30 FEEDS AND FEEDING 



teins, are digested and absorbed in the changes which these nutrients 

 undergo. 



55. The work of the digestive glands. The brilliant studies of the Rus- 

 sian physiologist, Pawlow* 3 and his associates, followed by others along 

 similar lines, have thrown much light upon the subjects of digestion, 

 appetite, and palatability. In order to study the processes of digestion, 

 operations such as the following were performed on many dogs: (1) 

 The ducts, or tubes, which deliver the saliva into the mouth were cut, 

 turned outward, and healed into the cut edges of the skin, so that when 

 saliva was secreted it poured out thru the opening and could be caught 

 in glass tubes attached to the dog's head. (2) The gullet, which carries 

 food from the mouth to the stomach, was cut across, led outward, and 

 healed in the skin at the throat, so that when food was swallowed it 

 would pass out at the severed end and fall back into the dish out of which 

 he was feeding. Food so eaten was called a ' ' false meal. ' ' In many cases 

 a dog with a gullet thus severed would chew and swallow the ' ' false 

 meal'* again and again with apparent satisfaction. (3) An opening was 

 made thru the side of a dog and into his stomach. On the healing of the 

 stomach wall with the cut in the skin, the investigator was enabled to 

 pass food directly into the stomach and study the processes of digestion 

 occurring within that organ. (4) A portion of the stomach was con- 

 stricted and made into a small separate chamber which likewise opened 

 out thru the side of the dog. Here the flow of juices coulcl be studied 

 independent of admixture with food placed in the other portion of the 

 stomach. (5) The small intestine was drawn to the side of the dog, and 

 an opening made in it the same as in the stomach. (6) The pancreatic 

 duct was cut and led outward, so that its secretion could likewise be 

 studied. The animals usually yielded readily to the operations and lived 

 comfortable lives, so that the results were normal. 



It was found that the sight, smell, or taste of food not only started 

 the flow of saliva in the mouth, but the gastric juice also began to pour 

 from the walls of the stomach in about five minutes, even when there was 

 no food in that organ. The gastric secretions which are brought forth 

 by the sight, taste, or smell of food were designated by Pawlow as ''psy- 

 chic secretions." For example, when a dog was given a false meal, and 

 the swallowed food fell out of the fistula, or opening, in the throat and 

 back into the dish out of which the dog was eating, the stomach would 

 nevertheless pour forth its fluids (psychic secretions), as tho the food had 

 reached it. The more eagerly the dog ate his false meal the greater was 

 the amount of gastric juice, and the richer it was in both acid and 

 pepsin. The gastric secretions were strongest and most abundant with 

 that food which was liked best, and food given in small portions called 

 forth stronger juices than when the whole ration was given at one time. 



These psychic secretions do not last long enough to explain the long 

 continued flow of gastric juice when a normal meal is eaten. 



8 The Work of the Digestive Glands. 



