Mammalia: Nervous System 699 



movement of the head must cause some flow of endolymph in the 

 canals and their ampullae. The amounts of flow in the several cavities 

 will be unequal, the degree of inequality depending upon the spatial 

 relation of the plane of each canal to the direction in which the head 

 moves. The result is a differential stimulation of the three cristae 

 acusticae. There is doubt as to the exact nature of the functions of the 

 sensory maculas of the mammalian utriculus and sacculus. It seems 

 most likely that they have to do with equilibration, probably register- 

 ing the sialic position of the animal, whereas the ampullar cristae 

 register changes of posit ion. There is evidence that fishes — at least some 

 of them — hear, probably by means of the maculas of the utriculus and 

 sacculus (possibly the sacculus only), because the lagena is very slightly 

 developed. It is possible that the macula of the mammalian sacculus 

 may serve for reception of vibrations which are of lower frequency than 

 those which stimulate the cochlear organ. 



Whether the average mammalian eye is superior to that of reptiles 

 may be open to question. The olfactory organ of mammals, judged by 

 its more elaborate structure, must be considerably more efficient than 

 that of reptiles. The mammalian ear as an auditory organ is unques- 

 tionably far superior to the reptilian ear. The high degree of elabora- 

 tion of the mammalian cochlear structures, even as compared to the 

 best reptilian cochlea (that of crocodilians), cannot be without physio- 

 logic meaning. Some reptiles may have a very keen sense of hearing 

 but they lack the mechanism requisite for any wide range of auditory 

 discrimination. 



The auditory capacities of vertebrates are, in general, correlated 

 with their vocalizing abilities. This seems to imply that the thing of 

 primary importance is that an animal should be able to hear another 

 of its own kind. Consistent with the greater elaboration of the mam- 

 malian cochlea, the vocal performances of cats, coyotes, and donkeys 

 and the chatter of primates far excel anything in the vocal repertory 

 of reptiles. The roar of a big alligator is terrific but it is totally lacking 

 in the tonal quality and emotional expressiveness which mark the 

 voices of dogs and cats. The alligator is perhaps not especially inter- 

 ested in hearing anything but another alligator. It must be admitted 

 that the correlation between vocalization and hearing is not to be 

 observed in all members of a vertebrate Class. There are many mam- 

 mals which seem to have very good ears but only feeble vocal ability. 



The human mammal far excels all others in capacity for pro- 

 ducing sounds. Structurally, his cochlea seems to be not markedly 

 better than that of many other mammals, but he at least makes much 

 more use of it. This is to be credited to his brain rather than to his ear. 



