788 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



tioii ( 042). These facts enable us to understand the influence of the tensor 

 tympani muscle, in augmenting the tension of the membrane, and thus en- 

 abling it to vibrate in reciprocation to sounds having a great variety of fun- 

 damental notes. It appears to be antagonized by the stapedius, the contrac- 

 tion of which seems to diminish the tension of the membrana tympani, and 

 to take off pressure from the fluid of the labyrinth. These two muscles con- 

 jointly may be considered to regulate the transmission of sonorous undula- 

 tions to the fluid of the internal sac, preventing it from being too violently 

 affected by loud sounds, in the same manner that the iris regulates the ad- 

 mission of light to the eye ( 612); and the analogy extends also to their 

 nervous supply, the stapedius being excited to action by a branch of the 

 Facial, whilst the tensor tympani receives its nerves from the Otic gan- 

 glion. 1 They are probably put into conjoint action when we are listening for 

 faint sounds, so as to bring the tympanum into the state of tension best 

 adapted to reciprocate them ; whilst by a like preparation, the concussive 

 effects of a loud sound that is anticipated, are more effectually moderated 

 than when it strikes the ear without warning. It is probably owing to an 

 imperfect action of these muscles, that some persons are deaf to grave sounds, 

 whilst they readily hear the more acute. Helmholtz 2 observes, that by the 

 transference of the vibrations of the tympanum to the much smaller mem- 

 brane of the fenestra ovalis, mechanical power is gained for the establish- 

 ment of vibrations in the comparatively incompressible fluid of the labyrinth. 

 He observes also that the inferior aspects of the articular surfaces of the 

 malleus and incus are furnished with small teeth, so arranged that when the 

 malleus moves outwards the articular surfaces easily separate as far as the 

 somewhat loose capsule will allow, whilst when the malleus moves inwards 

 these two bones become firmly locked. The result of this is, that when air 

 is blown into the tympanic cavity, the membrana tympani can move out- 

 wards without dragging the stapes out of the fenestra ovalis. 



646. The uses of the Tympanic Cavity are very obvious. One of its pur- 

 poses is to render the vibrations of the membrane quite free; and the other, 

 to isolate the chain of bones in such a manner as to prevent their vibrations 

 from being weakened by diffusion through the surrounding solid parts. As 

 to the objects of the Eustachian Tube, opinions have been much divided. 

 Many of these opinions, however, such as the one most commonly enter- 

 tained, that it serves the same purpose as the hole in an ordinary drum, 

 removing an impediment to the free vibration of the membrane that would 

 be offered by the complete inclosure of the air within, are at once negatived 

 by the fact, which seems to have been demonstrated by Mr. Toynbee and Dr. 

 Jago, 3 that the guttural orifice of the tube is usually closed, being only 

 opened during the act of swallowing. 4 The principal object of the Eusta- 

 chian tube (which is always found where there is a tympanic cavity) seems 

 to be the maintenance of equilibrium between the air within the tympanum 



of curves, representing complicated vibrations, not only of the vocals, but also of the 

 notes produced by the passage of these into consonants; of the sound colors of vari- 

 ous musical instruments, and of various notes, all of which present curves of appro- 

 priate form. See also Tyndall's work, On Sound, 18(57, for a full resume of all recent 

 re>c;irehes, also Squire on The Quality of Musical Sounds, Quart. Journ. of Sci., 



1 8(1:'), p. GOO. 



1 See Mr. C. Brooke in Lancet, 1843, p. 380; and Mr. Toynbee in Brit, and For. 

 Med.-Cliir. llev., vol. xi, p. 235. 



2 Uebcr die Mefhanik der Gehorknoehenehon, rfluger's Archiv. 18fi8. See Hum- 

 phry and Turner's Journal of Anatomy, vol. iii, 1868, p. 219; also Buck, New York 

 Med. Journ., June, 1874. 



3 See his very interesting essays in the Med.-Chir. Review, 1867, pp. 175 and 496. 



4 Loc. cit., and Proceedings of the lioyal Society, 1852. 



