416 ANATOiMY FOR NURSES [Chap. XX 



downward and backward. It is lined by a prolongation of the 

 skin, which in the outer half of the canal is very thick and not at 

 all sensitive, and in the inner half is thin and highly sensitive. 

 Near the orifice the skin is furnished with a few hairs, and far- 

 ther inward with modified sweat-glands, the ceruminous glands, 

 which secrete a yellow, pasty substance resembling wax. This 

 wax is thought, to be offensive to insects, and consequently a 

 defence against their intrusion. 



Middle ear. — The middle ear, or t\Tnpanum, is a small, ir- 

 regular bony cavity, situated in the petrous portion of the temporal 

 bone, and lined with mucous membrane. It is separated from the 

 external auditory canal by the drum-membrane (membrana 

 tympani), and from the internal ear by a very thin, bony wall 

 in which there are two small openings covered with membrane 

 — the oval window, or fenestra ovalis, and the round window, 

 or fenestra rotunda. The cavity of the middle ear is so small 

 that probably five or six drops of water would completely fill it. 

 It communicates below with the pharynx by the small passage 

 called the Eustachian tube.^ 



The function of this tube is to ventilate this cavity and keep the 

 atmospheric pressure equal on each side of the drum-membrane. 

 The middle ear also communicates above with a number of bony 

 cavities in the mastoid portion of the temporal bone. These 

 cavities, called mastoid cells, are lined with mucous membrane, 

 which is continuous with that covering the cavity of the tympa- 

 num. 



Membrana tympani (membrane of the drum). — It is a tough, 

 fibrous membrane set in the bony opening of the external audi- 

 tory canal. The degree of tension of the membrane is regulated 

 by the tensor tympani muscle. This muscle is lodged in a bony 

 canal that is above and parallel with the Eustachian tube. 



Ossicles. — Stretching across the tympanic cavity is a chain 

 of tiny, movable bones, three in number, and named from their 

 shape the malleus, or hammer, the incus, or anvil, and the stapes, 

 or stirrup. The handle of the hammer is attached to the drum- 

 membrane, and the opposite end or head of the hammer is at- 



' This direct connection between the ear and the pharynx is one of the impor- 

 tant reasons for the frequent cleansing of the mouth necessary in infectious and 

 contagious diseases. The Eustachian tube forms a passageway for germs to travel 

 from the mouth to the middle ear and there cause various infections. 



