232 DISEASES OF BONE. 



78 and 79, especially when they are compared with Fig. 80, 

 which represents a cannon and splint bone free from splint. 



PREDISPOSITION. — Evolution plays a large part in the for- 

 mation of splints, which, certainly, were seldom if ever present 

 in the remote five-toed or even three-toed ancestors of the horse. 

 If we compare our own hand to the lower portion of a horse's fore 

 leg, we shall see that our wrist corresponds to his knee, and our 

 middle finger to his pastern and foot. His ancestral digits (fingers 

 or toes) which would correspond to our thumb, index finger, ring 

 finger and little finger, have entirely disappeared during the course 

 of evolution ; and the bones (metacarpal) between the 'knee and 

 the first digit (thumb) and fifth digit (little finger) have also 

 vanished. The metacarpal bones of his second digit (our index 

 finger) and fourth digit (our ring finger) remain, however, in the 

 more or less decadent state of splint bones. 



Splint bones are often called rudimentary bones, but that is an 

 evident misnomer. 



We all know that this diminution in the number of the 

 digits of the horse is due to gradual alteration of surroundings. 

 His early ancestors no doubt found an expansive foot of five, or 

 even three, digits, useful for sustaining the weight of their bodies 

 in the more or less marshy ground which these animals inhabited. 

 But as their descendants adopted harder " going " for feeding and 

 roaming purposes, the third digit of all four legs became more 

 useful, with the result of increase in its size, and of diminution in 

 the volume of its fellows. 



By an examination of our own hand, we can see that there are 

 muscles which are placed between our metacarpal bones (the 

 bones between the wrist and fingers), and consequently we are able 

 to slightly alter the width of the palm of the hand, and to 

 separate our fingers one from the other, or draw them together. 

 The possible amount of separation of the metacarpal bones and 

 their respective fingers varies in proportion to the distance from 

 the wrist. There is a somewhat similar condition in the metacarpal 

 interosseous muscles of the dog. Decrease of this lateral play (or 

 fanning-out action with the wrist as a pivot) of the metacarpal 

 bones, is accompanied by diminution of the distance at which 

 these bones are apart. This curtailment of space has reached 

 its maximum extent in the case of the ox's third and fourth 

 metacarpal bones, which are fused into the one bone that forms 

 his cannon bone; and a similar condition exists in his hind legs, 

 namely, in his metatarsal bones. At first glance, the cannon bone 

 of the ox might appear to be a single bone, but as Milne Edwards 

 (Gaudr)^'s " Enchainements du Monde Animal ") points out, it 



