194 



How a 



part of the last century was fortunate 

 enough to see all this happening in a 

 human stomach. At an army post in 

 Michigan a trapper, Alexis St. Martin, 

 was shot through the stomach. It was a 

 large wound, right through the wall of 

 the stomach, large enough to put a fist 

 into. The man recovered but the hole 

 never completely closed and there was 

 left an opening through the body wall 

 between the ribs into the stomach. The 

 hole was so large that Beaumont could 

 look into the stomach. He could pour 

 water through the opening or introduce 

 food. He could suspend food in the 

 stomach for a certain length of time and 

 then recover it; he could siphon out di- 

 gested foods; he could measure the con- 

 tents of the stomach and could test the 

 juice chemically. While Beaumont was 

 performing all these experiments St. 

 Martin made his living by chopping 

 trees. 



Because the composition of gastric 

 juice is known, a rather good imitation 

 of it can be made in the laboratory. You 

 will be interested in trying the effect of 

 imitation gastric juice on the various 

 food substances. See Exercises 12, 13, 

 14, and 15. 



Gastric juice, like salivary juice, is 

 largely water. In it are hydrochloric (hy- 

 dro-klor'ric) acid, and several enzymes. 

 One of the enzymes, pepsin, changes in- 

 soluble protein, a very large molecule, 

 into the somewhat smaller molecules of a 

 substance called peptone. Peptones are 

 intermediate products of digestion. They 

 cannot be used by the body, nor will they 

 diffuse through the walls of the intestine. 

 Another enzyme, remiin, curdles milk 

 and helps in its later digestion. The third, 



Complex Animal Uses Food unit iv 



a fat-splitting enzyme, is of very little 

 importance in an adult. The hydrochloric 

 acid is not an enzyme. But it is of great 

 importance in digestion in at least two 

 ways. Unless it is present, protein is not 

 digested by pepsin. And besides, hydro- 

 chloric acid reacts with a number of 

 insoluble minerals, calcium phosphate 

 among others, producing soluble min- 

 erals. 



Scientists have long wondered why 

 gastric juice which digests proteins does 

 not also digest the cells of the stomach 

 since protoplasm is made up largely of 

 proteins. No satisfactory answer has been 

 found. Some protection to the lining of 

 the stomach is probably provided by the 

 mucus secreted in large amounts by some 

 of the hning cells. This mucus spreads 

 itself over the inside of the stomach. 



(Optional) Tissues making up the stom- 

 ach. The organs of the digestive tube are 

 complex organs composed of a variety 

 of tissues. You will find it helpful at 

 this point to review the section on tis- 

 sues in Problem 3 of Unit II. On the 

 outside of the stomach is a form of 

 epithelium, serous membrane. This thin 

 and very smooth, moist epithelial tissue 

 covers all the internal organs and lines 

 the body cavity. As the organs slide over 

 each other or against the inside of the 

 cavity there is little friction. The serous 

 membrane covers three distinct layers of 

 involuntary muscle. In one layer the 

 muscle fibers run lengthwise, in another 

 they are arranged in a circular fashion 

 around the stomach, and in the third 

 they are diagonal. In between these mus- 

 cle fibers there are fibers of connective 

 tissue which are always found in con- 

 junction with muscle cells. Lining the 



