64 Anatomy of Skeleton 



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 " corre- 

 spond " 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 imme- 

 diately 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, conies 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 " 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. 



