84 LAND MAMMALS IN THE WESTERN HEMISPHERE 



ber which is never exceeded in any normal terrestrial mammal ; 

 the members are numbered from the inner side, beginning with 

 the thumb or pollex, from I to V. Many mammals have 

 fewer than five metacarpals, which may number four, three, 

 two or only one ; the third is never lost, but any or all of the 

 others may be suppressed, and functionless rudiments of them 

 may long persist as splints or nodules. The metacarpals are 

 elongate, relatively slender and of more or less cylindrical 

 shape ; but the form varies considerably in different groups, 

 according to the way in which the hand is used. When em- 

 ployed for grasping, as in many arboreal animals and pre- 

 eminently in Man, the pollex is frequently opposable to the 

 other fingers and enjoys much freedom of motion. In the 

 camels and true ruminants the third and fourth metacarpals 

 are coossified to form a cannon-hone (see Fig. 43, p. 91), but the 

 marrow cavities and the joints for the phalanges remain 

 separate. 



The phalanges in land mammals never exceed three in each 

 digit, except the pollex, which, when present and fully developed, 

 has but two. The phalanges are usually slender in proportion 

 to their length, but in very heavy hoofed animals they are short 

 and massive. The terminal joint is the ungual phalanx, which 

 carries the nail, claw, or hoof, its shape varying accordingly. 



The hind leg is constituted in very much the same manner 

 as the fore, but with certain well-marked and constant dif- 

 ferences. The thigh-bone, or femur, is usually the longest and 

 stoutest of the limb-bones and in very large animals may be 

 extremely massive. At the upper end is the hemispherical 

 head, which is set upon a distinct neck and projects inward and 

 upward, fitting into the acetabulum of the hip-bone. Nearly 

 all land mammals have a small pit on the head of the femur, 

 in which is inserted one end of the round ligament, while the 

 other end is attached in a corresponding depression in the 

 floor of the acetabulum. This ligament helps to hold the thigh- 

 bone firmly in place and yet allows the necessary freedom of 



