DIGESTION AND ABSORPTION 1 75 



a lo per cent water solution of commercial pepsin and an approxi- 

 mate 0.5 per cent solution of hydrochloric acid made by adding 1.5 

 c.c. concentrated HCl to 100 c.c. water, (c) Take four test tubes and 

 place in (i) 5 c.c. pepsin solution + 5 c.c. water (neutral). In tube 

 (2) 5 c.c. pepsin solution + 5 c.c. 0.5 per cent hydrochloric acid solu- 

 tion (approximately 0.25 per cent acid). In tube (3) 5 c.c. pepsin 

 solution + 5 c.c. of 0.5 per cent sodium carbonate solution. Into 

 each test tube now place a test section of coagulated egg albumen. 

 Put the tubes in an incubator at 38° C. or in a beaker of water kept 

 at 38°-40° C, and let them remain for an hour or more, noting any 

 changes in the albumen. Digestion takes place only in tube (2). 

 (d) If desired make similar tests diluting the acid so as to make solu- 

 tions containing 0.2 per cent acid, o.i per cent acid and another to 

 which i.o c.c. of concentrated hydrochloric acid has been added. 

 Note most favorable acidity, (e) Repeat also with three tubes made 

 as tube (2) in (6), keeping one at room temperature, one at 38°-4o° C, 

 one heated to 100° C. in boiling water, and another immersed in cold 

 water io°-i5° C. Note at which temperature digestion is most 

 active. Warm the first and last of these tubes to 38°-40° C. and note 

 result. Cool the one heated to boihng down to 38°-4o° C. and note 

 result. Low temperatures inhibit the action of pepsin but do not 

 destroy the enzyme. Boiling temperature destroys the enzyme 

 completely. 



(2) Action of Rennin. Use a commercial rennet obtained from a 

 dairy laboratory or dissolve a rennet tablet in 10-20 c.c. water. Warm 

 100 c.c. of whole milk to 38°-40° C. and add 2 or 3 drops of the rennet 

 solution. Mix slightly and then allow to stand. Note coagulation 

 of milk casein. When a good coagulation has formed, break it up 

 with a glass rod; decant off the whey and collect the casein on a 

 folded filter paper. Press as dry as possible and test a portion of the 

 casein for protein by Millon's or Xantho-proteic test (Experiment 

 XVni, 4, a. b). Compare with Experiment XXIX. 



The Small Intestine 



The most important protein digestion takes place in the 

 small intestine by the action of several enzymes present in 

 two digestive juices, viz. pancreatic juice, secreted by the 

 pancreas, and intestinal juice, secreted by the cells of the mucous 



