THE SENSES. 



181 



It contains a chain of small bones (Ossicula auditus) which extends from 

 the membrana tympani to the fenestra ovalis. 



The membrana tympani is placed in a slanting direction at the bottom 

 of the external auditory canal, its plane being at an angle of about 45 

 with the lower wall of the canal. It is formed chiefly of a tough and 

 tense fibrous membrane, the edges of which are set in a bony groove; its 

 outer surface is covered with a continuation of the cutaneous lining of the 

 auditory canal, its inner surface with part of the 

 ciliated mucous membrane of the tympanum. 



The small bones or ossicles of the ear are three; 

 named malleus, incus, and stapes. The malleus, or 

 hammer-bone, is attached by a long slightly curved 

 process, called its handle, to the membrana tympani; 

 the line of attachment being vertical, including the 

 whole length of the handle, and extending from the 

 upper border to the centre of the membrane. The 

 head of the malleus is irregularly rounded; its neck, 

 or the line of boundary between it and the handle, 

 supports two processes; a short conical one, which 

 receives the insertion of the tensor tympani, and 

 a slender one, processus glacilis, which extends for- 

 ward, and to which the laxator tympani muscle is 

 attached. The incus, or anvil-bone, shaped like a 

 bicuspid molar tooth, is articulated by its broader 

 part, corresponding with the surface of the crown of 

 a tooth, to the malleus. Of its two fang-like pro- 

 cesses, one, directed backward, has a free end lodged 

 in a depression in the mastoid bone; the other, curved 

 downward and more pointed, articulates by means of a roundish tuber- 

 cle, formerly called os orUculare, with the stapes, a little bone shaped 

 exactly like a stirrup, of which the base or bar fits into the fenestra 

 ovalis. To the neck of the stapes, a short process, corresponding with 

 the loop of the stirrup, is attached the stapedius muscle. 



The Ossicula. The bones of the ear are covered with mucous 

 membrane reflected over them from the wall of the tympanum; and are 

 movable both altogether and one upon the other. The malleus moves 

 and vibrates with every movement and vibration of the membrana tympani, 

 and its movements are communicated through the incus to the stapes, 

 and through it to the membrane closing the fenestra ovalis. The malleus, 

 also, is movable in its articulation with the incus; and the membrana 

 tympani moving with it is altered in its degree of tension by the laxator 

 and tensor tympani muscles. The stapes is movable on the process of the 

 incus, when the stapedius muscle acting, draws it backward. The axis 

 round which the malleus and incus rotate is the line joining the processus 

 gracilis of the malleus and the posterior (short) process of the incus. 



(3.) Internal Ear. The proper organ of hearing is formed by the dis- 

 tribution of the auditory nerve within the internal ear, or labyrinth of 

 the ear, a set of cavities within the petrous portion of the temporal bone. 



FIG. 358. -Outer surface 

 of the pinna of the right 

 auricle. 1, helix: 2, fossa 

 of the helix; 3, antihelix; 

 4, fossa of the antihelix ; 5, 

 antitragus; 6, tragus; 7, 

 concha; 8, lobule. %. 



