456 ANATOMY OE VERTEBRATES. 



the distal end, K marks the trochlear surface for the radius, the 

 fore part of which bone, o, passes into the depression Z, when 

 fully flexed : k is the inner condyle, i the outer condyle ; m the 

 posterior fossa for the olecranon, when the antibrachium is ex- 

 tended. The ulna, represented by its olecranon, 5, and upper 

 part of the shaft, w, coalesces by the latter, in aged Horses, with 

 the radius : it presents a small articular surface, f, for the hu- 

 merus. The radius and ulna coalesce in Macrauchenia. The 

 equine carpus includes, in the proximal row, the scaphoid w, 

 lunare x, cuneiforme y, and pisiforme z, which latter is large and 

 prominent. The os magnum, 2, in the second series of carpal bones 

 is remarkable for its great breadth, corresponding to the enor- 

 mous developement of the metacarpal bone of the middle toe, 4, 5, 

 which forms the chief part of the foot. Splint-shaped rudiments 

 of the metacarj)als, answering to the second and fourth, 6, of the 

 pentadactyle foot, are articulated respectively to the trapezoides 

 and the reduced homologue of the unciforme, 3. The miocene 

 Hipparion retained stunted hoofs supported by the second and 

 fourth digits of the fore-foot, as in the hind-foot, fig. 194 : but all 

 modern and existing representatives of the genus Eqiius have the 

 digital developement concentrated on the medius : of which, in 

 fig. 190, 12-13 shows the proximal phalanx, called in Hippo- 

 tomy the ^ great pastern'; 14-15, the middle phalanx, called the 

 ^ small pastern'; 16, the ungual phalanx, called the ^coffin- 

 bone': 11 and 17 are ' sesamoids,' the latter being called the ^ nut- 

 bone.' 



The ilium of the Horse, fig. 300, 62, is longer and less ex- 

 panded superiorly than in the Tapir ; but it articulates by the 

 corresponding part to the sacrum, which renders it hammer- 

 shaped. The femur is characterized by the partial division of the 

 great trochanter, and, as in other Perissodactyles, has the third 

 trochanter. The medullary artery enters the middle of the shaft 

 at its postero-internal side, and inclines slightly upward. In fig. 

 195, a is the shaft, h the ' neck,' c the head ; d d, the great tro- 

 chanter, of which the upper division is called ' the spoke ; '/"is the 

 ^ third trochanter,' g marks the place of a deep fossa giving origin 

 to the gastrocnemius externus, h is the outer condyle. In the tibia, 

 s-io is the protuberance and ridge for the rotular ligament, v the 

 articular head of the bone, u the outer concavity. The distal end 

 is excavated by a deep oblique double trochlear cavity for the 

 astragalus, 5. The fibula is represented by its head, 1, and a 

 slender styliform portion of the shaft, ending in a point, at 2. 

 There is no representative of the distal end, as in Macrauchenia 



