THE IMSER TISSUES OF ANIMALS. 339 



subject to tensions and pressures, the nutritive liquid diffused 

 through the substance of the cartilage, compressed as it must 

 be, will tend to ooze from the surface of the cartilage, and to 

 return again when the stress is taken off. Such alternate 

 movements of the nutritive liquid, perpetually repeated, will 

 be apt to form channels. These, at first quite superficial and 

 inappreciable, will become more appreciable ; since, when 

 the}' are once commenced, any further additions of substance 

 to the surface will be prevented from closing their openings 

 by the alternate rushes of liquid ; and so a vascular layer 

 of appreciable thickness may gradually be formed. But 

 without doing more than hint this, 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 somewhat extensible, and 



spreads out laterally under pressure, but resumes its form 

 when relieved. How, then, will the capillaries traversing 

 such a substance 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 prosencbyma 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 il 

 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 taken off, the bone yields 

 again towards the same side when again longitudinally 

 pressed. Hence the substance of its concave side, never 

 rendered convex by a bend in the opposite direction, would 



