CHAPTER VI 

 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 lung. 



Logically, we ought to take metabolism after absorption and 

 before excretion, tracing the food through all its vicissitudes 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, 

 many of the intermediate steps of the process are as yet hidden from 

 us ; we know best 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 knowledge 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. 



SECTION I. PRELIMINARY ANATOMICAL AND CHEMICAL DATA. 



Comparative. In the lowest kinds of animals, such as the amoeba, 

 there is neither mouth, nor alimentary canal, nor anus: the food, 

 wrapped round by pseudopodia, is taken in at any part of the animal 

 with which it happens to come in contact. A vacuole is formed around 

 it. Acid is secreted into the vacuole, the food is digested within the 

 cell-substance, and the part of it which is useless for nutrition is cast 

 out again at any part of the surface. 



Coming a little higher, we find in the Ccelenterates a mouth and 

 alimentary tube, which opens into the body-cavity, where a certain 



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