COMPARISON OF THE HAND AND FOOT. 



143 



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

 about the permanent condition. The humerus is folded backwards against the trunk, and at 

 the same time undergoes a rotation outwards, so that the radial (preaxial) condyle becomes 

 external, and the extensor aspect of the elbow is directed backwards. The femur, on the 

 other hand, is inclined forwards and rotated inwards, thereby bringing the tibial (preaxial) 

 condyle to the inner side, and the extensor aspect of the knee forwards. The rotation of the 

 humerus outwards in man amounts to about 45, so that the axis of the elbow-joint is placed 

 obliquely to the median plane, but in quadrupeds the rotation takes place through 90. and thus 

 the axis of the elbow-joint becomes transverse. The rotation inwards of the femur is about 90, 

 and the axis of the knee-joint is therefore placed transversely, in both man and quadrupeds. 

 In the upper limb of man, the radius being in semipronation, no material change occurs in 

 the position of the hand, the thumb hanging naturally forwards ; but in animals destined to 

 rest on the palmar aspect of the hand or digits, important changes occur in the position of the 

 radius by which, as this bone is brought forwards upon the humerus, and its lower end carried 

 inwards, the manus or its elements are placed permanently in the prone position, with the 



td. 

 tm 



Fig. 167. DORSAL SURFACE OF THE EIGHT 

 CARPUS OF MAN. (Flower.) 



s, scaphoid ; I, lunar ; c, cuneiform or pyra- 

 midal ; p, pisiform ; tm, trapezium ; id, tra- 

 pezoid ; m, magnum ; u, unciform ; I V, five 

 metacarpals. 



Fig. 168. DORSAL SURFACE OF THE CARPUS 



OF A BABOON. (Flower. ) 



s, scaphoid ; I, lunar ; c, cuneiform or pyra- 

 midal ; p, pisiform ; tm, trapezium ; tf) ', trape- 

 zoid ; m. magnum ; u, unciform ; rs, radial sesa- 

 moid ; ce. os cent-rale ; I V, five metacarpals. 



first or radial digit inwards. In the foot no such change is required, as already by the internal 

 rotation of the femur at its upper part, the conditions for plantar support have been secured, 

 and the first or tibial digit is on the inner side. Farther, in man, as the body attains its full 

 development, both limbs are extended in a line parallel to the axis of the trunk, the upper 

 dropping loosely from the shoulder-joint with the greatest freedom of motion ; the lower more 

 closely articulated in the hip- joint, and suited to give firm support to the body in the erect 

 posture. 



Hand and foot. The similarity of the digital and metacarpal bones of the hand 

 with those of the foot in number, form, and connections is so great that the homodynamous 

 correspondence of these bones is immediately recognised. The main differences between them 

 consist in the greater length of the fingers, and the opposability of the thumb to the 

 other digits through its mobility at the carpo-metacarpal articulation, in adaptation to the 

 function of the hand as a grasping organ. 



The morphological- construction of the carpus and tarsus is only to be understood by 

 reference to a more generalized form, such as is met with in the carpus of the water-tortoise 

 (fig. 166) and in the tarsus of certain amphibia. Here nine elements are seen, arranged in a 

 proximal series of three, which are named respectively radiale or tibiale, intermedium, and 

 ulnare or fibulare, a distal series of five, named catyjalia or tarsalia I V, counting from the 

 preaxial border, and an interposed centrale. 1 In man, as in nearly all mammals, the distal 

 row of the carpus and tarsus comprises only four bones, the unciform and cuboid, which 

 support the postaxial two digits, being generally regarded as formed by the fusion of the 

 fourth and fifth elements. Of the proximal elements of the carpus, the radiale is represented 

 in the scaphoid, the intermedium in the lunar, and the ulnare in the pyramidal. The centrale 

 appears to be absent in the carpus of man, although it occurs as a distinct bone in nearly all 

 apes. In the human foetus, however, at the latter part of the second month, a rudiment of 

 the centrale is present as a small cartilaginous nodule, which in the course of the third month 

 becomes fused with the cartilage of the scaphoid. 2 The latter bone must therefore be regarded 



1 Gegenbaur, " Untersuch. zur vergleich. Anat. ; I. Carpus und Tarsus," Leipzig, 1864. 



2 Leboucq, " Recherches sur la morphologic du carpe chez les mammiferes," Arch, de Biologie, 

 v, 35. 



