OF HEAEDTG. 



71 



is constructed with a view to collect and augment the sonorous 

 vibrations of the atmosphere, and convey them to the acoustic 

 or auditory nerve (fig. 45, o), which arises from the posterior 

 part of the brain (fig. 20). 



146. The ears never exceed two in number, and are placed, 

 in all the vertebrata, at the hinder part of the head. In a 

 large pro- 

 portion of 

 animals, 

 as the dog, 

 horse, rab- 

 bit, and TO 

 most of the 

 mammals, 

 the exter- 

 nal parts of 

 the ear are 

 generally 

 quite con- 

 spicuous, 

 and as they 

 are at the 

 same time 

 moveable, 

 they be- 

 come one 

 of the pro- 

 minent 

 features of 



Fig. 45. Vertical Section of the Organ of Hearing in 

 Man. The internal parts are enlarged, to make them more 



the physi- evident, a, b, c, the external ear ; d, the entrance to the 



ognomy. 



147. 

 These ex- 

 ternal ap- 

 pendages, 

 however, 

 do not, 

 properly 

 speaking, 

 constitute 



auditory canal,/; e, e, petrous portion of the temporal hone, 

 in which the internal ear is excavated ; g, membrane of the 

 tympanum ; h, cavity of the tympanum, the chain of bones 

 being removed ; t, openings from the cavity into the cells, 

 /, excavated in the bone ; on the side opposite the mem- 

 brana tympani are seen the foramen ovale and foramen ro- 

 tundum ; k, the Eustachian tube ; I, the vestibule ; m, the se- 

 micircular canals ; n, the cochlea ; o, auditory nerve ; p, the 

 canal for the passage of the carotid artery to the brain ; g, 

 part of the glenoid fossa, for receiving the head of the lower 

 jaw ; r, the style-like process of the temporal bone, which 

 gives attachment to muscles ; s, the mastoid process of the 



the organ temporal bone. 



