Limbs : Upper Limb 63 



only concerned with a theoretical extension dorsally and ventrally on the wall of the body, as shown 

 in Fig. 49, so that we would obtain the rudiments of pelvic and pectoral arches similar in structure 

 and consisting of dorsal and ventral parts supporting at their junction the skeleton of the free limb. 

 The ventral portion was double, consisting of two bars articulating perhaps with a central ventral 

 part of the trunk skeleton (? sternum) or with their fellows of the opposite side : thus greater 

 stability was secured for the action of the free limb. The dorsal part was broad and single and 

 could reach the dorsal skeleton of the trunk by extension. In this we have the foreshadowing of 

 the human pelvic arch at least, the dorsal ilium and the two ventral bars, pubis and ischium : the 

 pectoral girdle had a dorsal " scapula," and ventral " coracoid " and " pre-coracoid " bars. 



If we now imagine that these strengthened limbs are being used for purposes of support on 

 land as well as propulsion in the water, we can understand that the slight differences in the con- 



ventral . 



FIG. 49. I and 2 are intended to illustrate the hypothetical growth of the bony supports of 

 the limb in the vertebrates. In i a short lateral appendage is shown on the left side, 

 made more useful by thickened and resistant tissues in its centre ; on the right side the 

 limb is lengthened and in consequence its supports are strengthened and made bony, these 

 are segmented to allow of movement in the limb, and a point d'appui for the action of 

 the whole limb is provided in the form of a skeletal support in the body-wall. Such 

 supports extend dorsally and ventrally. 2 shows the animal from the side with the dorsal 

 and ventral extensions in the body-wall in the pectoral and pelvic regions ; the limits of 

 the limb attachments are seen and the shaded areas at the dorso-ventral junctions mark 

 articular surfaces lor the skeleton of the free limbs. 3 contrasts the corresponding parts 

 of the two limbs. Dorsal view. Pre., Post, preaxial and postaxial borders ; /). dorsal 

 aspect ; X. thumb and big toe. The common type is altered on the right as the limbs 

 come into position, and the ventral surface, V ., looks forward in the fore limb and back 

 in the hind limb. H. points to the form of the human (or plantigrade) foot derived from 

 this. 



ditions of action of the limbs will begin to graft differences of detail on the fundamentally similar 

 structure of these parts, and these will be accentuated as the specialisation in function of the fore 

 limbs progresses. Thus we might consider the fixation of the pelvic girdle and great mobility of the 

 pectoral girdle as being characters secondarily acquired, in common with lesser differences in 

 structural detail, and grafted on the fundamentally similar construction of the limbs. 



Looked at in this way, it becomes doubtful whether such a thing as true homology can be said 

 to exist between the upper and lower limbs ; the likeness between them is the result of similarity 

 of relations and not of genetic similarity the dorsal scapula and the dorsal ilium, for instance, have 

 been called into being by the same necessities of function, and not because they both come from the 

 same stock, so to speak, and similar views might be held about the other corresponding structures. 



In other words, there is an analogous resemblance rather than an homologous one between the 

 corresponding parts of the limbs, and, where the functions have become most divergent, there the 



