THE EAR. 



71 



THE EAR. 



THE organ of hearing is divisible into three parts : the external ear (fig. 78, 

 1, 2), the tympanum or middle ear (3), and the labyrinth or internal ear (G). The 

 

 Fig. 78. DIAGRAMMATIC VIKW FROM 



BEFORE OF THE PARTS COMPOSING 

 THE ORGAN OF HEARING OF THE 



LEFT SIDE (after Arnold). 



The temporal bone of the left side, 

 with the accompanying soft parts, has 

 been detached from the head, and a 

 section has been carried obliquely 

 through it so as to remove the front 

 of the meatus externus, half the 

 tympanic membrane, and the upper 

 and anterior wall of the tympanum 

 and Eustachian tube. The meatus 

 internus has also been opened, and 

 the bony labyrinth exposed by the 

 removal of the surrounding parts of 

 the petrous bone. 1, the pinna and 

 lobule ; 2 to 2', meatus externus ; 2', 

 membrana tympani ; 3, cavity of the 

 tympanum ; above 3, the chain of 

 small bones ; 3', opening into the 

 mastoid cells ; 4, Eustachian tube ; 

 5, meatus internus, containing the 

 facial (uppermost) and auditory 

 nerves ; 6, placed on the vestibule of 

 the labyrinth above the feuestra 

 pvalis ; a, apex of the petrous bone ; b, internal carotid artery ; c, styloid process ; d, facial nerve 

 issuing from the stylo-mastoid foramen ; e. mastoid process ; /, squarnous part of the bone. 



first two of these are to be considered as accessories to the third, which is the 

 portion of the organ to which the fibres of the eighth or auditory nerve are 

 distributed. 



THE EXTERNAL EAR. 



In the external ear are included the pinna the part of the outer ear which 

 projects from the side of the head and the meatus or passage which leads thence 

 to the tympanum, and which is closed at its inner extremity by a membrane inter- 

 posed between it and the middle ear. 



THE PINNA. 



The general form of the pinna or auricle, as seen from the outside, is concave, 

 to fit it for collecting and concentrating the undulations of sound, but it is thrown 

 into various elevations and hollows, to which distinct names have been given 

 (fig. 79). The largest and deepest concavity is called the concha ; it surrounds the 

 entrance to the meatus, and is interrupted at its upper and anterior part by a ridge, 

 which is the beginning of the helix. In front of the concha, and projecting back- 

 wards over the meatus, is a conical prominence, the trayus (fig. 79), covered 

 usually with hairs. Its upper part sometimes forms a rounded prominence (tuber- 

 culum supratragicum, His). Behind the tragus, and separated from it by a deep notch, 

 is another smaller elevation, the antitragus. Below the antitragus, and forming the 

 lower end of the auricle, is the lobule, which is devoid of the firmness and 

 elasticity that characterise the rest of the pinna. The thinner and larger portion of 



