. VARIABILITY OF THE SKELETON. 89 



bone is actually placed, a fresh articulation is formed, to which 

 nothing will be wanting in the course of a few months, neither 

 articular cartilages, synovial fluid, nor the ligaments which 

 retain the bones in their place. Here again, according to the 

 expression which we used just now, function has produced 

 the organ. 



So much for the furrows formed in the bone. But how 

 can we attribute to external influences those decided promi- 

 nences which we observe everywhere on the surface of the 

 skeleton, those apophyses, as they are called, to which each 

 muscle is attached. 



The answer is not less easy ; it is sufficient to account for 

 the formation of projections on the face of the bone, if we call 

 into play an influence contrary to that which we know to be 

 capable of hollowing out the indentations. We must admit 

 that traction has been exercised on the portion of the bone 

 where the projection is observed. 



The existence of traction on all the points in the skeleton to 

 which muscles are attached is absolutely evident ; it is clear 

 that the intensity of these tractions is proportional to the force 

 of the muscles which produce them. Thus, it is precisely in 

 the tendinous attachments of the stronger muscles that we find 

 the more projecting apophyses ; a proof that the prominences 

 in the bone are intimately connected with the intensity of the 

 effort acting upon them. The right arm, more frequently used 

 than the left, acquires more decided projections on its bony 

 structure. When paralysis of a limb suppresses the action of 

 the muscles, its skeleton is no longer under the influence of 

 muscular power, and the apophyses become less prominent ; 

 in fact, if paralysis dates from birth, the bone remains nearly 

 in its foetal form, which function has not supervened to 

 modify. 



Comparative anatomy also confirms this general law that 

 the longer the apophysis is, the greater energy it reveals on 

 the part of the muscle which was inserted into it. 



Mens. Durand de Gros has clearly shown the influences of 

 muscular function on the form of the torsion of the humerus 

 in different species of fossil and recent animals. Thus the 

 humerus in the mole, the ant-eater, and several other burrow- 



