94 THE STRUCTURE OF MAN 
For the further study of the processes by which the limbs 
are displaced during development, I must refer the reader to the 
works of von Kolliker, Holl, and others. It should, however, be 
remarked once more that the twisting of the hind-limb occurs 
at the hip-joint merely, [and affects the ihmb as a whole, its 
originally ventral surface becoming posterior and its dorsal 
anterior in position, and that in the fore-limb the twisting most 
conspicuously affects the manus and the forearm, the radius under- 
going a marked inward rotation upon the ulna. The humeral 
segment more nearly retains in the adult its original position], and 
the rotation and retroflexion which it ultimately exhibits chiefly 
result from a twisting of the shoulder girdle, with accompanying 
modifications of its articular head. 
These changes in position of the shoulder girdle are connected 
with the development of the thorax. As long as the latter retains 
the laterally compressed form characteristic of most Mammals, 
and is not expanded dorsally, the scapula hes at its side. Later, 
when transverse enlargement and consequent dorsal expansion of 
the thorax are effected (cf. ante, p. 36), the scapula comes to lie 
upon (ze. dorsad of) it. This change in the thorax plays a leading 
part in altering the position of the shoulder girdle as a whole, 
and of the limb attached to it. 
If we wish to homologise the two pairs of limbs scientifically, 
we can only do so by tracing their displacements back towards 
their embryonic positions. 
CHANGES OF POSITION OF THE LIMBS IN RELATION 
TO THE TRUNK 
A comparison of the fore-limb of Man with that of the lower 
Vertebrates, and especially of the Fishes and Amphibians, and a 
careful analysis of the courses and relationships of its muscles and 
nerves with respect to the trunk and the spinal cord, lead us to 
the conclusion that the shoulder girdle and its associated lmb 
originally lay farther forwards, z.e. nearer the head. ‘The dis- 
placement backwards most probably took place, as has already 
been shown (ante, p. 44), simultaneously with the disappearance of 
the cervical ribs—indeed the loss of the latter certainly helped 
to bring this about, by compelling the scapula and clavicle to 
find points of attachment farther back on the thorax. 
Whereas this shifting of the fore-limb takes place from before 
backwards, that undergone by the hind-limb is from behind 
