CHAPTER V. 



DIGESTION. 



HAVING now seen in outline what the food of the body is and should 

 be like, our next search is in what manner it becomes made into tissue- 

 protoplasm which is alive again. Food, as we have seen, is largely either 

 animal or vegetable. Substances of these origins are, by definition, forms 

 of protoplasm. The present chapter, then, deals with that portion of the 

 reanimation of food called digestion. 



The whole process by which food proper is converted into tissue or 

 made a source of energy to the organism may conveniently be divided 

 for purposes of description into four stages, which we shall discuss under 

 the arbitrary names digestion, absorption, nutrition, and excretion. The 

 last process, excretion, is quite as essential as the others, for most of the 

 products of katabolism are of such a nature that unless promptly removed, 

 they quickly bring about protoplasmic and organic death. In our sketch 

 of protoplasm we have already seen an outline of the general nature of 

 this extensive vital function of nutrition. Here we extend it and apply 

 it to man. So great is the complexity of animal function, whether taken 

 as a whole, or in any of its parts, and so interdependent are its various 

 portions, that sometimes, in fact, there is no such sharp discrimination 

 between functions as, for convenience of description and ease of learning, 

 we describe. We find an example of this circumstance, so common in 

 science, in the latter limit of "digestion." Arbitrarily we shall leave the 

 nutritive materials in this chapter when they are all ready to be absorbed 

 into the complex circulation of the blood and lymph. It is the work of 

 digestion proper to prepare the food for absorption. When, however, one 

 considers that part of the work of digestion is apparently done as the 

 nutritive substances pass through the thin wall of the intestine, and that 

 in the villi both processes may take place, it is clear that the two subjects 

 are much less sharply discriminated by the organism than by the ana- 

 lyzing mind describing them. This general interdependence of organic 

 functions, this perfect unification, more or less evident, of the structural 

 and functional multitude making up the individual, is an important 

 matter for anyone to remember who would understand man in any way 

 adequately. Only so, indeed, will his view be broad enough and accurate 

 enough to rightly interpret things. We encounter repeatedly reminders 

 of this principle that description implies a misleading separateness in 

 structure and function when in reality the process is often a greatly 

 involved continuity. 



As was seen to be the case with the function of respiration, digestion is 



