408 THE NERVOUS SYSTEM AND ORGANS OF SENSE 



Inc. 



time of a heavy concussion and thus prevent the rupture of the delicate 

 tympanic membrane. Placed directly against the tympanic membrane 

 and connecting it with another membrane, separating the middle from 

 the inner ear, is a chain of three tiny bones, the smallest bones of the body. 

 The outermost is called the hammer; the next the incus, or anvil; the 

 third the stirrup. All three bones are so called from their resemblances 

 in shape to the articles for which they are named. These bones are held 



in place by very small 

 muscles which are deli- 

 cately adjusted so as 

 to tighten or relax the 

 membranes guarding 

 the middle and inner 

 ear. 



The Inner Ear. - 

 The inner ear is one 

 of the most compli- 

 cated, as well as one 

 of the most delicate, 

 organs of the body. 

 Deep within the tem- 

 poral bone there are 

 found two parts, one 

 of which is called, col- 

 lectively, the semicir- 

 cular canal region, the 

 other the cochlea, or 

 organ of hearing. Both 

 of these organs consist 



of membranous bags lying in a fluid which partially fills the bony cavity 

 which incloses them. These membranous structures themselves also con- 

 tain a fluid. The semicircular canals are connected with the cochlea on one 

 side, and are separated from the middle ear only by a membrane and the 

 fluid which surrounds them. There are three semicircular canals, delicate 

 membranous bags lying in a watery fluid .and surrounded by bone. 



It has been discovered by experimenting with fish, in which the semi- 

 circular canal region forms the chief part of the ear, that this region has 

 to do with the equilibrium or balancing of the body. We gain in part our 

 knowledge of our position and movements in space by means of the semi- 

 circular canals. 



That part of the ear which receives sound waves is known as the cochlea, 

 or snail shell, because of its shape. This very complicated organ is lined 

 with sensory cells provided with cilia. The cavity of the cochlea is filled 

 with a fluid. It is believed that somewhat as a stone thrown into water 

 causes ripples to emanate from the spot where it strikes, so sound waves 



Section of ear : E.M., auditory canal ; Ty.M., tympanic 

 membrane ; Eu., Eustachian tube ; Ty, middle ear ; 

 Coc., A.S.C., E.S.C., etc., internal ear. 



