438 MAN BECOMES THE CONQUEROR OF THE WORLD 



the organ in longitudinal section looks somewhat like an onion 

 cut lengthwise. 



Four kinds of substances may be distinguished by the sense of 

 taste. These are sweet, sour, bitter, and salt. Certain taste 

 cells located near the back of the tongue are stimulated only by a 

 bitter taste. Sweet substances are perceived by cells near the 

 tip of the tongue, sour substances along the sides, and salt about 

 equally all over the surface. Taste and smell are often confused 

 and many things which we believe we taste are in reality perceived 

 by the sense of smell. 



Smell. The sense of smell is located in the membrane lining 

 the upper part of the nose. Here are found a large number of rod- 

 shaped cells which are con- 



nected with the fore brain 

 by means of the olfactory 

 nerve. In order to perceive 

 odors, it is necessary to have 

 them, either as minute par- 

 ticles of solid matter or as 



-olfcKttar^" cett 



Olfactory cells. 



Where is the sense of smell 

 located ? 



gases, diffused in the air. 

 If we wish to smell particularly well, we sniff so as to draw the air 

 higher in the nasal chambers and nearer the olfactory cells. 



Hearing. The organ of hearing is the ear. The outer ear 

 consists of a funnel-like organ composed largely of cartilage which 

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

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

 tympanic membrane. The function of the tympanic membrane 

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

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

 ear, to the inner ear. 



Middle ear. The middle ear is a cavity inclosed by the temporal 

 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 tympanic membrane. For 

 this reason we open the mouth at the time of a heavy explosion 

 and thus prevent the rupture of the delicate tympanic membrane. 



