28 OSTEOLOGY 



such as the horse and ox, in which the foreUmbs are used only for support and locomotion, the 

 clavicle is absent. Other animals which use these limbs for grasping, burrowing, climbing, etc. 

 (e.^.,man, apes, moles), have well-developed clavicles which connect the scapula with the sternum. 



The arm (Brachium) contains a single long bone, the humerus or arm bone. 



In the forearm (Antibrachium) are two long bones, the radius and ulna. 

 These vary in relative size and mobility. In the horse and ox the two bones are 

 fused, and the lower part of the limb is fixed in the position of pronation. The 

 radius is placed in front and supports the weight. The ulna is well developed only 

 in its upper part, which forms a lever for the extensor muscles of the elbow. In 

 the pig the ulna is the larger and longer of the two bones, but is closely attached to 

 the back of the radius. In the dog the ulna is also well developed and a small 

 amount of movement is possible between the two bones. 



The forefoot or hand (Manus) consists of three subdivisions, viz., the carpus, 

 metacarpus, and digit or digits. 



The carpus, popularly termed the "knee" in animals, and homologous with 

 the wrist of man, contains a group of short bones (Ossa carpi). These are typically 

 eight in number and are arranged in two transverse rows — a proximal or anti- 

 brachial, and a distal or metacarpal. The bones of the proximal row, named from 

 the radial to the ulnar side {i. e., from within outw^ard), are the radial, intermediate, 

 ulnar, and accessory carpal bones. The bones of the distal row are designated 

 numerically, in the same direction, as first, second, third, and fourth carpal bones. 



This nomenclature, introduced by Gegenbaur, and now used largely by comparative anat- 

 omists, seems decidedly preferable to the variety of terms borrowed from human anatomy and 

 based on the form of the bones in man. The following table of synonyms in common use is ap- 

 pended for comparison. The Latin terms and abbreviated notations are given in parenthesis. 



Radial (Os carpi radiale, Cr) Scaphoid 



Intermediate (Os carpi intermedium, Ci) Semilunar 



Ulnar (Os carpi ulnare, Cu) Cuneiform 



Accessory (Os carpi accessorium, Ca) Pisiform 



First carpal (Os carpale primum, CI) Trapezium 



Second carpal (Os carpale secundum, C2) Trapezoid 



Third carpal (Os carpale tertium, C3) Os magnum 



Fourth carpal (Os carpale quartum, C4) Unciform 



The central carpal bone (Os carpi centrale) is omitted, since it is not a separate element in 

 the animals under consideration here. 



The metacarpus contains typically five metacarpal bones (Ossa metacarpalia 

 I-V), one for each digit; they are long bones and are designated numerically from 

 within outward. This arrangement occurs in the dog, although the first meta- 

 carpal is much smaller than the others, and the second and fifth are somewhat 

 reduced. Further reduction has taken place in the other animals, resulting in 

 the perissodactyl and artiodactyl forms. In the horse the first and fifth metacarpals 

 are absent, the third is the large supporting metacarpal bone and carries the single 

 digit, while the second and fourth are nmch reduced. In artiodactyls {e. g., ox, 

 sheep, pig) the third and fourth are the chief metacarpals and carry the well 

 developed digits; they are fused in the ox and sheep. The others are variously 

 reduced or absent as noted in the special descriptions to follow. 



The fossil remains of the ancestors of the existing Equida? illustrate in a most complete man- 

 ner the reduction which has occurred in this respect. The earhest known ancestor of the horse, 

 Eohippus or Hyracotherium of the Lower Eocene, had four well developed metacarpal bones, 

 each of which carried a digit; the first metacarpal bone was small. Eleven intermediate stages 

 show the gradual evolution of the race from this primitive animal, which was about the size of 

 the domestic cat. There is reason to believe that earlier forms had five digits. 



The digits (Digiti manus) are homologous with the fingers of man, and are 

 typically five in number. They are designated numerically from the radial to 

 the ulnar side, in correspondence with the metacarpus. The full number is present 

 in the dog. In the ox and pig the third and fourth are well developed and support 



