THE SHAPES OF VERTEBRATE SKELETONS. 215 



is without any divisions, there are definite divisions of the 

 substance forming the animal's sides? I think we have. A 

 glance at the distribution of forces under the transverse 

 strain, as represented in the foregoing diagrams, will show 

 how much more severe is the strain on the outer parts than 

 on the inner parts ; and how, consequently, any modifications 

 of structure eventually necessitated, will arise peripherally 

 before they arise centrally. The perception of this may be 

 enforced by a simple experiment. Take a stick of sealing- 

 wax and warm it slowly and moderately before the fire, so as 

 to give it a little flexibility. Then bend it gently until it is 

 curved into a semi-circle. On the convex surface small 

 cracks will be seen, and on the concave surface wrinkles; 

 while between the two the substance remains undistorted. 

 If the bend be reversed and re-reversed, time after time, 

 these cracks and wrinkles will become fissures which gradu- 

 ally deepen. But now, if changes of this class, entailed by 

 alternating transverse strains, commence superficially, as they 

 manifestly must; there arise the further questions What 

 will be the special modifications produced under these special 

 conditions? and through what stages will these modifica- 

 tions progress? Every one has literally at hand an example 

 of the way in which a flexible external layer that is now 

 extended and now compressed, by the bending of the mass it 

 covers, becomes creased; and a glance at the palms and the 

 fingers will show that the creases are near one another 

 where the skin is thin, and far apart where the skin is thick. 

 Between this familiar case and the case of the rhinoceros- 

 hide, in which there are but a few large folds, various grada- 

 tions may be traced. Now the like must happen with the 

 increasing layers of contractile fibres forming the sides of 

 the muscular tunic in such a type as that supposed. The 

 bendings will produce in them small wrinkles while they are 

 thin, but more decided and comparatively distant fissures 

 as they become thick. Fig. 289, which is a horizontal longi- 

 tudinal section, shows how these thickening layers will 



