123 



MORPHOLOGY OF THE BOXES OF THE LIMBS. 



towards the taiL To these borders of the limits. Huxley and Flower have given 

 the names of prcaxial and jjostaxial respectively, as indicating- their position 

 before and behind the axis of the limbs. Wlren at a somewhat later stage of 

 development, the divisions of the limbs make their appearance, and more especially 

 Avhen the quinquifid division of the digits in the hand and foot becomes percep- 

 til)le. it is obvious that the thumb and radius in the one limb, and the great toe 

 and tibia in the other, occupy correspondmg 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 ; and thus the radial condyle of the humerus with 

 the great tuberosity are preaxial. while the lesser tuberosity, ulnar condyle, uhia, 

 and little finger are postaxial. In the lower limb, the lesser trochanter, internal 

 conlyle, tibia and great toe are preaxial. while the great trochanter, external 

 •condyle, fibula, and lesser toe. are postaxial. And at the same time the dorsal or 

 extensor surface of the limbs becomes external, and the ventral or flexor sm-face 

 Jjecomes internal. 



FiK. 112. 



Fig. 112. — Lateral view of 

 THE Human Embryo of 



ABOUT SEVEN WEEKS, SHOW- 

 ING THE Rudimentary 

 Limbs in their second 

 Position. (A. T.) Mag- 

 nified 7 diameters. 



r, preaxial or radial and 

 pollex border of tlie thoracic 

 limb ; u, its postaxial ornluv.r 

 and little finger border ; t, 

 jireaxial or tibial and hallux 

 border of tlie pelvic Hnib ; /, 

 its postaxial or fibidar and 

 little toe bordei". 



Very soon, however, in the higher animals and in man, farther changes operate 

 in bringing aljout the permanent form. First, there is the eversion of the humeras 

 ■so as to place the radial condyle outwards, and the inversion of the femur so as to 

 place the tibial condyle m wards. In the upper limli of man. the radius being in 

 :semipronation, no material change occurs in the position of the hand, the thumb 

 hanging naturally forward ; but in animals, destined to rest on the palmar 

 .aspect of the hand or digits, important chang-es occur in the position of the 

 radius by which, as this bone is brought forward upon the humerus, and its 

 lower end carried inwards, the manus or its elements are placed pennanently 

 in the iirone position, with the first or radial digit inwards. In the 

 foot no such change is required, as already by the mtemal rotation of the 

 femur at its upper pait. the conditions for plantar suj^port have been secured, 

 and the first or tibial digit is on the inner side. Further, in man, as the body 

 attains its full development, lioth limbs are extended in a line parallel to the 

 axis of the trunk, the upper droi^prng loosely from the shoulder- joint ^^■ith the 

 greatest freedom of motion : the lower more closely articulated in the hip- joint, 

 .and suited to give finn support to the body in the erect jiosture. 



It is proper to mention here a very ingenious view of the homologies of the 

 limbs which has been suggested by Martins, according to which the humerus 

 is to be regarded as virtually twisted upon itself to the extent of 90° at the 

 neck, and 90° more from that part downwards, or to the extent of 180° in 

 its whole length. By this torsion, IMartms accounts for the deviation of the 

 ■external condyle of the humerus from the origmal or typical jiosition which he 

 considers to remain in the femur, and thus he endeavours to show, and it must 

 be admitted with some appearance of probability, how. by supposing the humerus 

 to be untwisted, an exact correspondence of the surfaces and borders can be 

 established between the humems and femur. Gegenbaur has adopted this 

 view, and has added some facts in illustration of it. (Ch. Martins, " Nouv. 



