LIFE A^'D HEALTH OF CELLS. 227 



ing over them by ciliary action. If we imagine this sponge 

 placed in a vessel of water we will have no very unfair picture 

 of the body of a mammal. The little ma'^ses of protoplasm in 

 the sponge may be compared to the cells which torm our 

 bodies, the vessel to our skin, the water to the intercellular 

 fluid or lymph in which our tissues are bathed, and its surface 

 whereby oxygen is absorbed from the air to our lungs. Just 

 as the sponge cannot 'be said to live in air, so we do not really 

 live in air. We — that is, the tissues which compose our bodies 

 — live in what Claude Bernard* has termed an internal medium, 

 viz. the intercellular fluid which exudes from the blood-vessels, 

 gives nutriment and oxygen to the tissues, and returns again to 

 the general circulation by the lymphatics and veins. 



It will simplify our conception of this subject if we fix in our 

 mind's eye one little mass of protoplasm or cell, and consider 

 what changes will be produced in it by different conditions. 

 Any alteration in the amount of the nutrient fluid, or in its 

 composition, will necessarily produce a change in the nutrition 

 of the living matter to which it is supplied. If nutriment be 

 withdrawn, the cell will begin to burn away. If oxygen be 

 withheld,! or the products of waste be not removed, combustion 

 will cease, and the cell will die. If nutriment or oxygen be 

 supplied in insufficient quantity, or the products of waste only 

 partially removed, the cell may adapt itself to the altered cir- 

 cumstances, and its nutritive and functional processes go on in 

 the same way, but to a less extent than before ; or they may 

 become deranged — that is to say, the cell becomes diseased. 

 The limits within which the cell can adapt itself to changes in 

 nutrition are the limits of its health. The liigher animals, how- 

 ever, are no mere aggregation of cells, each nourishing itself 

 independently of the others ; for each cell has its own peculiar 

 function, each its special kind and amount of nourishment ; and 

 none must do either too much or too little work, none must have 

 too much or too little nourishment, or the nutrition and func- 

 tional activity of the body as a whole cannot be properly main- 

 tained. This delicate adjustment of the several parts to one 



* Bernard, Revue des Cotcrs Scientifiques, 1863-4, p. 278. 

 f Kulme, Vntersuchungen uher Froiop''as'n:a u. ContractUitdt, p. 53. 



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