life: its nature, origin, and maintenance SCHAFER, 513 



MAINTENANCE OF LIFE OF CELL AGGREGATE IN HIGHER ANIMALS. 



We have seen that in the simplest multicellular organisms, where 

 one cell of the aggregate differs but little from another, the condi- 

 tions for the maintenance of the life of the whole are nearly as simple 

 as those for individual cells. But the life of a cell aggregate such as 

 composes the bodies of the higher animals is maintained not only by 

 the conditions for the maintenance of the life of the individual cell 

 being kept favorable, but also by the coordination of the varied 

 activities of the cells which form the aggregate. Whereas in the 

 lowest Metazoa all cells of the aggregate are alike in structure and 

 function and perform and share everything in common, in higher 

 animals (and for that matter in the higher plants also) the cells have 

 become specialized, and each is only adapted for the performance of 

 a particular function. Thus the cells of the gastric glands are only 

 adapted for the secretion of gastric juice, the cells of the villi for the 

 absorption of digested matters from the intestine, the cells of the 

 Iddney for the removal of waste products and superfluous water from 

 the blood, those of the heart for pumping blood through the vessels. 

 Each of these cells has its individual life and performs its individual 

 functions. But unless there were some sort of cooperation and sub- 

 ordination to the needs of the body generally, there would be some- 

 times too little, sometimes too much, gastric juice secreted; some- 

 times too tardy, sometimes too rapid, an absorption from the intestine; 

 sometimes too little, sometimes too much, blood pumped into the 

 arteries, and so on. As the result of such lack of cooperation the 

 life of the whole would cease to be normal and would eventually 

 cease to be maintained. 



We have already seen what are the conditions which are favorable 

 for the maintenance of life of the individual cell, no matter where 

 situated. The principal condition is that it must be bathed by a 

 nutrient fluid of suitable and constant composition. In higher ani- 

 mals this fluid is the lymph, which bathes the tissue elements and is 

 itself constantly supplied with fresh nutriment and oxygen by the 

 blood. Some tissue cells are directly bathed by blood; and in inver- 

 tebrates, in which there is no special system of lymph vessels, all the 

 tissues arc thus nourished. All cells both take from and give to the 

 blood, but not the same materials or to an equal extent. Some, 

 such as the absorbing cells of the villi, almost exclusively give; 

 others, such as the cells of the renal tubules, almost exclusively take. 

 Nevertheless, the resultant of all the give and take throughout the 

 body serves to maintain the composition of the blood constant under 

 all circumstances. In this way the first condition of the maintenance 

 of tlie life of the aggregate is fulfill(Hl ])y insuring that the life of the 

 individual cells composing it is kept normal. 



