186 SENSORIAL FUNCTIONS. 



known that when one listens to a scarcely audible sound 

 he instinctively opens his mouth. When this tube is entirely 

 closed, total deafness is often the result. 



576. Bones of the Ear. Behind, or within the interior 

 side of the tympanum, there is a chain of very minute 

 moveable bones, of peculiar shapes, seen of the natural 

 sizes at b. One end of this chain is fastened to the tym- 

 panum, and the other to a part called the fenestra ovalis, 

 or oval window. The latter part is a membrane situated 

 in the cavity of the tympanum, opposite to the orifice of the 

 eustachian tube, and covering a cell in the bone, called a 

 mastoid cell, which cell is filled with air. 



577. These bones, called the tympanic ossicula, or little 

 bones of the drum, are represented separate, arid twice 

 their natural size, by Fig. 115. Fig. 115. 



Their names have been deri- 

 ved from their shapes, rather 

 than the offices they perform. 

 The first, m, is Ihe malleus, or 

 hammer, the long handle h, of 4 

 which is affixed to the ear- 

 drum; the second, i, is the 

 incus, or anvil, which somewhat resembles in shape a 

 molar tooth, the crown of which is attached to the head 

 of the hammer ; the third, o, is the round, or orbicular 

 bone. This is the smallest bone in the human skeleton, 

 being no larger than a millet seed, and is situated be- 

 tween the long process of the anvil and the next bone in 

 the number. The fourth and last bone in the chain is the 

 stapes, or stirrup, s, which is fastened by its base, or 

 widest part to the fenestra ovalis. 



578. The bones are regularly articulated with each 

 other so as to allow of motion between each two, and their 

 office appears to be, to transmit the vibrations of the ear- 

 drum to the fenestra ovalis, and probably also to increase 

 the force of these vibrations. 



579. Labyrinth of the Ear. We have thus given a sum- 



Where are the bones of the ear situated? To what parts are these 

 bones attached ? What are the names of the tympanic ossicula ? 



