96 HUMAN BIOLOGY 



peptone (P. B., 53). This chemical change is brought about 

 in our bodies to some extent by the gastric juice. 



132. Digestion of proteins. Optional laboratory demonstra- 

 tion. 



Materials: Boiled egg, powdered pepsin (which should be ob- 

 tained fresh or kept in a tightly stoppered bottle), hydrochloric 

 acid, water ; test tubes. Each of the following experiments should 

 be kept throughout the whole time as nearly as possible at the 

 temperature of the body (98.6 F.). 



A. To prove that protein requires digestion after it is swallowed. 



1. .Shave off with a knife and cut into the finest pieces possible a 



part of the white of a boiled egg (or better, grate the 

 egg). The solid constituents of egg are largely pro- 

 tein. Put into a test tube a small amount (about twice 

 the size of a pea) of this minced egg, add water, and 

 shake. Label the test tube No. 1, and allow the mix- 

 ture to stand for a day or two as nearly as possible at 

 a temperature of 98.6 F. (which is the normal tem- 

 perature of the interior of our bodies). 



a. Has all the egg been made liquid or digested by the 



water ? How do you know ? 



b. Pour off some of the clear liquid into a test tube, and add 



nitric acid and boil. Has any of the protein been 

 digested ? How do you know ? 



2. Into another test tube put the same amount of minced egg, 



add a spoonful or more of saliva. Label it test tube 

 No. 2. Shake and allow it to stand for a day or two 

 beside test tube No. 1. 



a. Is protein digested by saliva ? How do you know ? 



b. What do you therefore conclude in regard to the possibility 



of protein-digestion by the saliva ? 



B. To prove that gastric juice digests protein. 



1. Into a third test tube put a small amount of the minced egg. 

 Half fill the tube with water, add powdered pepsin to 



