160 



COMPARATIVE ANATOMY 



As the function of the limb is now no longer simply to propel 

 the body, but also to lift it up from the ground, the firmly con- 

 nected elements of its skeleton are placed at an angle to one 

 another (elbow and knee, in which the angle is directed backwards 

 and forwards respectively), definite articulations being formed 

 between them in a proximo-distal direction. The fore-limb serves 

 in typical cases mainly for pulling and the hind-limb for pushing 

 the body along the ground, and on this fact depend the various 

 differences between the two as regards their relation as a whole to 



the trunk and of their various parts 

 to one another. Instead of project- 

 ing horizontally outwards, the limb 

 extends downwards, and thus the 

 angle between it and the median 

 plane of the trunk is gradually re- 

 duced, until in Mammals eventually, 

 the longitudinal axis of the limb, 

 when at rest, is parallel with the 

 median plane of the body. In the 

 higher types this is more particularly 

 the case as regards the posterior 

 THE extremities, the anterior limbs under- 

 THE going the most varied adaptative 

 modifications, and giving rise to 

 prehensile or to flying organs or, 

 as in aquatic Mammals, becoming 

 once more converted into paddles. 

 The fore-limbs and hind-limbs of all 

 Vertebrates above Fishes may, how- 

 ever, be reduced to a single ground- type. 



A division into four principal sections can always be recog- 

 nised : in the case of the fore- limb these are spoken of as upper 

 arm (Irachium), fore-arm (antibrachiuni), wrist (carpus), and hand 

 (mantis') ; and in the hind-limb as thigh (femur}, shank (cms), 

 ankle (tarsus), and foot (pcs) (Fig. 123). The bone of the upper 

 arm (humerus'), like that of the thigh (femur) is always unpaired, 

 but two bones are present in the fore-arm and shank. The former 

 are called radius and ulna, and the latter tibia and fibula. The 

 hand and foot are also respectively divisible into two sections, a 

 proximal metacarpus and metatarsus, and a distal series of 

 phalanges, which form the skeleton of the fingers and toes (digits). 

 Both manus and pes are made up of several series of cylin- 

 drical bones. There are never more than five complete series, 

 which except as regards number present essentially similar 

 primary relations throughout the higher Vertebrates. The 

 skeleton of the carpus and tarsus, each of which typically consists 

 of a series of small cartilages or bones, shows much variation ; but 

 the following arrangement may be taken as typical (Fig. 123). 



FIG. 122. DIAGRAMMATIC FIGURES 

 TO SHOW THE RELATIONS OF 

 ANTERIOR EXTREMITY TO 

 TRUNK IN FISHES (A), AND THE 

 HIGHER VERTEBRATES (B). 



Mt, metapterygium ; I'd, radialia 

 in A, radius in B ; S, pectoral 

 arch : Ul, ulna ; proximally to Ul 

 and Ed is the humerus. 



arrangement 



