THE SKELETON. 81 



simplicity; as both may be regarded as modifications of the 

 same original structure, they are homologous. The pelvic 

 girdle clearly corresponds generally to the pectoral arch, the 

 tibia and fibula to the radius and ulna; the five metatarsal 

 bones to the five metacarpal, and the phalanges of the toes to 

 those of the thumb and fingers. On the other hand, there is 

 in the arm no separate bone at the elbow-joint corresponding 

 to the patella at the knee, but the ulna bears above a bony 

 process, the olecranon (0, Fig. 35), which at first is a separate 

 bone and is the representative of the pafcella. There are in 

 the carpus eight bones and in the tarsus but seven. The 



FIG. 36. Diagram showing the relation of the pectoral arch to the axial 

 skeleton. 



astragalus of the tarsus (Ta, Fig. 38) represents, however, two 

 bones which have grown together. The elbow-joint bends 

 ventrally and the knee-joint dorsally. 



Comparing the limbs as a whole, greater differences come 

 to light, differences which are 

 mainly correlated with the dif- 

 ferent uses of the two limbs. 

 The arms, serving as prehensile ,A 

 organs, have all their parts as 

 movable as is consistent with 

 the requisite strength, while 



,1 , TIT- i FIG. 37. Diagram showing the at- 



tne lower limbs, having to bear tachment of the pelvic arch to the axial 



the whole weight of the Body, s 



require to have their parts much more firmly knit together. 

 Accordingly we find the shoulder-girdle, represented red in 

 the diagram (Fig. 36), only directly attached to the axial 

 skeleton by the union of the inner ends of the clavicles with 

 the sternum, and capable of considerable independent move- 

 ment, as seen, for instance, in " shrugging the shoulders." 



