SECRETION. 



241 



to the intensity of the appetite or enjoyment of the food, and may last for 

 several hours even though the aetnal period of eating has been short (five min- 

 utes). It is this secretion that first acts upon the food received into the stomach. 

 Later its action is supplemented by an augmented secretion, caused by stimuli 

 of a chemical nature originating in the food ingested. Some foods contain 

 substances ready formed that are capable of acting in this way. Investigation 

 of various articles of diet showed that meat extracts, juices, and soups contain 

 these substances in largest amounts. Milk and aqueous solutions of gelatin 

 act in the same way, although less powerfully. Water also, if in sufficient 

 quantity, acts as a direct stimu- 

 lant. Other common articles 

 of food, such as bread or white 

 of egg, do not contain these 

 stimulating substances. Food 

 of the latter character, when 

 introduced directly into a dog's 

 stomach through a fistula, pro- 

 vokes not a drop of secretion 

 and undergoes no digestion, 

 if it has been introduced in 

 such a way as to avoid arous- 

 ing the psychical secretion, as, 

 for instance, at times when the 

 animal is dozing. If, how- 

 ever, this latter class of foods 

 undergo digestion, as would 

 happen in normal feeding in 

 consequence of the action of 

 the " psychical secretion," sub- 

 stances capable of stimulating 

 the stomach to secretion are 

 developed, and their action 

 keeps up the flow of secretion 

 after the effect of the psychical 

 factor has become weakened. 

 The nature of these chemical 

 stimuli remains entirely undetermined. Pawlow's first statement that pep- 

 tone constituted at least one member of this group lie now finds is erroneous. 

 It is assumed that these substances act through the secretory nerves, and it 

 has been shown also that other substances may have the contrary effeel of 

 retarding or inhibiting the gastric secretion. This has been proved tor fats 

 at least. Oils of various kinds decrease the secretion of gastric juice, while 

 they augment the pancreatic secretion. Another mosl suggestive result of 

 Pawlow's work is the proof that the quantity and characteristics of the secre- 

 tion vary with the food. Apparently the quantity of the secretion varies, other 



Vol. I.— 16 



Fig. 63.— Diagram showing the variation in quantity of 

 gastric secretion in the dog after a mixed meal: also the 

 variations In acidity and In digestive power (after Ehigine). 



