ORGAN OF HEARING. 177 



is about an inch long; narrower in its middle than extremities; longer 

 inferiorly than superiorly, owing to the obliquity of the membrana 

 tympani; and slightly curved upwards about its middle. The outer 

 orifice is furnished with down or hairs vibrissse like the orifices of 

 certain other canals. The meatus is osseous, for the space of half an 

 inch, and penetrates the temporal bone. More externally, it is formed 

 of fibro-cartilage, a prolongation of that of the concha. It is lined by 

 an extension of the skin, which becomes gradually thinner as it proceeds 

 inwards, and is ultimately reflected over -the outer surface of the mem- 

 brana tympani. Beneath this skin, numerous sebaceous glands or folli- 

 cles are situate, which secrete the bitter humour, called cerumen. This 

 humour occasionally becomes inspissated; obstructs the canal ; prevents 

 sonorous vibrations from reaching the membrana tympani, and is thus 

 the cause of deafness. Softening it, by means of warm water or oil, 

 or soap and water dropped into the meatus, and removing it by means 

 of the syringe, restores the hearing. 



The portion of the auditory apparatus arbitrarily termed the exter- 

 ternal ear, is a complete cul-de-sac, formed by a prolongation of the 

 common integument. There is no opening communicating with the 

 next portion the middle ear ; the membrana tympani, with its der- 

 moid envelopes, forming at once the 

 medium of union and separation Fi s- 66 - 



between the two. A knowledge of A B 



this fact would somewhat diminish 

 the alarm in cases where insects or 

 other extraneous bodies get into the 

 meatus. The pain is excruciating, 

 owing to the great general sensibi- 

 lity of this portion of the auditory 

 apparatus ; but the chief dread en- 

 tertained is, that the irritating sub- 

 stance may pass into the head. It 



* * , T ! Membrana tympani. 2. Malleus. 3. Sta- 



cannot proceed further than the pes. 4. incus. 



membrana tympani, and even if it 



were able to clear this obstacle, insuperable impediments would exist 



to its farther progress inwardly. 



2. The middle ear includes the cavity of the tympanum, the small 

 bones contained in the cavity, the mastoid cells, Eustachian tube, &c. 

 Like the last, it belongs to the physical portion of the ear. The 

 cavity of the tympanum or drum of the ear has the shape of a portion 

 of an irregular cylinder. Its name is, indeed, not inappropriate. It 

 bears some resemblance to a drum ; not only in form, but, as will be 

 seen, in function. The outer extremity is closed, as in a drum, by the 

 membrana tympani. This membrane is not situate vertically in the 

 meatus; but obliquely downwards and inwards; so that the cavity is 

 broader above than below. It is very thin and transparent, and con- 

 sists of three layers, the outermost formed by the membrane lining 

 the meatus auditorius externus ; the innermost belonging to the mem- 

 brane of the cavity of the tympanum ; and the middle the membrane 

 proper. On its inner side passes the nerve called chorda tympani; 



VOL. I. 12 



