THE ENDOSKELETON: GIRDLES, THE STERNUM, AND APPENDAGES 85 



front of the astragalus is the navicular or scaphoid (centrale), a curved bone 

 reaching to the medial side of the foot. Directly in front of the calcaneus is the 

 cuboid (fourth and fifth tar sales fused), which articulates with the fourth and fifth 

 metatarsals. Medial to the cuboid is the third or lateral cuneiform (third 

 tar sale), articulating with the third metatarsal. 

 Medial to this is the second or intermediate 

 cuneiform (second tar sale), articulating with the 

 second metatarsal. In the cat there is a first or 

 medial cuneiform (first tarsale) along the medial 

 border of the anterior part of the ankle in front 

 of the navicular. It articulates with the small 

 rudimentary first metatarsal which lies directly in 

 front of it. In the rabbit the first cuneiform is 

 fused to the proximal end of the second meta- 

 tarsal. The homology of these ankle bones with 

 those given in the primitive vertebrate plan (Fig. 

 24) is quite evident. The sole consists of four 

 long metatarsals and one rudimentary one (the 

 first) on the medial or ventral side of the proximal 

 end of the second metatarsal. The terminal 

 phalanges of the digits are curiously beak shaped, 

 for the support of the horny claws, and in the cat 

 have sheaths at their bases into which the bases 

 of the horny claws fit. 



The joint between foot and shank in mam- 

 mals is between the ankle bones and the malleoli 

 of the tibia and fibula, unlike the condition seen 

 in birds and reptiles, where the joint lies between 

 the distal and proximal ankle bones. The gait 

 of the cat and rabbit is chiefly digitigrade, al- 

 though the hind legs assume the plantigrade pos- 

 ture when the animal sits down. 



Attention was called in Section HI of the 

 manual to the torsion which the mammalian 

 hind limb has undergone with the result that 

 the toes, originally pointing laterally as in Nec- 



turus now point anteriorly. The statements made at that place should be 

 reviewed here. 



C. THE PECTORAL GIRDLE, THE STERNUM, AND THE ANTERIOR 



PAIRED APPENDAGES 



i. The origin of the sternum. Since the origin of the pectoral girdle and appendages is the 

 same as that of the pelvic girdle and appendages, it remains to discuss the formation of the 



sternum 



FIG. 26. Diagrams to illustrate 

 the theory of the origin of the ster- 

 num from the pectoral girdle. A, 

 pectoral girdle of an elasmobranch. 

 B, central portion of the girdle 

 beginning to separate. C, central 

 portion of the girdle completely 

 separated, forming the sternum. 



