PEPTIC DIGESTION 55 



<^lls into the secretion, which is then discharged into the lumen of 

 the gland. The most important substance in a digestive secretion is 

 the ferment. In the case of a gastric juice this is pepsin. We 

 can trace an intermediate step in this process by the presence of the 

 granules. The granules are not, however, composed of pepsin, but 

 of a mother-substance which is readily converted into pepsin.. We 

 shall find a similar ferment precursor in the cells of the pancreas, 

 and the term zymogen is apphed to these ferment percursors. The 

 zjTnogen in the gastric ceUs is called pepsinogen. The rennet- 

 ferment or rennin that causes the curdling of milk is distinct from 

 pepsin, and is preceded by another zymogen ; it is, however, formed 

 by the same cells. 



The parietal ceUs undergo merely a change of size during secre- 

 tion ; at first they are somewhat enlarged, and after secretion they 

 shrink. They are also called oxyntic cells, because it is beheved 

 that they secrete the hydrochloric acid of the juice. Heidenhain 

 succeeded in making in one dog a cnl-de-sac of the fundus, in another 

 of the pyloric region of the stomach; the former secreted a juice 

 containing both acid and pepsin ; the latter, parietal cells being 

 absent, secreted a viscid alkaline juice containing pepsin. The for- 

 mation of a free acid from the alkaline blood and lymph is an 

 important but puzzling problem. There is no doubt that it is formed 

 from the chlorides of the blood and lymph, and of the many theories 

 advanced as to how this is done, Maly's is, on the whole, the most 

 satisfactory. He considers that the acid originates by the interac- 

 don of the calcium chloride with the disodium hydrogen phosphate 

 of the blood, thus : — 



2Na2HP04 -I- 3CaGl2=Ca3(P04)2 -I- 4NaCl + 2HC1 



[disodium [calcium [calcium [sodium [hydro- 



hydrogen chloride] pho^hate] chloride] chloric 



phosphate] acid] 



or more simply by the interaction of sodium chloride and sodiimi di- 

 hydrogen phosphate, as is shown in the following equation : — 



NaH.,P04 -I- NaCl=Na.2HP04 -f HCl 



[sodium di- [sodium [disodium [hydro- 



hydrogen chloride] hydrogen chloric 



phosphate] phosphate] acid] 



The sodium dihydrogen phosphate in the above equation is pro- 

 bably derived from the interaction of the disodium hydrogen phos- 

 phate and the carbonic acid of the blood, thus : — 



Na.,HP04 -f- COo -|-H20=NaHC03 -I- NaHoP04 



But, as Professor Gamgee has pointed out, these reactions can 



