478 



APPLIED BIOLOGY 



cerning the effect of pepsin and acid on the particular protein used 

 in these experiments ? 



If time permits, other proteins may be tested in the same way. 

 Try a hard-boiled egg grated or minced into fine particles, shreds of 

 lean meat, or cheese. Also, the acid may be omitted, in order to 

 show that pepsin alone will not digest proteins. 



(D) The action of the rennin may be illustrated by adding some 

 commercial liquid rennet or junket tablets to milk. Place some of 

 the curds on filter-paper, wash with water, place in a test-tube with 

 some water, add strong nitric acid, heat. What nutrient is abundant 

 in the curds ? Pepsin and acid will digest them. 



Summarizing, gastric juice can digest proteins, coagulate 

 the proteins of milk, and then digest them, and digest a 



small amount of fat ; but 

 it does not digest starch. 

 Fat meat breaks up ex- 

 tensively when kept for 

 some hours in gastric 

 juice, but only the pro- 

 tein walls of the cells have 

 been digested, allowing 

 the contained oil to es- 

 cape undigested. 



Fat-cells. (D) Place a 

 small piece of fat meat or 

 suet on an object-slide in a 

 drop of glycerine, and with 

 a pair of needles tease the 

 meat into as small particles 

 as possible. Put on cover- 

 glass, and examine with low 

 and high power of a micro- 

 scope. Note the spherical 

 cells filled with fat, which 



FIG. 158. Part of a cross section of intes- 

 tine of a mammal, x, marks center of 

 cavity of intestine ; g, intestinal gland 

 between villi ; /, lymph gland ; v, villus ; 

 ra, muscle layers, the outer being longi- 

 tudinal and the inner circular in arrange- 

 ment of fibers (cells) ; 6, blood-vessels ; 

 ly, a lacteal or lymph-vessel in center 

 of a villus (v) ; a, peritoneum. Compare 

 with Fig. 19 of frog. (From Wieder- 

 sheim.~) 



may be in the form of needle- 

 shaped crystals at the ordinary temperatures. If a small piece of fat 

 meat be soaked for a time in ether to dissolve the fat, and then placed 

 iniodine-eosm or other stains, the nuclei of the fat-cells will be stained. 



