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 limb 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 lies 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 (i.e. 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 limb 

 originally lay farther forwards, i.e. nearei^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 



