9. Of what special protective value is the sense of smell?
PRACTICAL WORK
To demonstrate the Pacinian Corpuscles.—Spread out the mesentery from the intestine of a cat and hold it between the eye and the light: Pacinian corpuscles will appear as small translucent bodies having the general form of grains of wheat. Secure a portion of the mesentery over a circular opening in a thin piece of cork and examine it with a microscope of low power. Follow the course of the nerve fiber to the nerve from which it branches.
To show Relative Sensitiveness of Different Parts of the Skin.—Holding a bristle between the fingers, bring the end in contact with the skin, noting the amount of pressure necessary to cause a sensation of[pg 349] touch. Test the lips, tongue, tips of fingers, and palm and back of hand, trying different sizes of bristles. Has the degree of sensitiveness any relation to the thickness of the cuticle?
To show Perceptive Differences of Different Portions of the Skin.—Place the points of a pair of dividers on the back of the hand of one who looks in the opposite direction. Is one point felt or two? Repeat several times, changing the distance between the points until it is fully determined how near the two points must be placed in order to be felt as one. In like manner test other parts of the body, as the tips of the fingers and the back of the neck. Compare results obtained at different places.
To locate Warm and Cold Sensation Spots.—Slowly and evenly draw a blunt-pointed piece of metal over the back of the neck. If it be of the same temperature as the skin, only touch sensations will be experienced. If it be a little colder (the temperature of the room) sensations of cold will be felt at certain spots. If slightly warmer than the body, heat sensation spots will be found on other parts of the skin. If the heat and cold sensation spots be marked and tested from day to day they will be found to remain constant as to position. Inference.
CHAPTER XXI - THE LARYNX AND THE EAR
Man is a social being. His inclinations are not to live alone, but to be a part of that great human organization known as society. For men to work together, to be mutually helpful one to another, requires the ability to exchange ideas and this in turn requires some means of communication.[115] One means of communication is found in certain movements of the atmosphere, known as sound waves. In the exchange of ideas by this means there are employed two of the most interesting divisions of the body—the larynx and the ear. The first is an instrument for the production of sound waves; the second is the sense organ which enables the sound waves to act as stimuli to the nervous system.