CHAPTER IV. 

 DIGESTION. 



IN the last chapter we have described the manner in which 

 the interchange of gases between the tissues and the air is 

 carried out. We have now to consider the digestion and 

 absorption of the solid and liquid food, its further fate in 

 relation to the chemical changes or metabolism of the 

 tissues, and finally the excretion of the waste products by 

 other channels than the lungs. 



Logically, we ought to take metabolism after absorption 

 and before excretion, tracing the food through all its vicissi- 

 tudes from the moment when it enters the blood or lymph 

 till it is cast out as useless matter by the various excretory 

 organs. Unfortunately, however, the steps of the process 

 are as yet almost entirely hidden from us ; we know only 

 the beginning and the end. We can follow the food from 

 the time it enters the alimentary canal till it is taken up by 

 the tissues of absorption ; and we have really a fair know- 

 ledge of this part of its course. We can collect the end 

 products as they escape in the urine, or in the breath, or in 

 the sweat ; and our knowledge of them and of the manner 

 in which they are excreted is considerable. But of the 

 wonderful pathway by which the dead molecules of the food 

 mount up into life, and then descend again into death, we 

 catch only a glimpse here and there. Only the introduction 

 and the conclusion of the story of metabolism are at present 

 in our possession in fairly continuous and legible form. We 

 will read these before we try to decipher the handful of torn 

 leaves which represents the rest, 



