GASTRIC DIGESTION. 



359 



of the oesophagus. Here, also, the secretion poured out by the glands 

 of this locality is alkaline, and yet contains pepsin, while a watery 

 infusion of this part of the frog's stomach is also highly peptic. On the 

 other hand, the hydrochloric acid is formed, according to Heidenhain, by 

 the parietal or associate cells, which are found only in the glands of the 

 fundus. This statement, also, has been proved b}' the determination 

 that the secretion alone of the fundus during active digestion has an 

 acid reaction, while that of the pylorus never acquires any acidity. So, 



Fig. 152.— Glands of the Fundus of the Stomach. (ITrldcnhain.) 



A and At, during fasting ; B, first stage of digestion : enlargement of the chief cells and commencing turbidity ; C and 



D, second stage of digestion: the chief cells become smaller in size and more and more turbid. 



again, in the frog these associate cells are found in the stomach, and not 

 in the tubular glands of the oesophagus, and it is known that in the 

 stomach alone free acid is formed. Again, in hibernating animals the 

 chief cells disappear, and in these animals the secretion of gastric juice 

 never acquires an acid reaction. The milk-curdling ferment is, probably, 

 also formed by the central cells, especially those of the pyloric extremity, 



