THE NERVOUS SYSTEM AND ORGANS OF SENSE 407 



these projections on the top and back part of the tongue, are located 

 the organs of taste. These organs are called taste buds. 



Each taste bud consists of a collection of spindle-shaped nerve cells, 

 each cell tipped at its outer end with a hairlike projection. These cells 

 send inward fibers to other cells, the fibers from which ultimately reach 

 the brain. The sensory cells are surrounded by a number of protecting 

 cells which are arranged in layers about them. Thus the organ in longi- 

 tudinal section looks somewhat like an onion cut lengthwise. 



How we Taste. 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. A substance must be 

 dissolved in fluid in order to be tasted. 

 Many things which we believe we Section of circumvallate papilla, 

 taste are in reality perceived by the 



sense of smell. Such are spicy sauces and flavors of meats and vege- 

 tables. This may easily be proved by holding the nose and chewing, 

 with closed eyes, several different substances, such as an apple, an onion, 

 and a raw potato. 



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 connected with the forebrain by means of the olfactory nerve. In 

 order to perceive odors, it is necessary to have them diffused in the ah* ; 

 hence we sniff so as to draw in more air over the olfactory cells. 



The Organ of Hearing. The organ of hearing is the ear. In the fish, 

 frog, and reptile, the outer ear, so prominent in man, is entirely lacking. 

 The outer ear consists of a funnel-like organ composed largely of cartilage 

 which is of use in collecting sound waves. This part of the ear incloses the 

 auditory canal, which is closed at the inner end by a tightly stretched mem- 

 brane, the tympanic membrane. We have seen the tympanic membrane 

 of the frog on the outer surface of the head. The function of the tympanic 

 membrane is to receive sound waves, for all sound is caused by vibrations 

 in the air, these vibrations being transmitted, by the means of a com- 

 plicated apparatus found in the middle ear, to the real organ of hearing 

 located in the inner ear. 



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 



