MOKPHOLOGY OF THE BONES OF THE LIMBS. 



ilium may be regarded as consisting essentially of three-sided prismatic rods, in the primitive 

 position of which an internal or vertebral surface is separated from two external surfaces by 

 preaxial and postaxial ridges, and the two external surfaces are separated by an external 

 ridge, which descends from the dorsal extremity of the bone to the joint-socket. It is 

 on this external ridge, glenoid in the scapula and cotyloid in the ilium, that in each case 

 the long head of the great extensor muscle of the limb is attached. The primitive arrange- 

 ment is modified in two ways, viz., 1st, by the outgrowth to a greater or less extent of the 

 edges of the rod, thus giving rise to fossae in the situation of the original surfaces, and 2nd, 

 by the occurrence of a rotation in different directions in the two limbs, the scapula outwards, 



Fig. 165. DIAGRAM OP AN EARLY HUMAN 



EMBRYO, SHOWING THE RUDIMENTARY 

 LIMBS IN THEIR SECOND POSITION. (Allen 



Thomson. ) 



r, preaxial or radial and pollex border of 

 the thoracic limb ; u, its postaxial or ulnar 

 and little finger border ; t, preaxial or 

 tibial and hallux border of the pelvic limb : 

 f, its postaxial or fibular and little toe 

 border. 



the ilium inwards, in accordance with the rotation which takes place during the course of 

 development in the free part of the limb. The primitive vertebral surf ace of the scapula 

 becomes the prescapular or supraspinous fossa, while in the ilium the corresponding surface 

 is the sacral, which, being connected to the vertebral column, undergoes but little change in 

 position. The postscapular or infraspinous fossa of the scapula is accordingly represented by 

 the iliac fossa of the ilium, and the subscapular fossa by the gluteal surface. (See the table 

 of homologous parts below.) 



Bones of the limbs. In making the comparison of the bones composing the limbs them- 

 selves it is necessary to revert to the simpler relations subsisting between the limbs and the 

 trunk or vertebral axis of the body in embryonic life. In the earliest stage the limbs may be 

 said to bud out from the side of the trunk as fiattish semilunar flaps, so that they present a 



Fig. 166. DORSAL SURFACE OF THE RIGHT MANUS OF A WATER 

 TORTOISE. ( Gegenbaur. ) 



R, radius ; TJ. ulna ; r, radiale ; i, intermedium ; u, ulnare ; c, centrale ; 

 1 5, five carpal bones of the distal row ; m 1 m 5 , five metacarpals. 



dorsal and a ventral surface, coinciding with these respective 

 surfaces of the trunk ; but in the next stage, when the limbs 

 come to be folded against the body in the ventral direction, 

 although the original relation to the trunk is undisturbed, 

 their axes have now come to lie nearly perpendicularly to the 

 coronal plane of the vertebral axis, and the position of the limbs 

 is such that in each there is one border which looks towards 

 the head, and another which looks towards the tail. To these 

 borders of the limbs the names of preaxial and postaxial are 

 given respectively, as indicating their position before and 

 behind the limb-axis. When at a somewhat later stage of 

 development the divisions of the limbs make their appearance, 

 and more especially when the quinquifid division of the digits 



in the hand and foot becomes perceptible, it is obvious that the thumb and radius in the one 

 limb and the great toe and tibia in the other, occupy corresponding cephalic and preaxial 

 situations ; and it is not difficult to trace from these the corresponding relations of the parts 

 in the upper division of the limbs. Thus, the radial condyle of the humerus with the great 

 tuberosity are preaxial, while the lesser tuberosity, ulnar condyle. "ulna, and little finger are 

 postaxial. In the lower limb, the small trochanter, internal condyle, tibia and great toe are 

 preaxial, while the great trochanter, external condyle, fibula, and little toe, are postaxial. 

 And at the same time the dorsal or extensor surface of the limbs is external, and the ventral 

 or flexor surface is internal. 



Journ. Anat., v, 67 ; Sabatier, " Comparaison des ceintures et des membres anterieurs et posterieuvs 

 dans la serie des vertebras," Montpellier, 1880. 



