232 FIRST YEAR SCIENCE 



upon the ear drum. The ear drum is a thin membrane 

 stretched tightly across a bony opening and vibrating when 

 the air waves strike it, as a drum does when struck by the 

 drumstick. On its inner side the drum is attached to the 

 inner ear by a chain of three bones. The sensitive cells 

 of the inner ear transmit the impressions made by the 

 sound vibrations through the auditory nerve to the brain, 

 where they are interpreted into the sensation of sound. 



On the tongue and in the nose are cells which transmit to 

 the brain the impressions produced upon them by different 

 qualities, the one of solutions and the other of gases. The 

 sensations thus produced are called taste and smell. 



The sensation of touch originates in the skin and is much 

 more acute in some portions than in others. The tips of 

 the fingers in the blind are often trained to such delicate 

 perception that they, in a great degree, take the place of 

 the lacking sense organ. These sensations, like all others, 

 are carried to the brain by the nerves and there inter- 

 preted into the sensation of touch. 



109. Food. Experiment 117. Chop a piece of the white of a 

 hard-boiled egg into pieces about as large as the head of a pin and 

 place in a test tube. Chop up another piece much finer than this and 

 place it in a second test tube. Make a mixture of 100 cc. of water, 5 cc. 

 of essence of pepsin and 2 cc. of hydrochloric acid. Pour into each 

 test tube enough of this mixture to cover the white of egg to a con- 

 siderable depth. Shake thoroughly and put in a place where the tem- 

 perature can be maintained at 37 C. or 98 F. A fireless cooker or a 

 bucket of warm water is good for this. Allow to stand for several 

 hours, keeping the temperature constant. The white of egg is dis- 

 solved, the action being more rapid in the second tube. Try the same 

 experiment using water; using dilute hydrochloric acid. Do these 

 have the same effect as when used with the pepsin ? The pepsin 

 solution is an artificial gastric juice. 



In order that the work of the body may be carried on, 

 food is required. This food may be supplied by either 



