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 earpi radiale, Cr). : Mere cretactha nt caret . Seaphoid 
Intermediate (Os carpi intermedium, Ci) desi daa cdarskacbiaaPreoes .. Semilunar 
Ulnar (Os carpi ulnare, Cu)... ... tea nace teenie dae .... Cuneiform 
Accessory (Os earpi accessorium, Ca). . ks haa © oe Sere eusif orm 
First carpal (Os carpale primum, C1) fee. ite ..... Trapezium 
Second carpal (Os carpale secundum, C2) a ..... Trapezoid 
Third carpal (Os carpale tertium, C3) Os magnum 
Fourth earpal (Os carpale quartum, C4) 3 .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 (7. e., from within outward). This arrangement occurs 
in the dog, although the first metac arpal 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 deseriptions to 
follow. 
The fossil remains of the ancestors of the existing Equide 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 pentadaectyl 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 prime) 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 tertiz) 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. Ace 
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 (syndactylism) 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 pelvine) consists of the os coxe (or 
