THE SENSE OF HEARING. 



527 



FIG. 196. 



SENSE OF HEARING. 



Anatomy of the Organ of Hearing. 



For descriptive purposes, the ear, or organ of hearing, is 

 divided into three parts, the external, the middle, and the in- 

 ternal ear. The two first are only ac- 

 cessory to the third or internal ear, 

 which contains the essential parts of an 

 organ of hearing. The accompanying 

 figure shows very well the relation of 

 these divisions one to the other (Fig. 

 196). 



The external ear consists of the pinna 

 or auricle, and the external auditory 

 canal or meatm. The principal parts 

 of the pinna are two prominent rims 

 inclosed one within the other (helix and 

 antihelix}, and inclosing a central hol- 

 low named the concha ; in front of the 

 concha, a prominence directed back- 

 wards, the tragus, and opposite to this, 

 one directed forwards, the antitragus. 

 From the concha, the auditory canal, 

 with a slight arch directed upwards, 

 passes inwards and a little forwards to 

 the membrani tympani, to which it thus 

 serves to convey the vibrating air. Its 

 outer part consists of fibro-cartilage 

 continued from the concha ; its inner part of bone. Both 

 are lined by skin continuous with that of the pinna, and ex- 

 tending over the outer part of the membrana tympani. Towards 

 the outer part of the canal are fine hairs and sebaceous glands, 

 while deeper in the canal are small glands, resembling the 

 sweat-glands in structure, which secrete a peculiar yellow sub- 

 stance called cerumen, or ear-wax. 



The middle ear, or tympanum (b, Fig. 197) is separated by 

 the membrana tympani from the external auditory canal. It 

 is a cavity in the temporal bone, opening through its anterior 

 and inner wall into the Eustachian tube, a cylindriform 

 flattened canal, dilated at both ends, composed partly of bone 

 and partly of cartilage, lined with mucous membrane, and 

 forming a communication between the tympanum and the 

 pharynx. It opens into the cavity of the pharynx just behind 



Outer surface of the pin- 

 na of the right auricle. %. 

 1, helix; 2, fossa of the 

 helix ; 3, antihelix ; 4, fossa 

 of the antihelix; 5, anti- 

 tragus; 6, tragus; 7, con- 

 cha ; 8, lobule. 



