398 • HITCHCOCK'S ANATOMY 



piece, while in the other mammalia it consists of three. But 

 the most important part of this organ, the labyrinth, agrees 

 in its structure throughout the whole class. The greatest 

 variation that exists is in the semicircular canals and cochlea. 

 In the cat the canals form the segment of a circle, while in 

 the goat they furnish a portion of an ellipse, and in the horse 

 a parabolic curve. The cochlea varies in the number of its 

 coils. In the whale and dolphin it makes but one and a half 

 turns ; in man two and a half; in the ornithorynchus half a 

 coil much resembling that of a bird ; while in the squirrel it 

 makes nearly four turns. No physiological effect of these 

 variations has as yet been given. In man and the apes the 

 whole cavity of the tympanum is concealed in a portion of 

 the temporal bone, but in all other mammals we find a pecu- 

 liar tympanic bone which varies exceedingly in the different 

 classes. 



727. Ear of Birds. — There are but few birds which have 

 any thing like an external ear. One species of owl, however, 

 has a membranous concentric fold which can be used as a 

 valve. The cochlea has the form of an obtuse cone, which at 

 its extremity swells into an oval tubercle. The tympanic 

 cavity communicates with cavities in the cranial bones, which 

 greatly increases the resonance of sound, in the same manner 

 as the sounding-board in the piano, or the main body of a 

 violin or violincello. 



728. Hearing in Reptiles —Although reptiles present a 

 more imperfect form of the organ of hearing than mammals 

 or birds yet they possess the tympanum, a membrana tym- 

 pani, and a chain of ossicles or little bones. Also a cochlea 

 is found, which is most fully developed in crocodiles and formed 

 almost as in birds. 



How do the canals differ in different animals ? What variation in the cochlea ? What 

 is the os quadratum ? 727. What bird has external ears ? What is said of the cochlea 

 in birds? What does the tympanum communicate with? What is the use of this an 

 rangement ? 728. What is said of the organs of hearing in reptiles ? 



