PROPAGATION OF PHYSIOLOGICAL CHANGE 485 



chemical change resulting from the exudation of a new corrosive 

 substance from the excited cells, or is it a physical consequence 

 of repeated pressure upon the homogeneous jelly ? It is clear 

 that the mass primarily stimulated is the jelly, which from its 

 consistence must be held to protect the cells enclosed within 

 it. Again, the irruption of bone-forming cells takes place, made 

 possible by the liquefactive change. Each one of these new 

 cells secretes an exudate which then "sets." The directions 

 taken by the fibres precipitated in the setting bone-matrix at 

 once suggest the notion that they are the result of the stresses 

 and strains to which the material is subjected. Calcium salts 

 also make their appearance passing from a state of solution to 

 be precipitated as granules. In the case, then, of bone-formation 

 it is again the change of physical state which at once claims our 

 interest. Now it may be objectedi that there is no proof that 

 these changes are not of chemical rank, perhaps even attended 

 with an output of waste products. However, even if that 

 possibility be admitted, the contrary hypothesis is seen to 

 be at least as admissible, and therefore to require definite 

 examination. 



But if a case can be made out for the examination of such 

 possibilities when dealing with extracellular material, the same 

 possibility must also be introduced within the limits of the cell. 

 The difference between similar changes within and without the 

 cell may be no more than a difference of opportunity and 

 advantage. Outside the cell the sole sources of energy are 

 those provided by stimulation and the heat of the circulating 

 solutions of the body. The cell has the advantage here. 

 Inside the cell recovery may thus be more usual than outside 

 of it. Within the cell, as for example in the musculature, is the 

 source of heat production whence the heat of these circulating 

 solutions is derived. Within the muscle-fibre itself this chemical 

 source of energy may play no greater part in many important 

 internal processes than is played by the heat of the forge in 

 the production of steel. If the muscle distributes heat to other 

 tissues of the body which is made use of in this way, what limits 

 are to be set to its own participation in similar advantages ? It 

 is a possibility often lost sight of, but none the less important, 

 that the source of energy within the muscle may possibly be 

 responsible for physical states, for physical stores of potential 

 energy, the disturbance of which in their turn may give rise 



