VARIABILITY OF THE SKELETON. 



87 



has shown that the humerus of a man or of an animal is the 

 homologue of the femur, but of a femur twisted on its axis, so 

 that the knee turned behind becomes an elbow, zoologists 

 have replied that this torsion was purely virtual. Instead of 

 being the effect of a muscular effort, whose slow and gradual 

 action has reversed the axis of the bone, this singular form 

 is, in their opinion, the result of a pre-established arrange- 

 ment of the organism; for the embryo shows a contorted 

 humerus, before muscular action has been sufficiently developed 

 to produce such a modification of its skeleton. 



We might, with greater show of reason, argue in a directly 

 opposite manner. 



No one denies at the present day that the bony system is 

 perfectly yielding in its character. These organs, which are 

 so compact and so hard in the dead skeleton, are, on the con- 

 trary, essentially capable of being modified while the organism 

 is living. If we exert upon a bone a pressure or a tension, 

 however slight it might be, yet if prolonged for a considerable 

 time, it can produce the strangest changes of form ; the bone 

 is like soft wax which yields to all external forces ; and we 

 may say of the skeleton, reversing the proposition to which we 

 have just alluded, that it is completely under the influence of 

 the other organs, and that its form is that which the soft parts 

 with which it is surrounded permit it to assume. 



We are indebted to medicine and surgery for the knowledge 

 of important facts, of which many examples could easily be 

 given. Thus, when an aneurism of the aorta is developed, 

 and it happens to meet in its course the sternum or the clavicle, 

 it does not stop at this barrier of bone, but perforates it 

 in a few months. The substance of the bone is absorbed and 

 disappears under the pressure of the aneurism ; it certainly 

 resists less the effort of the invading tumour than do the softer 

 parts — the skin, for example. 



But this pressure of the aneurism differs in no respect 

 from that of the arterial blood; the force with which the 

 aneurismal sac compresses and perforates the bones, is present 

 in every part where an artery touches a bone. The same ab- 

 sorption of the bony material still goes on, so that the artery 

 hollows out for itself a furrow in which it lodges with its dif- 



