332 ZOOLOGY 



The limbs 5. The limbs originate as paired fins. In the limbs 

 of fishes we find a series of bones, supporting a large 

 number of rays. In the terrestrial vertebrates the 

 number of parts is reduced and, as it were, stereotyped, 

 so that five is the typical and maximum number of toes 

 or digits. In the frog the hind foot has a rudimentary 

 sixth toe, a relic of the earlier condition when these parts 

 exceeded five. Occasionally in man and other animals 

 an extra digit appears as an abnormality. In the horse, 

 on the other hand, there is only a single functional digit 

 on each foot, the enlarged toenail being the hoof. 



The anterior In the anterior limb the first long bone, articulating 

 with the scapula, is the humerus. It is followed by two 

 less robust bones, side by side, the inner being the radius, 

 the outer or posterior the ulna. We commonly feel our 

 pulse in the radial artery, close to the lower (distal) end 

 of the radius. In the wrist is a group of small bones, 

 collectively known as the carpus. The more primitive 

 carpus (as in the turtles) consists of a central bone, the 

 os centrale, three basal bones, and five apical, the last 

 standing at the bases of the five digits. In man the 

 three basal bones are preserved, but the centrale has 

 disappeared, and the fourth and fifth of the apical row 

 have united to form the unciforme. The accessory 

 pisiform (pealike) bone has nothing to do with the primi- 

 tive carpus. 



Following the carpus is the series of five digits. The 

 first bones (in ourselves supporting the palm of the 

 hand) are called metacarpals ; the others are the pha- 

 langes. 



The In the posterior limb we have corresponding parts : 



the first long bone is the femur ; then follow the tibia and 

 fibula (the tibia being the stout shin bone). The small 

 bones of the ankle are collectively called the tarsus, and 



