DIGESTION. 77 



pepsin through the influence of the acid. To this intermediate 

 substance has been given the name pepsinogen. 



The large oval or parietal cells also contain granules which are 

 very few in number and small in size, though quite distinct. These 

 are very constant in quantity, the cells showing mainly differences in 

 size. Thus, before secretion, they are swollen; afterward, shrunken. 

 They are frequently termed oxyniic, as they are thought to secrete 

 hydrochloric acid, one of the essential compounds of the gastric secre- 

 tion. The exact process, however, is still shrouded in mystery. It 

 is thought to result from a simple process of diffusion in the parietal 

 cells of chlorides taken from the blood, for during secretion the 

 quantity of chlorides leaving the blood through the kidneys is dimin- 

 ished. Maly's theory with regard to this is very satisfactory. In it 

 he claims that the acid originates by the interaction of the calcium 

 chloride with the disodium hydrogen phosphate of the blood. The 

 interaction is simplified by the following equation of Maly's: 



2Na 2 HPO, + 3CaCl 2 = Ca 3 (P0 4 ) 2 + 4NaCl + 2HC1 



Disodinm Calcium Calcium Sodium Hydro- 



hydrogen chloride. phosphate. chloride. chloric 



phosphate. acid. 



The stimulus to the secretion of HC1 is the presence of free 

 chlorine ions on the inner side of the stomach's glands. If chlorine 

 ions are absent in the stomach then no HC1 is formed. If the ani- 

 mal be fed on bromides instead of chlorides, then hydrobromic acid 

 is formed in place of HC1. The glands of the stomach do not permit 

 chlorine ions to go through them, whilst free hydrogen ions which 

 exist in the blood go through the glands into the stomach and HC1 

 is formed. 



Formed in the central cells is another zymogen than pepsinogen, 

 which, when mixed with acid, produces an enzyme, or ferment, known 

 as rennin. This ferment has the power to coagulate milk, forming 

 casein. Eennin is found wherever pepsin is manufactured, although 

 distinctly different in character and action. 



The fluid is not poured out at the same rate from the beginning 

 to the end of digestion. The Mett method of preparing the proteid is 

 to fill a glass tube, one to two millimeters in diameter, with egg- 

 albumin and coagulate it at 95 C. The tube is then cut into small 

 pieces and placed in 1 or 2 cubic centimeters of the juice to be investi- 

 gated. The law of Schuetz is as follows : the quantity of pepsin in 

 the compared liquids is proportionate to the square of the rapidity of 

 digestion; that is, the square of the column of proteid in a Mett tube 



