CHAPTER VI. 



THE MECHANISM OF DIGESTION. 



THE acts of digestion may be divided into mechanical and 

 chemical processes. Under the mechanical processes come the ar- 

 rangements for the subdivision, onward movement, and general 

 mixture of the food. The chief objects of the chemical changes 

 may be said to be the change from the insoluble to the soluble 

 form of certain kinds of food-stuffs (starch, proteids) and the 

 finer subdivision of others, such as fats, which do not dissolve in 

 the intestinal or body juices. 



Attention has already been called to the fact that there are 

 different kinds of contracting textures, and that they are capable 

 of different kinds of motion, some slow and steady, some rhyth- 

 mical, some sharp, short and sudden. It must also be remem- 

 bered that the more energetic and sudden the motions are, the 

 more marked becomes the differentiation of the tissue. Thus the 

 active, quick-contracting skeletal muscles and the rhythmically 

 acting heart, are made up of tissue which is very distinct in 

 structure and in mode of action from that of the contracting cells 

 composed of ordinary protoplasm, while in the slowly moving 

 internal organs we meet tissue elements which, in different ani- 

 mals, show many stages of gradation between simple, indifferen- 

 tiated protoplasm and the special striated muscle tissue. 



It is necessary that in the first stages of alimentation the mo- 

 tions should be quick and energetic; so the mouth, pharynx, and 

 upper part of the oesophagus are supplied with striated muscle 

 tissue, which differs in function and structure from that of the 

 rest of the alimentary canal. In the stomach and intestines the 

 slower and more gradual kinds of motion are required, and here 

 we find a good example of non-striated muscle tissue. 



