BODY CONTROL AND HABIT FORMATION 359 



J^ail. 



JEJ.M. 



Middle Ear. — The middle ear in man is a cavity inclosed by the tem- 

 poral bone, and separated* from the outer ear by the tympanic membrane. 

 A little tube called the Eustachian tube connects the inner ear with the 

 mouth cavity. By allowing air to enter from the mouth, the air pressure 

 is equalized on the ear drum. For this reason, we open the mouth at the 

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

 tympanic membrane. 

 Placed directly against 

 the tympanic mem- 

 brane and connecting 

 it with 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 

 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 delicately 

 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 complicated, as 

 well as one of the most delicate, organs of the body. Deep within the 

 temporal bone there are found two parts, one of which is called, collec- 

 tively, the semicircular canal region, the other the cochlea, or organ of hear- 

 ing. 



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 compHcated organ is lined 

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



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. 



