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 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, 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 absorp- 

 tion ; 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 consider- 

 able. But of the wonderful pathway by which the dead mole- 

 cules of the food mount up into life, and then descend again into 

 death, we catch only a glimpse here and there. Only the intro- 

 duction 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. 



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 nutri- 

 tion is cast out again at any part of the surface. 



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



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