344 PHYSIOLOGICAL DEVELOPMENT. 



bends it, the severest compression falls on the substance of 

 its concave side. Each time, then, the capillaries running 

 through this part of its substance are violently squeezed 

 far more squeezed than they or any other of the capillaries 

 would have been, had the bone remained straight. Hence, 

 on every repetition of the strain, these capillaries near the 

 concave surface have their contents forced out in more 

 than normal abundance. The materials for the formation of 

 tissue are supplied in quantity greater than can be assimi- 

 lated by the tissue already formed ; and from the excess of 

 exuded plasma, new tissue arises. A layer of organizable 

 material accumulates between the concave surface and the 

 periosteum ; in this, according to the ordinary course of 

 tissue-growth, new capillaries appear ; and the added layer 

 presently assumes the histological character of the layer from 

 which it has grown. What next happens ? This added 

 layer, further from the neutral axis than that which has 

 thrown it out, is now the most severely compressed, and its 

 capillaries are the most severely squeezed. The place of 

 greatest exudation and most rapid deposit of matter, is there- 

 fore transferred to this new layer ; and at the same time that 

 "active nutrition increases its density, the excess of organizable 

 material forms another layer external to it : the successive 

 layers so added, encroaching on the space between the concave 

 surface of the bone and the chord of its arc. What 



limits the encroachment on this space ? what stops the pro- 

 cess of filling it up ? The answer to this question will be 

 manifest on observing that there comes into play a cause 

 which gradually diminishes the forces falling on each new 

 layer. For the transverse sectional area is step by step 

 increased ; and an increase of the area over which the weight 

 borne is distributed, implies a relatively smaller pressure 

 upon each part of it. Further, as the transverse dimensions 

 of the bone increase, the materials composing its convex and 

 concave layers, becoming further from the neutral axis, 

 become better placed for resisting the strains to be borne 



