CELL-CONTENTS AND CELL-WALLS. 47 



(i) Place some sections in a- watch-glass containing 

 either pepsin or trypsin, e.g. liquor pepticus ( 54) or 

 liquor pancreaticus ( 57) ; for comparison, place others 

 in a watch-glass of water. Set both in a warm place, and 

 note that the ground substance of the protein grains is 

 first dissolved, then the crystalloid more slowly, while the 

 limiting membrane of the vacuole occupied by the grain 

 persists. 



53. Digestion of Proteins. In mammals the pri- 

 mary proteins are acted upon by the gastric juice of the 

 stomach and by the pancreatic juice and the intestinal 

 juice (succus entericus) of the small intestine. The hy- 

 drolysis of the proteins is effected by the three enzymes, 

 pepsin, trypsin, and erepsin, present in these three juices 

 respectively. Pepsin hydrolyses the primary proteins into 

 peptones ; trypsin also acts upon the primary proteins, but 

 it carries the hydrolysing process further and changes the 

 peptones into amino-acids ; erepsin is peculiar in that it 

 does not attack the primary proteins, but is only capable 

 of acting upon proteoses and peptones, changing them into 

 amino-acids. 



For our purposes we may regard the proteolytic enzymes 

 of plants as corresponding to trypsin in their mode of 

 action. The vegetable trypsin called papain is obtainable 

 commercially, being used in medicine, but for the follow- 

 ing experiments we may use either pepsin prepared from 

 gastric juice, or preparations of pancreas containing the 

 enzymes diastase and lipase in addition to trypsin. 



54. Preparation of Pepsin. (a) Pepsin may be purchased in 

 the form of Beiiger's " liquor pepticus," or the dried pepsin (Bur- 

 roughs and Wellcome). (6) Artificial gastric juice may be prepared 

 as follows Get a fresh pig's stomach from the butcher, cut it open, 

 rinse with water, cut out the cardiac (broader) end, spread it out, 

 scrape the mucous (inner) surface, rub up the scrapings in a mortar 

 with sand, add water, rub up again, and filter ; the filtrate is to be 

 used. Another method is to scrape the mucous membrane off, dry 

 the scrapings between folds of blotting-paper, put them in a 

 bottle, and cover with glycerine which will dissolve out the pepsin ; 

 after a day, filter, and use the filtrate (glycerine extract), 



