374 DIGESTION 



must be secreted by the only other glandular elements present, the 

 parietal or ' border ' cells. And, indeed, the glands in the fundus 

 of the frog's stomach, which are composed only of ovoid cells, whilst 

 secreting much acid, also form some pepsin, although far less than 

 the oesophageal glands. During winter sleep (in the marmot) the 

 production of hydrochloric acid in the parietal cells stops altogether, 

 while the chief cells continue to accumulate granules of pepsinogen. 



That some pepsin is secreted by the pyloric end of the stomach 

 is not difficult to prove. The secretion collected from the isolated 

 pyloric portion is, indeed, like the secretion of the Brunner's glands 

 in the duodenum, quite unable to digest protein until dilute hydro- 

 chloric acid is added. But this is because in both cases the juice as 

 it flows from the glands is slightly alkaline, and, as we have already 

 seen, pepsin only acts in the presence of an acid. The milk-curdling 

 action of these two juices also unfolds itself only when the secretions 

 are first acidulated, and later on again neutralized; in other words, 

 the ferments must be activated by the addition of an acid. In normal 

 digestion the pepsin of the (in itself) alkaline secretion of the pyloric 

 end of the stomach becomes a constituent of the acid gastric juice; 

 and it may perhaps be considered a morphological accident, so to 

 speak, that the oxyntic cells of the fundus should mingle their acid 

 products with the (presumedly) alkaline secretion of the chief cells 

 in the lumen of each gland- tube, instead of being massed as a 

 separate organ with a special duct. 



We are not without other examples of digestive juices fitted or 

 destined to act in a medium with an opposite reaction to their own. 

 The ' saliva ' of the cephalopod Octopus macropus, strongly acid 

 though it is, contains a proteolytic ferment which in vitro acts, like 

 trypsin, better in an alkaline than in an acid solution. And trypsin, 

 whose precursor is a constituent of the alkaline pancreatic juice, while 

 the enterokinase which activates it is a constituent of the alkaline 

 succus entericus, performs a part at least of its work in an acid 

 medium. 



Attempts made to demonstrate an acid reaction in the border cells 

 have hitherto failed. Harvey and Bensley on the basis of experiments 

 with dyes (cyanimin and neutral red), which give different colours 

 according to whether the reaction is acid, alkaline, or neutral, have 

 concluded that free acid exists only on the internal surface of the 

 stomach, or at most also in the necks of the glands. The parietal cells 

 they find alkaline. They suggest that these cells form in some way, 

 of course ultimately from the chlorides of the blood, a chloride of an 

 organic base which does not decompose so as to yield free hydrochloric 

 acid until it reaches the mouth of the gland. The nature of this decom- 

 position, if it occurs, is unknown. It may be mentioned, although only 

 as a matter of historical interest, that some observers have denied that 

 the acid is secreted in the depths of any cell from the chlorides of the 

 blood, and have asserted that it is formed at the surface of contact of 

 the stomach-wall with the gastric contents from the sodium chloride of 

 the food by an exchange of sodium ions (p. 422) for hydrogen ions from 



