The Process of Life. 27 



The materials thus absorbed are either taken up into 

 the blood-stream or pass into a separate system of vessels 

 called laeteals. All the blood which comes away from the 

 alimentary canal passes into the liver, and there undergoes 

 a good deal of elaboration in that great chemical labora- 

 tory of the body. The fluid in the laeteals passes through 

 lymphatic glands, in which it too undergoes some elabora- 

 tion before it passes into the blood-stream by a large vessel 

 or duct. 



Thus the blood, which we have seen to be enriched with 

 oxygen in the lungs, is also enriched with prepared nutri- 

 tive material through the processes of digestion and absorp- 

 tion in the alimentary organs and elaboration in the liver 

 and lymphatic glands. 



Here let us again notice that the details of the process 

 of nutrition vary very much in different forms of life. In 

 some mammals the organs of digestion are specially fitted 

 to deal with a flesh diet ; in others they are suited for a 

 diet of herbs. In the graminivorous birds the grain is 

 swallowed whole, and pounded up in the gizzard. The 

 leech swallows nothing but blood. The earthworm pours 

 out a secretion on the leaves, by which they are partially 

 digested before they enter the body. Many parasitic 

 organisms have no digestive canal, the nutritive juices of 

 their host being absorbed by the general external surface 

 of the body. But the essential life-process is in all cases 

 the same the absorption of nutritive matter to be supplied 

 to the cell or cells of which the organism is built up. 



Thus in the mammal the blood, enriched with oxygen 

 in the lungs, and enriched also with nutritive fluids, is 

 brought, in the course of its circulation, into direct or 

 indirect contact with all the myriads of living cells in the 

 body. 



In the first place, the material thus supplied is utilized 

 for and ministers to the growth of the organs and tissues. 

 This growth is effected by the multiplication of the con- 

 stituent cells. The cells themselves have a very limited 

 power of growth. But, especially in the early stages of 



