30 OSTEOLOGY 



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

 animals under consideration here. 



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 rnagnum 



Fourth carpal (Os carpale quartum, C4) Unciform 



The metacarpus contains typically five metacarpal bones (Ossa metacarpalia 

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

 the radial to the ulnar side (i. e., from within outward). This arrangement occurs 

 in the dog, although the first metacarpal 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 meta- 

 carpal bone and carries the single digit, while the second and fourth are much re- 

 duced. 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 Equidae illustrate in a most complete man- 

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

 Eohippus of the Lower Eocene, had four well developed metacarpal bones, each of which carried 

 a digit; the first metacarpal bone was small. Intermediate forms 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 

 the weight, while the second and fifth are reduced. The existing horse has a single 

 digit, the third of his pentadactyl ancestors. The skeleton of each fully developed 

 digit consists of three phalanges and certain sesamoid bones. The first phalanx 

 (Phalanx prima) articulates with the corresponding metacarpal bone above and 

 with the second phalanx (Phalanx secunda) below. The third phalanx (Phalanx 

 tertia) is inclosed in the hoof or claw, and is modified to conform to the latter. The 

 sesamoid bones (Ossa sesamoidea) are developed along the course of tendons or in 

 the joint capsules at points where there is increased pressure. Two proximal 

 sesamoids (Ossa sesamoidea phalangis primse) occur at the flexor side of the meta- 

 carpo-phalangeal joint and form a pulley for the flexor tendon. The distal sesa- 

 moid (Os sesamoideum phalangis tertite) is similarly placed between the deep 

 flexor tendon and the joint between the second and third phalanx; it is absent in 

 the dog, which has a small sesamoid on the extensor side of the metacarpo-phalan- 

 geal joint, and often at the proximal interphalangeal joint also. 



Numerous cases are recorded of the occurrence of supernumerary digits (hyperdactylism) 

 in the horse and other animals. In some pigs, on the other hand, the two chief digits are fused, 

 and the condition (syndactyUsm) appears to be inherited. 



THE BONES OF THE PELVIC LIMB 



The pelvic limb (Extremitas pelvina), like the thoracic, consists of four seg- 

 ments, viz., the pelvic girdle, thigh, leg, and the pes; the last is subdivided into 

 tarsus, metatarsus, and digits. 



The pelvic girdle (Cingulum extremitatis pelvinae) consists of the os coxae (or 



