460 



ORGAN SYSTEMS OF MAN 



food directly into the stomach without the 

 dog seeing or smehing it, about one-half of 

 the normal flow of gastric juice took place. 

 While the flow in this case was somewhat 

 reduced by cutting the nerves to the stom- 

 ach, a considerable secretory activity contin- 

 ued. This could mean only that there must 

 be a hormone produced by the lining of the 

 stomach which circulates in the blood and 

 stimulates the gastric glands, at least in 

 part, to secrete. This hormone, gastrin, has 

 since been identified. With this type of ex- 

 periment Pavlov established the principle 

 of "conditioned reflex" which is the basis 

 for much of modern psychology. 



The stomach contents are very acidic 

 (pH 1.2-0.3) because of the high concen- 

 tration of hydrochloric acid in the gastric 

 juice. This is apparent to anyone who has 

 eaten too heartily and sometime later com- 

 plains of a "sour" stomach. The same taste 

 is even more evident when food is regurgi- 

 tated. The acid functions in providing a 

 favorable medium in which the enzyme, 

 pepsin, can do its best work, and it prob- 

 ably has a bactericidal effect on detrimental 

 bacteria which may be taken in with the 

 food. 



When pepsin leaves the gastric glands it 

 is in the inactive state, called pepsinogen. 

 When this substance comes in contact with 

 an acid medium, it is transformed into pep- 

 sin and is then able to attack the large pro- 

 tein molecules, breaking them down into 

 smaller ones. Pepsin is called a protease 

 (the suffix ase is used to identify an en- 

 zyme) because it breaks proteins down 

 into proteoses and peptones which are sol- 

 uble. The reaction may be expressed thus: 



pepsin 



Protein -|- water > proteoses and peptones 



Pepsin, like all digestive enzymes, functions 

 by adding water ( hydrolysis ) to the protein 

 molecules, thus breaking them into small 

 molecules. This is the initial stage of pro- 

 tein digestion. The ultimate goal is the 

 amino acid stage, but that must wait until 

 the food reaches the small intestine. 



Mammals possess another enzyme, ren- 

 nin, in their gastric juice which has a very 

 specific action, namely, the curdling of milk. 

 This has been evolved because the young 

 mammal depends on milk for the early part 

 of its post-natal existence, and were there 

 no curdling of the milk in its stomach, much 

 of the protein (caseinogen) would pass far 

 into the intestine, just as any other fluid 

 does, before protein digestion could start. 

 Rennin, therefore, converts the soluble pro- 

 tein caseinogen into insoluble casein, which 

 permits its normal digestion in the stomach 

 and later in the small intestine. 



Like other parts of the body, the stomach 

 is subject to many ills but it seems to suffer 

 especially from emotional strain. In some 

 people happiness seems to center around 

 the contentment of their stomachs. Every- 

 one is familiar with the difficulty experi- 

 enced in eating immediately following the 

 reception of good or bad news. This is ow- 

 ing to stimulation coming through the auto- 

 nomic nervous system. Continued emotional 

 strife can even produce organic damage, 

 such as erosion of small areas of the stom- 

 ach lining ( gastric ulcers ) . Frequently this 

 malady clears up "miraculously" when the 

 emotional strain is removed. In stubborn 

 cases of ulcers or cancer, parts, or even the 

 entire stomach in extreme cases, may be 

 successfully removed. This type of surgeiy 

 has progressed remarkably in the last two 

 decades but, needless to say, not to the 

 stage where a goat's stomach can replace 

 that of man — contrary to an idea staunchly 

 believed by many people. 



THE SMALL INTESTINE 



The small intestine is a narrow tube 

 with a length of over 23 feet which extends 

 from the stomach to the colon or large in- 

 testine. Although it is possible to get along 

 without a part or all of the stomach, it is 

 absolutely necessary that the small intestine 

 remains essentially intact, because it is here 

 that all digestion is completed and absorp- 



