PROPAGATION OF PHYSIOLOGICAL CHANGE 487 



of a chemical change as merely adding to the number of molecules 

 in solution, of the cell limits as permitting or restraining the 

 motion of dissolved material. None of the important activities 

 underlying these terms are made obvious by the contemplation 

 of " biogen molecules." 



Let us examine an instance a grade more complex. The 

 salivary glands are formed of similar pockets, from which on 

 the arrival of a nervous impulse water again gushes out on 

 to the surface of the body containing in solution organic and 

 inorganic matter. The same explanation will temporarily 

 cover the output of water. But why these other substances 

 also ? Now there is no doubt that most of these other materials 

 have lain within the cell long before the arrival of the nervous 

 impulse, having been manufactured during hours of rest from 

 secretion. They are now merely washed out in the rush of 

 water. In addition to the chemical work done on receipt of 

 the impulse we have here to consider specialised chemical work 

 performed quite independently of this, such as is done by gland- 

 cells to which no such impulses come. Is it of advantage to 

 consider the molecules engaged in the one change and in the 

 other as necessarily the same ? Is it not simpler to think of 

 molecules of diverse kinds in solution side by side but under- 

 going a different cycle of change with intensities that reach their 

 maxima at different times? We have been compelled to intro- 

 duce the concept "solution" to explain the transit of water. 

 There can be no doubt now, as the result of manifold experi- 

 ment, that this concept has to be introduced in case of every 

 cell ; nor even any doubt, to make the matter physically more 

 complex, that the variations of phase within these solutions 

 undergoing change w^ith every kind of alterations in physical 

 circumstance have all to be taken into consideration as modifying 

 the activities of the cell, and therefore as variable physical states 

 of functional importance. 



Now it may be said that, after all, these instances have done 

 no more than attribute important results to chemical change 

 as the primary source of energy directly involved in the pro- 

 cesses considered. The carbonic acid production has been dealt 

 with as the source of the change. But what if we select as 

 our new molecule adding to osmotic pressure not carbonic acid, 

 so obviously the outcome of an oxidation process, but some 

 inorganic salt?. In the case of the salivary secretion certain 



