THE INNER TISSUES OF ANIMALS. 353 



far more squeezed than they or any other of the canals 

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

 on every repetition of the strain, these canals near the con- 

 cave 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 assimilated 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 peri- 

 osteum; in this, according to 'the ordinary course of tissue- 

 growth, new vessels 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 vessels 

 are the most severely squeezed. The place of greatest exuda- 

 tion and most rapid deposit of matter, is therefore 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 process of 

 filling it up? The answer to this question will be manifest 

 when 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 



* To this implied inference it is objected that " excess of nutritive mate- 

 rial does not necessarily lead to correspondingly increased growth." My reply 

 is that a concomitant factor is activity of the tissue, and that in its absence 

 growth is not to be expected. 

 69 



