644 MANUAL OF ZOOLOGY. 



brachial or ulnar artery are protected on their way down the 

 arm by a canal placed a little above the elbow, and formed by 

 a process the " supra- condyloid" process which is some- 

 times present in man as an abnormality. 



In the fore-arm of all Mammals the ulna and radius are re- 

 cognisable, but they are not necessarily distinct ; and the radius, 

 as being the bone which mainly supports the hand, is the only 

 one which is always well developed, the ulna being often rudi- 

 mentary. In the Cetacea the ulna and radius are anchylosed 

 together; and in most of the Hoofed Quadrupeds they are 

 anchylosed towards their distal extremities. In the flying 

 Mammals or Bats the ulna is hardly recognisable, being re- 

 duced to its upper third and fused with the radius. The fore- 

 arm attains its greatest perfection in man, in whom the radius 

 can rotate upon the ulna, so as to allow the back of the hand 

 to be placed upwards or downwards, these movements being 

 known respectively as " pronation " and " supination." In the 

 Monkeys only is there any approach to this power of rotation. 



The fore-arm is succeeded by the small bones which com- 

 pose the wrist or "carpus." These are eight in number in 

 Man, but vary in different Mammals from five to eleven. 



The metacarpus in Man and in most Mammals consists of 

 five cylindrical bones, articulating proximally with the carpus, 

 and distally with the phalanges of the fingers. The most re- 

 markable modification of this normal state of things occurs in 

 the Ruminants and in the Horse. In the Ruminants, in which 

 the foot is cleft, and consists of two perfect toes only, there are 

 two metacarpal bones in the embryo; but these are anchylosed 

 together in the adult, and form a single mass which is known 

 as the "canon-bone" (fig. 354, c\ In the Horse, in which the 

 foot consists of no more than a single digit, there is only a 

 single metacarpal bone, on each side of which are two little 

 bony spines the so-called " splint-bones " which are at- 

 tached superiorly to the carpus, and are to be regarded as 

 rudimentary metacarpals. In most of the other Ungulates 

 there are at least three metacarpals, and in the elephants there 

 are five. 



The normal number of digits is five, but they vary from one 

 to five. The middle finger is the longest, and most persistent 

 of the digits of the fore-limb ; and in the Horse it is the only 

 one which is left (fig. 355). The thumb is very frequently 

 absent. In the Ruminants there are only two fingers which 

 are functionally useful, these carrying the hoofs. In most 

 Ruminants, however, there are two rudimentary and function- 

 ally useless digits in addition. 



