764 THE SENSE OF HEARING 



The external auditory meatus of man is a tubular passage 21-26 

 mm. in length, 8-9 mm. in height, and 6-8 mm. in width. It pursues a 

 slight spiral course forward, inward and upward, but may be straight- 

 ened very easily by pulling the pinna upward and backward. This 

 is made possible by the fact that the wall of this canal is cartilaginous 

 and movable for a distance of about one-half inch, while internally to 

 this point it becomes osseous. The delicate skin lining this canal 

 contains numerous sebaceous and ceruminous glands which furnish 

 the cerumen, a yellowish wax-like secretion, possessing a bitter taste 

 and peculiar odor. This secretion is lubricating and protective in 

 its function, because it prevents, in conjunction with the hairs, the 



entrance of dust and larger foreign 

 particles. Its excessive formation 

 and subsequent drying frequently 

 lead to the formation of chips and 

 plugs which greatly impair the pas- 

 sage of the sound waves, thereby 

 diminishing the acuity of hearing. 



The Middle Ear or Tympanum. 

 The middle ear consists of an ir- 

 regular cavity hollowed out of the 

 petrous portion of the temporal bone. 

 It is broader above and behind than 

 below and in front, and is shut off 

 from the external auditory meatus 



FIG. 3 8 2. DIAGRAMMATIC REPRE- , . 



SENTATION OF THE MIDDLE EAR OR TYM- by the eardrum or tympanic mem- 



PANIC CAVITY. brane. Anteriorly, it communicates 



i, External auditory meatus; 2, the w ith the pharynx by means of a 



ear drum or tympanic membrane; 3, i i , i i_- i_ i 



malleus, with its manubrium resting long and narrow tube which is known 



against the internal surface of the ear as the Eustachian tube, while pOS- 

 drum; 4, incus; 5, stapes resting against terioiiy. it IS connected with the 

 the membrane of the fenestra ovalis; 6, .*" ii ,- 



vestibule of the internal ear; 7, fenestra complex System of Small CaVltlCS in 

 rotunda; 8, Eustachian tube; 9, saccule; the mastoid bone, known as the 

 10, central canal of the cochlea; 11, magtoid ant rum and mastoid cells. 

 utncle; 12, muse, tensor tympam. 



Its inner wall, which is formed by 



the bony septum of the internal ear, is perforated in two places. In- 

 asmuch as one of these openings is oval in shape and the other round, 

 they are designated as the fenestra ovalis and fenestra rotunda. Both 

 are closed by a membrane, the outer surface of which lies in contact 

 with the air of the tympanum, while their inner surface borders upon 

 the lymphatic fluid filling the labyrinthine spaces. The tympanic cav- 

 ity is occupied by three small bones known as the ossicles, which are 

 arranged in series between the inner surface of the eardrum and the 

 outer surface of the membrane closing the fenestra ovalis. These os- 

 sicles are freely suspended in this space and are held in position by 

 ligamentous bands attached to different points of the wall, as well as 



