VARIABILITY OF THE SKELETON. 



91 



The curvature of the bones, or their contortion on their 

 axis, is a phenomenon which is frequently observed. I have 

 mentioned that Mons. Ch. Martin has demonstrated that in 

 all the mammalia, the humerus is a contorted femur, whose 

 axis has made half a turn upon itself ; this contortion, accord- 

 ing to Gegenbaiier, is less in the foetus than in the infant, 

 and becomes still more marked in process of age. It is 

 therefore partly effected by causes which are in action during 

 life ; and if it be true that every foetus brings into the world 

 a contorted humerus, it is not less true that this form may 

 be considered as the effect of muscular action accumulated 

 from generation to generation in terrestrial mammals. 



Articular surfaces are particularly interesting to study when 

 we wish to ascertain the influence of function over the organs. 

 If w^ admit that the friction of these surfaces has polished 

 them, and given them their curvature, it is easy, when we 

 consider the movement which takes place in each articulation, 

 to foresee the form which these surfaces ought to possess. 



The surfaces whose curvature has the greater number of 

 degrees, will correspond with the more extensive movements. 

 Moderate movements, on the contrary, will only produce sur- 

 faces whose curvature will correspond with an arc of but few 

 degrees. As a necessary consequence, the radius of curvature 

 in the articular surfaces will be very short, if the move- 

 ments are very extended ; it will be very long if the movement 

 is moderate. 



Let us examine, from this point of view, the articulations 

 of the foot in man ; we see in the tibio-tarsal articulation a 

 curvature of small radius, on account of the considerable move- 

 ment of the foot on the leg. In the tarsus the radius of the 

 curvature increases in proportion as the mobility of the bones 

 diminishes. The scaphoid shows articular surfaces of a great 

 radius ; the radius increases still more in the tarso-metatarsal 

 articulations, in which the movements are very limited ; then 

 it diminishes again in the articulations of the metatarsals 

 with the phalanges, and of the phalanges with each other, at 

 which point there is great mobility. 



Everyone knows that if the articular movement is only 

 effected in one direction, the surfaces will curve only in that 



