METABOLISM 221 



secretion, moreover, the product of the cell-metabolism is more unlike 

 the secreting protoplasm than in the case of growth. The former process 

 is more like manufacture, while the latter more resembles reproduction. 

 A difference that is only apparent is that the product in growth is "solid" 

 and in secretion usually liquid. Witness, however, the liquidity of the 

 tissues generally and the solidity of the grains of glycogen in the liver- 

 cells. So far as the metabolic changes occurring in the living unit are 

 concerned, we can state no important differences between growth and 

 secretion save that the former is largely ariabolism and the latter mostly 

 katabolism. As secretion is the chief function of all the epithelial and 

 lymphoid tissue of the organism, it has an organ of its own, but growth 

 is common to all the tissues. 



Broadly speaking, however, secretion too is a function of all protoplasm, 

 inasmuch as the process involves only the production of some material 

 substance by an organic tissue. Thus, one sees amebse (the individuals of 

 which consist each of "only a drop of protoplasm") surrounding proteid 

 food-particles, absorbing them intimately into the homogeneous colloidal 

 matter of that part of the body that is by chance concerned, and then 

 soon enclosing them by a food-vacuole filled with digestive juices secreted 

 apparently from that part of the protoplasm that happens to be imme- 

 diately about it. 



It must not be imagined that even in man the tissues in general have 

 lost their power of secretion. It appears, on the contrary, that every 

 tissue produces certain particular enzymes that in part control its own 

 special sort of metabolism. The substances needed widely or in large 

 amounts or in certain organs are produced in special epithelia, but every 

 true cell of the body seems to be the secretor of its own metabolic deter- 

 minants, as ameba obviously is. The details of this matter, all new 

 since knowledge of the internal secretions began to accumulate, are 

 still unknown, and will largely remain so until at least the chemical 

 nature of protoplasm is better learned. In this broad meaning of the 

 term secretion there is included both absorption and excretion, and 

 metabolism is evidently nearly its synonym. 



Underlying all the secretory and absorptive functions of the body is 

 the process which is known to physics as osmosis. The nature of this 

 series of events must be somewhat understood as a basis for compre- 

 hending what goes on in the protoplasm of the general tissues. 



OSMOSIS (from the Greek "pushing") is the passing or mixing of 

 liquids through membranes immersed in them. Abbe' Nolle t first 

 noticed the phenomenon in 1748. He observed that a bladder full 

 of alcohol immersed in water soon became overdistended with the water 

 that passed into it, the water pushing inward through the membrane 

 faster than the alcohol moved outward. It has much more recently 

 been shown by Pfeffer, working with plant-cells, that the pressure 

 exerted by any solution whose molecules do not dissociate into ions 

 is equal to the gaseous pressure which would be created by a like mass 

 of the dissolved substance vaporized and confined in a space equal to 



