LIFE: ITS NATURE, ORIGIN AND MAINTENANCE 25 



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 



The maintenance of bei k t f avou rable, but also by the co-ordination of 



the life of the cell- , 



aggregate in the the varied activities of the cells which form the aggre- 



higher animals. g a t e . Whereas in the lowest Metazoa all cells of the 



mechanisms. 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 special- 

 ised, 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 kidney 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 

 co-operation and subordination to the needs of the body generally, there 

 would be sometimes too little, sometimes too much gastric juice secreted ; 

 sometimes too tardy, sometimes too rapid an absorption from the intes- 

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

 arteries, and so on. As the result of such lack of co-operation 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 favourable 

 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 animals 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 invertebrates, in which there is no special 

 system of lymph-vessels, all the tissues are 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 circum- 

 stances. In this way the first condition of the maintenance of the life of 

 the aggregate is fulfilled by insuring that the life of the individual cells 

 composing it is kept normal. 



The second essential condition for the maintenance of life of the cell- 

 aggregate is the co-ordination of its parts and the due regulation of their 

 activity, so that they may work together for the benefit of the whole. 

 In the animal body this is effected in two ways : first, through 

 the nervous system ; and second, by the action of specific chemical sub- 

 stances which are formed in certain organs and carried by the blood to 



