204 HOW BODY CONTROL IS BROUGHT ABOUT 



External ear Middle ear 



Inner ear 

 Semicircular canals 



is of use in collecting sound waves, and the auditory canal, which 

 is closed at the inner end by a tightly stretched membrane, the 

 tympan'ic membrane. The function of the tympanic membrane 

 is to receive sound waves or vibrations in the air, which are trans- 

 mitted, by the means of a complicated apparatus found in the 

 middle ear, to the inner ear. 



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

 temporal bone, and separated from the outer ear by the tj^mpanic 

 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 tympanic membrane. 

 For this reason we open the mouth at the time of a heavy concus- 

 sion and thus prevent the rupture of the delicate tympanic 



membrane. Placed di- 

 rectly against the tym- 

 panic membrane and 

 connecting it with an- 

 other membrane which 

 separates the middle from 

 the inner ear, is a chain 

 of three tiny bones, the 

 smallest of the body. 

 They 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, collectively, the semicircular canals, the other the cochlea 

 (kok'le-d). 



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

 semicircular canal region forms the chief part of the ear, that this 

 region has to do with the equilibrium or balancing of the body. 

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

 cochlea (Lat., snail shell) because of its shape. This complicated 

 organ is lined with sensory cells provided with cilia, and its cavity 

 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, 



■Tympanic membran 



Eustachian tub^---^ ' Cochlea 

 Diagram of section of the human ear. 



