238 OUTLINES OF EVOLUTIONARY BIOLOGY 



affords support to the five metacarpal bones in the palm of the 

 hand or the five corresponding metatarsal bones in the foot. Each 

 metacarpal or metatarsal bone in turn forms the support of a 

 series of phalanges which constitute the skeleton of the fingers or 

 toes. The thumb and great toe have each two phalanges and 

 each of the other digits has three. 



When the human arm is stretched out at right angles to the 

 long axis of the body, with the thumb towards the head, it is in 

 the primitive position of the pentadactyl limb. The thumb and 

 the radius then lie on what is called the preaxial or anterior border, 



FIG. 94. Skeleton of the fore Limb (A) and hind Limb (B) of Man, showing 

 the pentadactyl Structure. (From photographs.) 



c. carpals ; fe. femur; fi. fibula: hum. humerus; m.c. metacarpals; mt. metatarsals ; 

 ph. phalanges; r. radius; t. tarsals ; ti. tibia; ul. ulna; I V, digits. 



the little finger and the ulna on the postaxial or posterior border 

 of the limb. For convenience of comparison with other types 

 the digits are numbered from in front backwards, the thumb 

 being No. I and the little finger No. V. Similarly in the hind 

 limb, which in man has become permanently twisted out of its 

 primitive position, it is easy to show that the great toe is really 

 the preaxial digit (No. I) and the little toe the postaxial (No. V). 

 In Fig. 94 both limbs are represented in the primitive position. 



Starting from the primitive pentadactyl type let us next inquire 

 more closely how different groups of air-breathing vertebrates have 

 solved the problems of locomotion on land, in air and in water. 



In considering the modifications of the limbs adapted for 



