.398 SENSITIVE SYSTEM. 



not an exact oval, one side being somewliat flattened) rests 

 against the membrane filling up the fenestra ovalis, to which in 

 figure it precisely corresponds. By a small head at the other 

 extremity it articulates with the os orbiculare. 



Muscles. — The mechanism of the internal ear is such as to re- 

 quire in the cavity of the tympanum the presence of four small 

 muscles*; and these operate on the malleus and stapes. They 

 are the — 



Laxator Tympani, — A very minute muscle, arising from 

 the outside wall, close to the attachment of the membrana tym- 

 pani, and inserted into the handle of the malleus, near its root. 



M. EXTHKNUs MALLEI vel M. Processus Minoris, is by 

 some denied altogether — at least, to be of the nature of muscle. 

 It arises from the upper part of the tympanum, and is attached 

 by a small tendon to the shorter process of the malleus. 



Tensor tympani arises from the side of the Eustachian 

 tube, and is inserted into the handle of the malleus, upon its up- 

 per side. 



Stapedius. — In the horse this muscle is developed in a re- 

 markable degree. It takes its rise from a little eminence within 

 the tympanum called the pyramid, and is fixed to the head of 

 the stapesf. 



* Sound is the eflFect of impression upon the portio mollis of the seventh 

 pair of nerves — the true auditory nerve. This impression is produced by 

 vibrations of the air upon the membrana tympani, communicated therefrom 

 by the osseous chain extended between them to the membrane of the 

 fenestra ovalis, and thence to the expanded auditory nerve. Now, these 

 vibrations being once excited, do not immediately cease, but continue suc- 

 ceeding one another in great rapidity; as, in common speaking, every syl- 

 lable articulated produces a separate and distinct impulse or vibrating mo- 

 tion upon the ear. Consequently, to prevent confusion of sound, or rather 

 confused audition, some contrivance was found necessary to put a stop to 

 one vibration before another was communicated. This explains, in general 

 terms, the use of the muscles of the tympanum. 



t It may not, perhaps, be considered altogether out of place to make a 

 remark or two here on the motions of the bones of the ear, and on the me- 

 chanical advantages derived from their arrangement and relative position. 

 The manubrium of the malleus is, as we have seen, extended downward 

 to be attached to the tympanum, whose every vibration, in course, affects it. 

 The other slender process, issuing from the neck of the bone and abutting 

 against the wall of the tympanum, being much nearer the point at which the 

 impression is received than the one where the power resides, becomes the 

 centre of motion — the fulcrum ; so that the bone is set into action upon 

 the principle of the lever. The moving power is applied to the manu- 

 brium, the process from the neck becomes the fulcrum, and the head of 

 the bone is the part on which the eff'ect is to be produced ; it being a law 

 in mechanics, that in proportion as the distance of the power from the ful- 

 crum or prop exceeds that of the weight or resistance to be overcome, so 

 will prove the mechanical advantage. In tlic example before us, this dis- 



