356 



DIGESTION AND ABSORPTION 



cavity, called the pulp cavity, corresponds to the cavity containing marrow 

 in bones. In life it contains living material — the blood vessels, nerves, 

 and cells which build up the bony part of the tooth. The bulk of the 

 hard part of the tooth consists of a limy material called dentine. Outside 

 of this is a very hard substance called enamel; this substance, the hardest 

 in all the body, is thickest on the exposed surface or crown of the tooth. 

 Each tooth is held in its place in the jawbone by a thin layer of bony 

 substance called cement. 



Problem XL VI. How foods are chemically prepared for ab- 

 sorption into the hlood. {Laboratory Manual, Proh. XZ VI.) 



(a) In the mouth. 



(b) In the stomach. 



(o) In the small intestine. 



Salivary Glands. — We are all familiar with the substance 

 called saliva which acts as a lubricant in the mouth. Saliva is 

 manufactured in the cells of three pairs of glands which empty 

 into the mouth, and which are called, according to their position, 



the parotid (under the ear), the sub- 

 maxillary (under the jawbone), and 

 the sublingual (under the tongue). 

 Digestion of Starch. — If we col- 

 lect some saliva in a test tube, add 

 to it a little starch paste, place the 

 tube containing the mixtine for a 

 few minutes in tepid water, and then 

 test with Fehling's solution, we shall 

 find grape sugar present. Careful 

 tests of the starch paste and of the 

 saliva made separately will usually 

 show no grape sugar in either. 



If another test be made for grape 

 sugar, in a test tube containing 

 starch paste, saliva, and a few drops 

 of any weak acid, the starch will be found not to have changed. 

 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 



A B 



Experiment showing non-osmosis of 

 starch in tube A, and osmosis of 

 Bugar in tube B. 



