190 SBNSORIAL FUNCTIONS. 



he instinctively opens his mouth. When this tube is 

 entirely closed total deafness is often the result. 



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 ova- 

 Us, or oval window. This 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. 

 These bones, called the tympanic ossicula, or little 

 bones of the drum, are represented separate, and twice 



their natural size, by Fig. 115. 

 Their names have been deri- 

 ved from their shapes rather 

 than the offices they perform. 

 The first, m, is the malleus, or 

 hammer, the long handle h, of 

 which is affixed to the ear- 

 drum ; the second, i, is the 

 incus, or anvil, \vhich some- 

 what 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 mil- 

 let seed, and is situated between 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. 



These bones are regularly articulated with each othei^ 

 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. 



Labyrinth of the Ear. We have thus given a sun> 



Where are the bones of the ear situated 1 To what parts are these 

 bones attached 1 What are the names of the tympanic ossicula 1 



