DIGESTION AND RESPIRATION 525 



was left intact. If the vagus nerve was cut, this secretion did not occur. 

 This experiment proved that the control of gastric secretion was at 

 least partly nervous. Subsequently it was discovered that if the vagus 

 was cut but food was permitted to reach the stomach, a considerable 

 flow of gastric juice was produced. Obviously the vagus nerve is not the 

 only means of stimulating the gastric glands. Further investigation 

 revealed that when partly digested food reaches the pyloric region of 

 the stomach, certain of the mucosal cells produce the hormone gastrin, 

 which is absorbed into the blood through the stomach wall and ulti- 

 mately reaches the gastric glands, stimulating them to secrete. When 

 food, especially fats, enters the duodenum, the duodenal mucosa pro- 

 duces the hormone enterogastrone which, on reaching the stomach, 

 inhibits the secretion of the gastric glands and slows down the churn- 

 ing action of the stomach. The rate of digestion in the stomach is 

 reduced or stopped. This not only helps to prevent the stomach from 

 digesting its own lining, but also enables fatty foods to stay for a longer 

 period in the duodenum where they can be acted on by bile salts and 

 lipase. 



The first hormone to be discovered was secretin, which initiates 

 pancreatic secretion. In 1902 Bayliss and Starling were investigating the 

 current belief that the secretion of pancreatic juice was under nervous 

 control. They found that the pancreas secreted its juice when acid food 

 entered the small intestine even though the nerves to and from the in- 

 testine were cut. A stimulant of some sort apparently traveled in the 

 blood. The injection of acids into the blood stream had no effect, so 

 they reasoned that some stimulating principle must be produced by the 

 intestinal mucosa upon exposure to acid foods. When they injected ex- 

 tracts of such a mucosa into the circulatory system the pancreas secreted. 



Secretin has a side effect on the liver for it increases slightly the 

 rate of bile secretion. However, another hormone, cholecystokinin, 

 which is also produced by the duodenal mucosa when acid food is in 

 the duodenum, is largely responsible for causing the gall bladder to 

 contract and release the bile. Vagal stimulation also plays a role in 

 the release of bile. 



223. Use of Absorbed Materials 



The absorbed products may be used as raw materials for the syn- 

 thesis of the components of protoplasm and as a source of energy to 

 stoke the cellular fires, or they may be stored for later use. The energy 

 requirements of a young adult man vary from 1600 to 6000 or more 

 Calories a day, depending on whether he is at complete rest, not even 

 digesting foods, or doing heavy physical work. A person leading a 

 rather sedentary life requires 2500 to 3000 Calories a day. All kinds of 

 food yield energy, when metabolized, but not to the same extent. When 

 burned completely in a calorimeter, one gram of carbohydrate or 

 protein yields about 4 Calories, and one gram of fat 9.5 Calories. 

 Though carbohydrates do not contain as many Calories per gram as 

 fats, they constitute the major body fuel for most people. Normally 



