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 likeness becomes least traceable, but always can be found the essential correspondence that marks the early similarity in function.
So it does not seem advisable to attempt to establish resemblances more than the most general ; but such attempts have been made with great ingenuity-to homologise soft parts as well as the skeleton, and to establish the correspondence in detail. Reference will be made later to some of these views : it is sufficient at present to say that the " body " of the scapula and the Ilium " correspond " because they represent the dorsal moiety in their respective girdles, while the Ischium and pubis correspond in the same way with the coracoid process-which is the only ventral part left in the human shoulder girdle, and may be a true coracoid or a pre-coracoid. Similarly the femur and humerus are the bony supports of corresponding proximal segments of the free limbs.
If we now consider the development of the human limbs, we find that they go through stages that could be said to represent their evolution as just described so very shortly and generally. Each limb begins as a bud jutting out from the body-wall; this has its mesenchyme applied to the surface of the body segments. It elongates and shows signs of dividing into its various parts, and at the same time its mesenchyme commences to lay down the condensations that will form its skeleton.
The skeletal basis is mapped out first in the proximal part of the limb and in the tissue immediately carrying it: the distal part of the bud grows rapidly, and its skeletal basis is laid down rapidly, but the extension of the condensation of the limb girdles follows it a little later, spreading into the wall of the body. In the same way, chrondrification of the girdles is secondary to the change in the free limb. As development proceeds the girdle, which is at first very small compared with the free limb, comes nearer its proper size, and later still it joins the sacral region and its fellow in the case of the pelvis and in that of the shoulder settles into position on the thorax. Although we must not look on ontogenetic development as necessarily a recapitulation of phylogeny, yet it is interesting to observe that this short sketch of embryonic formation is in complete accord with the sketch of assumed evolution.
It is in the later stages of development that one must look for the importation of secondary modifications, and it is after the mapping out of their skeletal bases that differences begin to be apparent between the limbs, which are at first very much alike. It is not necessary to deal with these differences here, save in one or two particulars.
Fig. 49 (3) is intended to show how the limbs may be considered to have come into their present positions. They are to be thought of as at first lying in the same direction, at right angles to the long axis of the body : under these conditions each limb has a dorsal and a ventral surface and anterior or cephalic and posterior or caudal borders. From their relations to the axis of the limb these borders are termed preaxial and postaxial.
The conditions in the four-footed animal are attained by bringing the thigh forward so that its preaxial border comes against the body, and flexing the knee, while at the same time the upper arm is brought back with its postaxial border against the body and the elbow bent: thus it becomes evident that in the hind limb the preaxial border is now mesial or internal, but in the fore limb the postaxial border is internal. The orthograde type is easily attained from this by bringing the legs down and thus making a rotation through 180 degrees at the hip-joint. The preaxial digit is the thumb in the hand and the big toe in the foot, and in this way these digits are found on descriptively different sides of the limb, and the same may be said for the bones of the leg : the outer bone of the leg (fibula) corresponds with the inner (ulna) in the forearm. Moreover, the dorsal or extensor side of the original lower limb comes to look ventrally as a result of this rotation.
In this way the two limbs may be compared, but it is questionable if the comparison is worth anything, or if the terms 14 pre-axial " and " post-axial " have any value beyond that of any other merely descriptive term. If, however, it is thought that true homology exists between the limbs, the appreciation of these points becomes of great importance, and in any case the " correspondences" just indicated can be at once admitted without defining the nature or value of the relations between the structures.
It lias been pointed out more recently, that there is a mirror-relation between the limbs_one limb corresponding with the other " the other way round "-and this is a suggestive conception of the limbs that does not depend on homology : but the question is rather outside the province of this work, and it is not possible to devote further space to its consideration.
The primary elements of the girdles and of the free hmb skeletons are laid down in cartilage, and the process of ossification follows in a general way the evolution of the limb as sketched above-that is, the centres for the proximal segments of the free limbs appear first, followed shortly by those for the next segment, and at about the same time ossification begins in the girdles : in the extreme segments the process is variable in its onset and delayed in parts, as it is in some portions of the girdles.