96 BtlMAN 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 sohd 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 aU 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 thai 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 



