THE SKELETON 91 
development in other directions such as might counteract the 
present tendency must, however, be allowed for (Pfitzner).’ 
COMPARISON OF THE FORE- AND HIND-Limps 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 spiral rotation 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 humerus 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 1s 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 Walruses, 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 feetal state.—G. B. H.] 
