MAMMALIA— PHYSIOLOGY 



475 



on. 



the 1 60 color tones. In total color blindness one cannot distinguish 

 between any other colors than gray. Partial color blindness is the 

 inability to distinguish between a certain group of colors. 



Audition. — The mammalian ear receives sound waves and trans- 

 mits them by the tympanum to the ossicles and from the ossicles 

 by way of the vestibule 

 they pass to the liquid 

 in the cochlea. The 

 movement is continued 

 to the basilar membrane 

 where vibrations of the 

 fibers in unison with the 

 tone affect definite hair 

 cells and the sensation is ^ 

 transmitted to the brain 

 by way of the eighth 

 nerve. (See p. 468.) 



The Organ of Corti 



in the cochlea of the ear 



is the receptor involved 



in hearing. Numerous 



theories of the activa- , , 



c -i -^ ^ Fig. au. The vertebrate eye. a, aqueous hu- 



tion of the constituents ^-^ . . „ i u -^ ; ■,,;.. 



. ^ mour; c, conjunctiva; C, cornea; ch, choroid; /, ins; 



of this organ by the Vl- ^^ j^^^. ^_„^ ^p^j^, ^erve; P, pigment layer; p, pupil; 

 brations of fibers imbed- r^ retina; r, layer of rods; s, sclerotic; y, vitreous body. 

 ded in the basilar rnem- (From Kerr. Courtesy of Macmillan and Co., Ltd.) 

 brane have been ad- 

 vanced. The human ear is sensitive to vibrations with a frequency 

 of from thirty to thirty thousand per second. 



Hearing in Aquatic Mammals. — The whale., like the seal and the 

 porpoise, lacks external ears. It has blow-holes on the top of its 

 head, which are closed by means of elastic cartilage valves. The 

 cachalot has an external opening, about i inch long in the full-grown 

 whale. The external auditory tube is closed by bony growth, 

 protecting the ear drum from the pressure encountered at great 

 depths. It is claimed by Kellogg that the whale-bone whales now 

 actually hear through their noses, and that the ear drum is nori- 

 functional. The bulla, a structure coiled like a conch shell, is said 

 to receive the sound waves. Whales are able to detect sounds very 

 readily. 



V. 



