LESSON Yi 



THE FUNCTION OF ALIMENTATION 



Part 1. — Digestion and Absorption 



1. "Waste made good by Food.— We explained in the 

 first Lesson that a living active man is always expending 

 energy in the form of the mechanical (muscular) work he 

 performs and of the heat he gives oft' by his skin and 

 lungs. Further, we pointed out that the source from 

 which the energy is derived lies in that constant oxida- 

 tional breaking down of the tissues which results from 

 their being supplied with oxygen, introduced into the 

 body by the lungs. And further, it was shown that the 

 above processes result in a waste of substance corre- 

 sponding exactly to the amount of energy expended. If 

 the man's activity is to continue from day to day, this 

 continual waste of sul)stance must be made good. Now 

 the only channel, except the lungs, \>y which altogether 

 new material is introduced into the body, is the alimen- 

 tary canal, and we may use the word alimentation to 

 denote the sum total of its operations in this connection. 

 These fall naturally under three heads, viz the introduc- 

 tion of food as new material ; the reduction of this food 

 by digestion to a condition such that it can pass through 

 the delicate structures which form the walls of the vessels 

 of the alimentary canal ; and absorption, or the processes 

 by wiiich the digested material is jiassed from the cavity 

 of the canal into the blood-vessels and lymphatics by 



