THE SENSE OF HEARING. 



639 



S/ 



the adult condition can not be removed, owing to its consolidation 

 with the surrounding bone. This membrane consists primarily of 

 a layer of fibrous tissue which is covered externally by a thin layer 

 of skin continuous with that lining the auditory canal, and internally 

 by a thin mucous membrane. The tympanic membrane is placed 

 obliquely at the bottom of the auditory canal, inclining from above 

 and behind downward and forward at an angle of about forty-five 

 degrees. The external surface of this membrane presents a funnel- 

 shaped depression, the sides of which are slightly convex. 



The Ear-bones. Running across the tympanic cavity and form- 

 ing an irregular line of 

 joined levers is a chain 

 of bones, which articu- 

 late one with another at 

 their extremities. These 

 bones are known as 

 the malleus, incus, and 

 stapes. The form and 

 arrangement of these 

 bones are shown in 

 Figs. 310, 311. 



The malleus, or ham- 

 mer bone, consists of a 

 head, neck, and handle, 

 of which the latter is 

 attached to the inner 

 surface of the membrana 

 tympani. The incus or 

 anvil bone presents a 

 concave articular sur- 

 face which receives the 

 head of the malleus. 

 The stapes, or stirrup- 

 bone, articulates exter- 

 nally with the long pro- 

 cess of the incus, and internally, by its oval base, with the edges of 

 an oval opening, the foramen ovale. The entire chain is partially 

 supported by a ligament attached to the short process of the incus 

 and to the walls of the tympanic cavity. 



The Tensor Tympani Muscle. This is a delicate muscle, 

 about 15 mm. in length, situated in a narrow groove just above the 

 Eustachian tube (Fig. 312). It arises from the cartilaginous portion 

 of the Eustachian tube and the adjacent portion of the sphenoid bone. 

 From this origin it passes nearly horizontally backward to the tym- 

 panic cavity; just opposite to the foramen ovale, its tendon bends at 



FIG. 310. TYMPANIC MEMBRANE AND THE AUDI- 

 TORY OSSICLES (LEFT) SEEN FROM WITHIN, 

 i. e., FROM THE TYMPANIC CAVITY. M. Manu- 

 brium or handle of the malleus. T. Inser- 

 tion of the tensor tympani. h. Head. IF. 

 Long process of the malleus, a. Incus, with 

 the short (K) and the long (/) process. 5. 

 Plate of the stapes. Ax, Ax, is the common 

 axis of rotation of the auditory ossicles. S l . 

 The pinion-wheel arrangement between the 

 malleus and incus. (Landois.} 



