Page:Proceedings of the Royal Society of London Vol 1.djvu/22

Rh texture, the muscular ﬁbres appearing similar throughout the whole surface; without any central tendons as in the diaphragm, and chieﬂy forming the internal layer, on which side they appeared most conspicuous.

The blood-vessels of this membrane, the number of which is proportionate to its action, resemble in a great measure those of the iris, and are nearly as numerous. They anastomose with one another in a similar manner, and their general dissection is from the circumference of the membrane to the handle of the malleus. From near this handle a small trunk sends off branches in a radiated manner, which likewise anastomose with those that have an opposite course. This correspondence in the number and distribution of blood-vessels, between the membrana tympani and the iris, is given as a proof of that membrane being endowed with muscular action; and indeed the author henceforth speaks without hesitation of a radiated muscle distinct from the membrane. Having examined the structure of this muscle in various animals, Mr. Home found the application of a principle laid down in a former lecture, namely, that while the organization necessary for muscular contraction may in some measure exist in an apparent membrane, yet wherever muscular action is to overcome a resistance, a fasciculated structure becomes absolutely necessary. Thus in birds, where from the smallness of the organ the resistance is very trifling, this membrane is of a simple structure, like the coat of the hydatid; whereas in the elephant the ﬁbres forming fasciculi are very distinct and obvious.

This muscular structure in the membrana tympani serves to explain various phænomena in hearing, which were not hitherto clearly understood. It is observed that the sounds produced by percussion on an extended membrane like the drum and tambourine, cannot but be imperfect: since, the tension being from side to side, the instrument may be considered as consisting of a number of parallel strings varying in their lengths like the parallel chords of a circle, and bearing consequently no musical proportion to each other. Such also must have been the imperfection of the membrana tympani, according to the received opinion of its structure: but after the present discovery it will appear obvious that the radiated muscle proceeding from the circumference of the circle to a common centre, produces a system of strings all equal in length, and uniformly varying in musical effect, according to their tension.

In order to illustrate the manner in which this radiated muscle adapts the membrane tympani to different sounds, the author ﬁnds it necessary to enumerate the more important parts of the organ, and to point out the use commonly assigned to each of them.

Without entering into this detail, we can here only observe that, availing himself of the present discovery, he compares the tensor and radiated muscles of the membrana tympani to a monochord, of which the membrane is the string, the tensor miracle the screw giving the necessary tension to make the string perform its proper scale of vibrations, and the radiated muscle acting upon the membrane like the