CHAPTER LVII 



THE BIOCHEMICAL PROCESSES OF DIGESTION 



In a book designed primarily for clinical workers, it would be out of 

 place to enter into details concerning the biochemical processes taking 

 place during the digestive process. There is, however, a certain amount 

 of fundamental knowledge which it is essential that we should consider. 

 In the first place it should be borne in mind thait in the digestion of 

 carbohydrates and proteins, various intermediate stages are passed 

 through before the final absorption products are formed. The highly 

 complex molecule of which protein, for example, is composed, is first 

 of all broken down into several smaller but still highly complex mole- 

 cules, each of which then undergoes further disruption, until ultimately 

 the amino acids are set free. Certain enzymes, such as trypsin, can 

 carry this process from the beginning through the greater part of its 

 course without the assistance of other enzymes, but in the natural proc- 

 ess of digestion, as it occurs in the gastrointestinal tract, the different 

 stages of the disruption are controlled by different enzymes. One enzyme 

 prepares the food for action by the next. This interdependence of their 

 actions demands that some provision should be made whereby each en- 

 zyme is secreted at the proper time; that is, when the foodstuff has al- 

 ready been prepared for its action by that of its predecessor. Thus, it 

 would be useless after food is taken for the gastric and pancreatic 

 juices to be secreted at the same time. Instead, the gastric juice is 

 secreted first, and the pancreatic only after the food has been prepared 

 for its action. This correlation in function we have already seen to be 

 dependent largely on the action of hormones. 



DIGESTION IN THE STOMACH 



The gastric juice contains two important digestive agencies: (1) the 

 enzyme, pepsin, and (2) hydrochloric acid. It is particularly in juices 

 secreted in the cardiac end of the stomach that these two substances are 

 found present; towards the pyloric end the hydrochloric acid entirely 

 disappears, and the pepsin content becomes distinctly less. 



515 



