334 HUMAN PHYSIOLOGY 



Salivary Glands. — Besides the cells in the mouth which secrete 

 mucus, other collections of gland cells form a substance called 

 saliva. There are three pairs of the salivary glands. They are 

 named according to their position, the parotid (under the ear), the 

 submaxillary (under the jawbone), and the sublingual (under the 

 tongue) . 



Home Exercise. — Chew paraffin or India rubber and collect saliva in 

 a test tube. Answer the following questions. What is its color? Does 

 any sediment appear after standing ? Test with htmus paper for its chemical 

 reaction. Partly fill three test tubes, one with water, one with saliva, one 

 with saliva plus weak acetic acid. Place in each a small piece of cracker. 

 Leave the three overnight in a warm place (about 98° Fahrenheit). Next 

 morning test the contents of the tubes for starch and for grape sugar. 

 In which tube has some starch been changed to grape sugar? Chew a 

 piece of cracker slowly. Notice any change in taste of cracker. How do 

 you account for this ? 



Digestion of Starch. — The digestion of starch to grape sugar 

 is caused by the presence in the saliva of an enzyme, or digestive 

 ferment. You will remember that starch in the growing corn 

 grain was changed to grape sugar by an enzyme, called diastase. 

 Here the same action is caused by an enzyme called vtyalin. 

 This ferment, as we see, acts only in an alkaline medium at about 

 the temperature of the body. 



Openings from Buccal Cavity. — The mouth cavity of man, as that of 

 a frog, has four paired openings leading from it, — two into the nostril (the 

 'posterior nares), and two to the ear (the Eustachian tubes, named after their 

 discoverer, an Italian doctor). Three single openings also exist, — one to 

 the outside (the mouth), another to the lungs, and a third, the gullet. 



In man, the windpipe is easily felt. It is a cartilaginous tube, the upper 

 part forming the voice box, or larynx (Adam's apple); directly dorsal to 

 this is the gullet. Food, in order to reach the gullet from the mouth cavity, 

 must pass over the glottis, the opening into the trachea. In the frog, the 

 glottis is normally a closed slit. In man, it is proportionately much wider. 

 When food is in the course of being swallowed, the upper part of the larynx, 

 called the epiglottis, forms a trap door over the opening. When the epi- 

 glottis is not closed, and food "goes down the wrong way," we choke 

 and the food is expelled by coughing. 



The Gullet,- or Esophagus. — In man this part of the food tube is much 

 longer proportionately than in the frog. Like the rest of the food tube 

 it is lined by soft and moist mucous membrane. The wall is made up of 

 two sets of muscles, — the inside ones running around the tube; the outer 

 band of muscle taking a longitudinal course. After food leaves the mouth 



