242 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



conditions being the same, with the amount of the food to be digested. By 

 some means the apparatus is adjusted in this respect to work economically. 

 Different kinds of food produce secretions varying not only as regards quan- 

 tity, but also in their acidity and digestive action. The secretion produced 

 by bread, though less in quantity than that caused by meat, possesses a greater 

 digestive action. On a given diet the secretion will assume certain charac- 

 teristics, and Pawlow is convinced that further work will disclose the fact 

 that the secretion of the stomach is not caused normally by general stimuli 

 all affecting it alike, but by specific stimuli contained in the food or produced 

 during digestion, whose action is of such a kind as to produce the secretion 

 best adapted for the food ingested. 



One of the curves showing the effect of a mixed diet (milk, GOO cubic 

 centimeters ; meat, 100 grams ; bread, 100 grams) upon the gastric secretion, 

 as determined by Pawlow's method, is reproduced in Fig. 68. It will be 

 noticed that the secretion began shortly after the ingestion of the food (seven 

 minutes), and increased rapidly to a maximum that was reached in two hours. 

 After the second hour the flow decreased rapidly and. nearly uniformly to 

 about the tenth hour. The acidity rose slightly between the first and second 

 hours, and then fell gradually. The digestive power showed an increase 

 between the second and third hours. 



Histological Changes in the Gastric Glands during- Secretion. — The 

 cells of the gastric glands, especially the so-called chief-cells, show distinct 

 changes as the result of prolonged activity. Upon preserved specimens taken 

 from dogs fed at intervals of twenty-four hours, Heidenhain found that in the 

 fasting condition the chief-cells were large and clear, that during the first six 

 hours of digestion the chief-cells as well as the border-cells increased in size, 

 but that in a second period extending from the sixth to the fifteenth hour, the 

 chief-cells became gradually smaller, while the border-cells remained large or 

 even increased in size. After the fifteenth hour the chief-cells increased in 

 size, gradually passing back to the fasting condition (see Fig. 6*4). 



Langley ' has succeeded in following the changes in a more satisfactory 

 way by observations made directly upon the living gland. He finds that the 

 chief-cells in the fasting stage are charged with granules, and that during 

 digestion the granules are used up, disappearing first from the base of the 

 cell, which then becomes filled with a non-granular material. Observations 

 similar to those made upon the pancreas demonstrate that these granules 

 represent in all probability a preliminary material from which the gastric 

 enzymes are made during the act of secretion. The granules, therefore, as in 

 the other glands, may be spoken of as zymogen granules, the preliminary 

 material of the pepsin being known as pepsinogen and that of the rennin 

 sometimes as pexinogen. 



Glands of the Intestine. — At the very beginning of the intestine in the 

 immediate neighborhood of the pylorus is found a small area of mucous mem- 

 brane containing distinct tubular glands, known usually as the glands of 

 1 Journal <>j Physiology, 1880, vol. iii. p. 269. 



