68 INNERVATION. [CHAP, xvill. 



calculated to entangle particles of dust, or small insects, and to pre- 

 vent their entrance into the organ. These glands are principally 

 seated in the subcutaneous tissue, where the cartilage is deficient, 

 and do not extend into the osseous portion of the canal. The 

 cerumen is an oily, very bitter substance, of a yellow colour, and 

 contains, in addition to fat, albumen, and colouring matter, a bit- 

 ter principle analogous to that of the bile. If not removed from 

 time to time, it is liable to form hard pellets, which either impact 

 the passage, or come into contact with the membrana tympani, and 

 in either case seriously interfere with the transmission of sound to 

 the internal parts. These concretions are partially soluble in ether 

 and turpentine. 



The Middle Ear, or tympanic cavity, is a space filled with air, com- 

 municating with the pharynx by the Eustachian tube, and inter- 

 posed between the external meatus and the labyrinth. It opens 

 behind into the mastoid cells, which are also filled with air, and it 

 is traversed by a chain of moveable bones, connecting the membrana 

 tympani with the vestibule or common central cavity of the la- 

 byrinth. The tympanum is of irregular shape, compressed laterally, 

 and lined by a very delicate ciliated epithelium, prolonged from the 

 pharynx. 



The external wall of the tympanum is formed by the membrana 

 tympanij and a small extent of the surrounding bone. The mem- 

 brane is nearly oval, but wider above than below, and as already 

 stated, placed in a slanting direction, so as to form an obtuse angle 

 with the upper wall, and an acute one of about 45 with the floor 

 of the auditory canal. It consists of three laminae, an external, 

 middle, and internal. The external is derived from the cuticular 

 lining of the canal, and easily detaches itself with that structure 

 after maceration. The middle is strong and fibrous, perhaps analo- 

 gous to the dermal part of the integument, and attached through 

 the medium of a dense fibrous rim to the bone, which presents a 

 distinct groove for its reception, except above. The handle of the 

 malleus is firmly united to this layer of the membrane, in a vertical 

 direction as far down as the centre, and draws the membrane in- 

 wards along that line, so that its outer surface is concave, its inner 

 convex. The abundant small vessels supplying this part run along 

 the handle of the malleus, and thence radiate more or less directly 

 towards the border. The fibrous tissue is in part similarly disposed, 

 and thus seems to have led Sir E. Home to describe a radiating 

 muscle in the membrane, which does not appear to exist. Seen 

 from within, a concentric arrangement of the fibres is more obvious, 



