of the Vertebrate Skeleton. 269 



body when passed onward in successive generations ; and there 

 is no inevitable limit to this accumulation or loss of structure, 

 except the maintenance of harmony in the organic functions. 



Here, then, the question presents itself, What are the condi- 

 tions which produce these modifications of the dialyzing action 

 which are manifested in hypertrophy and atrophy ? This I 

 now will endeavour to answer. The question may be taken 

 in the abstract. Assuming the amount of nutrient material to 

 remain constant, the change of growth must obviously be due 

 to some change of the conditions which affect the part. Now the 

 only conditions which, while affecting the whole body, maybe 

 variable in the different parts, are the forces manifested by the 

 organs in the discharge of their several functions. These act 

 either from within or without ; and therefore, as will be gene- 

 rally admitted, every mechanical force acting on the elements 

 of the body is in its effect either of the nature of an impact or 

 of an explosion ; and these, with all other forces acting upon 

 and within the animal, can only produce alternations of pres- 

 sure and tension and rest *. These, therefore, are the stimu- 

 lants to growth. But growth, being a condition in which the 

 particles expand and increase externally, can only take place 

 when the pressui'e is removed. And since increase of size can 

 only be resisted by continuous pressure, that, therefore, is the 

 mechanical condition of atrophy. In other words, these me- 

 chanical changes are the phenomena which we speak of col- 

 lectively as exercise. Now the reason why these mechanical 

 actions should produce growth is not far to seek. They alter 

 the conditions of nutrition. Pressure upon a muscle squeezes 

 the blood which was in the veins out of that muscle more 

 rapidly than it usually circulates ; and the removal of pressure 

 causes the blood to rush into the part with more force than 

 usual. That is, the establishment in a part of the body of 

 alternate pressure or tension and rest, sets up there a local 

 pump-action which, in effect upon the circulation, is like an 

 additional heart added to that part. It brings more blood' to 

 the part, and circulates more food through it ; the dialyzing 

 action is carried on faster ; and the fibres or cells become plump 

 with abundant food, and new matter is thus fixed in the 

 tissue, and the part has grown. 



Therefore since growth, so far as it characterizes the 

 individual and is kinetic, is produced by these mechanical 

 actions, we have to look upon nature and see in what w^ays 

 the parts of an organism act mechanically upon each other. 

 And should the evidence be conclusive that such actions ac- 



* Annals of Nat. Hist. Nov. 1866, No. 107, vol. xviii. p. 347. 



