570 PHILOSOPHICAL TRANSACTIONS. • [ANNO 1800. 



and the fibres of the radiated muscle are not only attached to its end, but also la- 

 terally to nearly the whole length of its handle. 



This oval form of the membrana tympani, in those quadrupeds, and the very 

 extensive attachment of the fibres of the radiated muscle to the handle of the mal- 

 leus, may be the reason why their ears are not equally fitted to hear inarticulate 

 sounds, as the ears of birds and of man. Should this radiated muscle of the 

 membrana tympani, which is probably the smallest in the body that has a distinct 

 action, be thought too insignificant to have an office of so much consequence as- 

 signed to it, let it be remembered, that the size of muscles is no indication of 

 their importance, but only of the resistance to be overcome by their action; and 

 that the more delicate actions are performed universally in the body by very small 

 muscles, of which the iris in the eye furnishes a very conspicuous example. 



Before the mode in which the radiated muscle adapts the membrana tympani to 

 different sounds can be explained, it is necessary that the more important parts of 

 the organ should be enumerated, and the use commonly assigned to each of them 

 pointed out. In man and the more perfect quadrupeds, this organ consists of the 

 following parts: the membrana tympani, situated between the external passage and 

 the cavity of the tympanum; 4 small bones, which form a chain across the tympa- 

 num, connecting the membrana tympani with another membrane lining the fora- 

 men ovale, which opens into the vestibulum, a more internal part of the organ of 

 hearing. The bones are, the malleus, which is united to the membrana tympani 

 by a portion of its handle, and to the 2d bone or incus by its head. The incus, 

 which is connected to the malleus by a capsular ligament, forming a regular joint, 

 the surfaces of the bones being covered with cartilage, but they have only a tremu- 

 lous motion on each other. The incus is also attached to the side of the cavity 

 of the tympanum, where the mastoid cells open, by a ligament on which it moves 

 backward and forward: it is united by its long process to the orbicular bone, which 

 is the smallest in the body, and connects the incus to the 4th bone or stapes, which 

 has its base applied to the foramen ovale, or opening leading into the cavity of the 

 vestibulum. The cavity of the tympanum, in which these bones are situated, com- 

 municates with the external air by means of the Eustachian tube, so that there is 

 always air behind the membrana tympani. 



The malleus has 3 muscles, by which it is moved; one of them is called the 

 tensor, from its pulling the malleus inwards, and tightening the membrana tympani: 

 the other 2 act in an opposite direction, and relax the membrane; the largest of 

 these is called the obliquus, and is the antagonist of the tensor muscle; the other 

 is very small, and is called the lexator. The stapes has 1 muscle, which acts on 

 it by bringing its basis closer to the foramen ovale. The vestibulum, which is 

 completely separated from the tympanum, by the membrane that lines the foramen 

 ovale, communicates freely with the cochlea and semicircular canals; but these 

 cavities are filled with a watery liquor, and have no communication, as the tympa- 



