486 SCIENCE PROGRESS 



to a development of heat. Heat so produced is to be traced 

 finally to the combustion of food material as surely as that 

 directly produced by the occurrence of concomitant chemical 

 change. 



So far I have done no more than consider the possibility 

 that physical state and change may be of importance in 

 intracellular change. Let us consider a case where they must 

 of necessity be introduced — as for example in the secretion of 

 water as sweat at a point upon the surface of the skin. Below 

 this surface there extends a long tubular pocket lined by 

 epithelial cells. These cells are suddenly aroused from a state 

 of rest by the arrival of a nervous impulse dispatched from the 

 central nervous system ; after a short delay water wells from 

 the mouth of the pocket on to the skin. Whence does this 

 water come ? Certainly not from the side links of any "biogen 

 molecule" within the cell. The water has indubitably been 

 drawn through the cell from the solutions bathing its internal 

 surface. The cell has acted as a pump in which valves give 

 a direction to the work performed. Now how is this to be 

 dealt with if the physical arrangements of the materials forming 

 the cell are not to be placed in the foreground as matters of 

 importance ? Discuss these arrangements and it at least 

 becomes possible to conceive a line of experimental approach 

 to the phenomenon. 



Let us for a moment suppose that, as is most probable, the 

 internal and external surfaces have characters of a different kind. 

 In some epithelial cells such differences can be seen. In all, 

 even including the nerve-cell, such differences must be inferred. 

 Let us suppose that this cell, set upon the surface of the body, is 

 in the habit of passing some of its waste products, as for example 

 carbonic acid, not back into the solutions of the body, but out 

 on to its external surface. What more probable ? But then, if 

 so, we have only to imagine that our exciting nervous impulse 

 quickens up the chemical changes occurring within the cell and 

 so gives rise to the production of carbonic acid. These new 

 molecules, added to the physical value of the cell as a mass 

 of molecules in a state of solution, are the cause of an increased 

 osmotic pressure which is sufficient with the differences 

 postulated between the cell surfaces to account for the passage 

 of water out into the pocket of the little gland. Here we have 

 been forced to think of the matter within the cell as a solution, 



