed vision in all directions. Their highly developed 

 eyes serve them well. Most land animals inhabit environments which 

 offer a variety of opaque impediments to distant vision — rugged con- 

 tour of ground, rocks, trees, underbrush. Food or enemies may be close 

 at hand but unseen. To detect their presence by chemical means — i.e., 

 to smell them — is vitally important. 



The keenness of the sense of smell depends, in part, on the area of 

 the surface over which the sensory cells are distributed, assuming that 

 the number of sensory cells per unit of area is constant. In reptiles 

 the internal surface of each nasal cavity is increased by an extensive 

 thin plate of bone, the concha, which projects from the lateral wall 

 into the cavity. To most mammals, far exceeding reptiles in speed, 

 agility, diversity of activities, and propensity for roaming far afield, 

 smell must be vastly more important than to the relatively sluggish 

 and less migratory reptiles. It is therefore to be expected that the 

 mammalian olfactory organs should be developed to a much higher 

 point of efficiency than those of a reptile. This is accomplished by 

 increase of olfactory surface. 



The nasal cavities are relatively much larger in mammals than in 

 reptiles, but the main factor in the increase of surface is the addition 

 of numerous conchae and the utmost expansion and elaboration of 

 each of them. As in reptiles, the conchae or turbinals (as they are 

 more commonly called in mammals) develop as ingrowing longitudinal 

 folds of the external or lateral wall of each nasal cavity. Into each fold 



