58 THE SURGICAL ANATOMY OF THE HORSE 



the fact that the inferior extremity of the bone " springs " on pressure, 

 owing to the slight interval which exists between it and the large meta- 

 carpal. This is a point of considerable importance, since the button of 

 the small metacarpal bone is by some operators taken as a guide to the 

 seat of operation in plantar neurectomy. 



The inner small metacarpal is thicker and more powerful than the outer. 



Each bone usually ossifies from a single centre, but at times a separate 

 centre is present for the head. 



FRACTURE OF THE METACARPAL BONES 



It will be remembered from our superficial examination and dissection 

 that these bones are particularly exposed to risk of injury, since they are 

 for the greater part subcutaneous, and are not clothed by soft structures. 

 For the same reason fractures of the metacarpal bones are very frequently 

 compound, since the fractured bones readily pierce the skin. 



In complete fracture all three bones are usually involved, since in the 

 mature animal the small metacarpals are firmly and closely attached to 

 the large. It rarely happens that one or other of the small bones is 

 fractured alone. A blow received over the lower extremity may, how- 

 ever, cause the inferior free end, i.e., the button, and about an inch of 

 the shaft of the bone above it, to become snapped off ; but this is a matter 

 which is not of serious moment, for, failing reunion of the severed ends, 

 the broken pieces of bone may be easily removed. 



Occasionally we have a deferred fracture of the large metacarpal 

 bone. Another theory as to the completion of such a partial fracture 

 is that it is brought about by pressure of the inflammatory exudate 

 which is poured out into the line of partial fracture. 



Fractures are usually the result of blows, kicks, or falls, and in 

 colliery ponies are frequently caused by the animals being struck by a 

 passing coal tub. 



