302 THE SENSORIAL FUNCTIONS. 



Fig. 391, is occupied by a rounded eminence (p,) of a tri- 

 angular shape, termed the promontory; on each side of 

 which there is an opening in the bone, closed, however, by 

 the membrane lining the whole internal surface of the cavi- 

 ty. The opening (o,) which is situated at the upper edge 

 of the promontory, is called the fenestra ovalis, or oval 

 window; and that near the under edge (R,) is the fenestra 

 rotunda, or round window. 



Connected with the membrane of the ear-drum, at one 

 end, and with the fenestra ovalis at the other, there extends 

 a chain of very 'minute moveable bones, seen at (B,) in Fig. 

 390; but more distinctly in Fig. 392, which is drawn on a 

 somewhat larger scale, and in which as before (D) is the 

 ear-drum; (p) the promontory, (o,) the fenestra ovalis; and 

 (R) the fenestra rotunda. These bones, which may be called 

 the tympanic ossicula, are four in number, and are repre- 

 sented, enlarged to twice the natural size, in Fig. 393. The 

 names they have received are more descriptive of their 

 shape than of their office. The first is the malleus, or ham- 

 mer (M;) and its long handle (H) is affixed to the centre of 

 the ear-drum: the second is the incus, or anvil (i;) the 

 third, which is the smallest in the body, being about the 

 size of a millet seed, is the orbicular bone (o;)* and the 

 last is the stapes, or stirrup (s,) the base of which is applied 

 to the membrane of the fenestra ovalis. These bones are 

 regularly articulated together, with all the ordinary appa- 

 ratus of joints, and are moved by small muscles provided 

 for that purpose. Their office is apparently to transmit the 

 vibrations of the ear-drum to the membrane of the fenestra 

 ovalis, and probably, at the same time, to increase their 

 force. 



The more internal parts of the ear compose what is de- 



* Blumenbach, and other anatomists, consider this as not being a separate 

 bone, but only a process of the incurs, view of the subject which is sup- 

 ported by the observations of Mr..Shrapnell, detailed in the Medical Ga- 

 zette, xii., 172. 



