100 THE SKELETON OF THE HORSE 
proximal extremity unites with the shaft before birth, the distal extremity toward 
the middle of the second year. The small metacarpal bones ossify from two cen- 
ters, one of which is for the proximal extremity. Their distal ends are cartilaginous 
at birth. Fusion of the middle part of the shaft with the large metacarpal bone is 
common. 
THE PHALANGES 
THE FIRST PHALANX 
The first phalanx (Phalanx prima)! is a long bone, and is situated between the 
large metacarpal bone and the second phalanx. It is directed obliquely downward 
and forward, forming an angle of 50 to 55 degrees with the horizontal plane in well- 
formed limbs. It consists of a shaft and two extremities. 
Distal end of small meta- 
carpal bone 
Distal end of large metacarpal bone 
Proximal end of first phalanx 
Proximal sesamoid bone 
Distal sesamoid bone 
Dorsal groove 
Angle 
Distal border of third phalanx 
Fic. 78.—SKeE.teTron OF Dicit AND Distat Part or Meracarpus oF Horse; LATERAL VIEW. 
1-7, Eminences and depression for attachment of ligaments. Cartilage of third phalanx is removed. 
The shaft (Corpus) is wider and much thicker above than below, and presents 
two surfaces and two borders. The dorsal surface is convex from side to side and 
smooth. The volar surface is flattened, and bears a triangular rough area, bounded 
by ridges which begin at the proximal tuberosities and converge distally; this area 
furnishes attachment to the distal sesamoidean ligaments. The borders, medial 
and lateral, are rounded and have a rough area or a tubercle on their middle parts. 
The proximal extremity (Extremitas proximalis s. basis) is relatively large. 
It bears an articular surface adapted to the distal end of the large metacarpal bone, 
1 This bone is also called the large pastern bone or os suffraginis. 
