740 THE EAR. 



THE EAR. 



THE organ of hearing is divisible into three parts : the external ear, the 

 tympanum or middle ear, and the labyrinth or internal ear. The first two 

 of these are to be considered as accessories or appendages to the third, \\hich 

 is the sentient portion of the organ. 



Fig. 495. 



Fig. 495. DIAGRAMMATIC VIEW FROM BEFORE OF THE PARTS COMPOSING THE ORGAN OF 

 HEARING OF THE LEFT SIDE (after Arnold, and from nature). 



The temporal bone of the left side, with the accompanying soft parts, has been 

 detached from the head, and a section has been carried through it transversely so as to 

 remove the front of the meatus externus, half the tympanic membrane, 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 lobe ; 2, 2', meatus externus ; 2', membrana tympani ; 

 3, cavity of the tympanum ; IV, its opening backwards into the mastoid cells ; between 

 3 and 3', the chain of small bones ; 4, Eustachian tube ; 5, meatus internus containing 

 the facial (uppermost) and the auditory nerves ; 6, placed on the vestibule of the laby- 

 rinth above the fenestra ovalis ; a, apex of the petrous bone ; I, internal carotid artery ; 

 c, styloid process; d, facial nerve issuing from the stylo-mastoid foramen; e, mastoid 

 process ; /, squamous part of the bone covered by integument, &c. 



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 ; together with the meatus or passage which 

 leads thence to the tympanum, and is closed at its inner extremity by a 

 membrane (membrana tympani) interposed between it and the middle ear. 



THE PINNA. 



Superficial configuration. The general form of the pinna or auricle is con- 

 cave, as seen from the outside, to fit it for collecting and concentrating the 

 undulations of sound ; it is thrown into various elevations and hollows, to 

 which distinct names have been given. The largest and deepest concavity, 



