THE SKELETAL SYSTEM 



253 



is the navicular. Each of the seven tarsals has six surfaces. The tibia 

 rests upon and articulates with the talus. The largest bone of the foot is 

 the calcaneus, which articulates with the talus and cuboid. To the 

 posterior tuberosity of the calcaneus is attached the tendon of Achilles. 

 Since the calcaneus acts as a lever with the talus as a fulcrum, it is not 

 surprising that the size of the calf muscle which provides the power varies 

 inversely with the length of the heel bone. 



Metatarsus and Phalanges. The metatarsal and phalanx bones 

 differ little from the corresponding bones of the hand, except that the 



NEURAL PROCESS! 



Fig. 210. — Stages in the development of the appendicular skeleton of man. A, Left 

 lateral aspect of arm in 11 mm. embryo; B, left lateral aspect of arm in 16 mm. embryo; 

 C, left lateral aspect of arm in 20 mm. embryo; D, left lateral aspect of leg in 11 mm. 

 embryo; E, left lateral aspect of leg in 14 mm. embryo; F, left lateral aspect of leg in 

 20 mm. embryo. (Redrawn after Bardeen and W. H. Lewis from Keibel and Mall.) 



phalanges are considerably shorter in the foot and the great toe has much 

 less freedom of movement than the thumb. 



Homologies of the Limb Bones. The striking similarity of the bones 

 of the upper and lower limbs, notwithstanding their great diversity of 

 function, is interpreted by morphologists as indicating a primary similarity 

 in use. Their present differences in form, size, and function have arisen 

 secondarily and adaptively. 



Development of the Appendicular Skeleton. The paired appendages 

 of vertebrates arise from two Wolffian folds, which extend along the 

 sides of the embryo at approximately the level where the hypomere 

 connects with the mesomere. Only the end portions of these folds, 



