348 PHYSIOLOGICAL DEVELOPMENT. 



the same actions go on in such ways as to produce the ob- 

 served effects. At the outset we are met by what seems a 

 fatal difficulty — cartilage is. a non-vascular tissue : this sub- 

 stance of which unossified bones consist is not permeated 

 by minute canals carrying nutritive liquid, and cannot, 

 therefore, be a seat of actions such as those assigned. 

 This apparent difficulty, however, furnishes a confirmation. 

 For cartilage that is wholly without permeating canals does 

 not ossify: ossification takes place only at those parts of 

 it into which the canals penetrate. Hence, we get ad- 

 ditional reason for suspecting that bone-formation is due 

 to the alleged cause; since it occurs where mechanical 

 strains can produce the actions described, but does not occur 

 where mechanical strains cannot produce them. Let us 

 consider more closely what the several factors are. It will 

 suffice for the argument if we commence with the external 

 vascular layer as already existing, and consider what 

 will take place in it. Cartilage is elastic — is some- 



what extensible, and spreads out laterally under pressure, 

 but resumes its form when relieved. How, then, will the 

 minute channels traversing it in all directions be affected at 

 the places where it is strained by a bend? Those on the 

 convex side will be laterally squeezed, in the same way that 

 we saw the sap-vessels on the convex side of a bent branch 

 are squeezed; and as exudation of the sap into the adjacent 

 prosenchyma will be caused in the one case, so, in the other, 

 there will be caused exudation of serum into the adjacent 

 cartilage: extra nutrition and increase of strength resulting 

 in both cases. The parallel ceases here, however. In 'the 

 shoot of a plant, bent in various directions by the wind, the 

 side which was lately compressed is now extended; and 

 hence that squeezing of the sap-vessels which results from 

 extension, suffices to feed and harden the tissue on all sides 

 of the shoot. But it is not so with a bone. Having yielded 

 on one side under longitudinal pressure, and resumed as 

 nearly as may be its previous shape when the pressure is 



