THE SKELETON 91 



development in other directions such as might counteract the 

 present tendency must, however, be allowed for (Pfitzner). 1 



COMPARISON OF THE FORE- AND HIND-LIMBS OF MAN 



In comparing the opposite extremities of the adult two 

 difficulties have to be met, the first being that the knee and elbow- 

 joint bend in exactly opposite directions, and the second that, 

 owing to the inward rotation of the fore-limb, the homologous 

 bones of the fore-arm and fore-leg (radius and tibia, ulna and 

 fibula) are differently disposed. 



Martins and Gegenbaur have endeavoured to explain these 

 difficulties by spiraljrotation of the humerus during development 

 said to be effected by alteration in growth of the epiphysial 

 cartilage, with the addition of bony tissue at some points and its 

 resorption at others. The distal end of this bone has its 

 original ventral surface turned dorsally and vice versd. By 

 comparing the position of the hiunerus in embryos and adults it 

 is found to rotate through an angle of about 35 (Gegenbaur). 



Spiral rotation of the humerus actually takes place, not only 

 in Man, but very commonly in other Vertebrates. It can further 

 be proved that it progressively increases as we pass from 

 the lower to the Caucasian races ; and Broca affirms that an 

 increase is to be found at different epochs within the same race. 



But although the torsio humeri is an undoubted ontogenetic 

 fact, according to more recent authors, it is questionable whether 

 it affords any explanation of the difference between the fore- and 

 hind-limbs. This subject is so important that we must enter 

 into it at some length, referring especially to the works of Hatschek 

 and Holl. The first of these investigators has rightly taken for 

 comparison the lowest terrestrial Vertebrata, the tailed Amphibia, 

 and he lays emphasis upon the fact that in these animals the 

 position of the fore- and hind-limbs in relation to the trunk is 

 almost identical. Both stand out at right angles to the long axis 



1 [It appears to me that the occasional longitudinal subdivision of the human 

 hallux-tarsal (ento-cuneiform) into two distinct bones may be not improbably a 

 phenomenon akin to that of the double ossification of the supra - occipital under 

 expansion (cf. ante, p. 60), if not an actual index of progressive development 

 now at work. My friend Professor Arthur Thomson informs me that, from a study 

 of the articular surfaces of this bone, he believes the tendency towards duplication 

 to be more general than is customarily assumed ; and it would be most interesting 

 to inquire whether among the Seals and Wall-uses, in which the inner and outer 

 digits are one or both similarly dominant over the rest, indications of a correspond- 

 ing variation might not be forthcoming in the foetal state. G. B. H.] 



