292 VERTEBRATE SKELETON 



The fore arm is long in bats and a few rodents. Both ulna and 

 radius are present and usually well developed. The most important 

 articulation with the humerus is that of the ulna, which has a sig- 

 moid groove for the trochlea, and proximal to this is the olecranon, 

 (rudimentary in Ungulates) which makes the ulna longer than "the 

 radius. Both bones articulate, as a rule, with the carpus, but where 

 much rotation is possible, the radius is expanded distally, giving 

 a larger surface for articulation with the carpus, and it may have 

 a styloid process on its outer border. 



The ulna is reduced in several mammals belonging to different orders. Thus 

 in bats it is slender, and here, as in horses, camels and some Ruminants it may 

 fuse with the radius, its proximal part being lost in the horse, its distal part fusing 

 with the radius in bats, and little more than the olecranon remaining in 

 Ruminants. 



The number of carpal bones ranges from ten or eleven in the 

 embryo to as few as five, arranged in two rows, in some adults. This 

 reduction is due, in part, to the absence of one or more cartilages in 

 the embryo, more to the fusion of elements, and still more to loss 

 accompanying the loss of digits. Radiale, intermedium and ulnare 

 usually retain their individuaHty and are the larger carpal bones, 

 but radiale and intermedium often unite as a scapho-lunatiun. Some 

 embryos have two centraha, but usually only the radial of these is 

 present, while in some species no centrale is known. The ulnar 

 centrale may fuse with intermedium or with carpale 2 or 3 ; the radial 

 centrale commonly unites with the radiale. Except in*whales, car- 

 paha 4 and 5 unite as a hamatum. The fusions and reductions of 

 the carpal bones is greatest in Ungulates. 



The metacarpals are usually long and slender and undergo re- 

 ductions closely related to those of the digits. These latter vary 

 from the normal five, to one. Typically the phalangeal formula is 

 2, 3, 3, 3, 3, and only rarely, when all digits are present, fingers 2, 3 

 and 4 may have two phalanges each. The terminal phalanx is 

 often split or grooved on the dorsal side for the support of a claw. 



Besides these normal parts other bones may develop on both radial and ulnar 

 sides — prepollex and postminimus (p. 277), the most common being the pisiforme 

 on the ulnar side which in several rodents, may have two joints. The prepollex 

 is less common, except in digging or swimming species with broad hands. It is 

 two jointed in a few species. 



