318 



TUL INTERNAL ENVIRONMENT OF THE BODY 



Part III 



Fig. 17.6. The external ears of tree-shrew and man are strikingly similar in hav- 

 ing the rolled edge that is associated with their reduction in size. In some mammals, 

 i.e. hats, there are muscles by which the flap (tragus) can be pulled down over the 

 passage to the eardrum; in the human ear unfortunately this passage can only be 

 stopped with the fingers. The tree-shrew {Tiipaia tana) of southeast Asia is a small 

 generalized mammal that originated about 100 million years ago and is believed to 

 be an ancestor of the gorillas, man, and other primates. 



horses, or drop one ear and lift the other toward the danger as rabbits do. 

 The sizes and patterns of auricles are correlated with the habits of their 

 owners, and picturesquely so, small in the burrowing chipmunks and wood- 

 chucks, large in horses and giraffes that gather sound waves on the open 

 plains, largest of all in African elephants, and most elaborate in bats that 

 are aware of ultrasonic sounds (Figs. 17.7, 17.8). A number of mammals, 

 especially seals and others living in the water, can close the entrance to the 



Fig. 17.7. The enormous external ears of insectivorous bats. Left, European 

 long-eared bat; right, pallid cave bat of U.S.A. Bats hear ultrasonic sounds, wholly 

 inaudible to human ears. (After Allen: Bats. Cambridge, Mass., Harvard Univer- 

 sity Press, 1939.) 



