134 PHYSIOLOGY OF FARM ANIMALS [CH. 



into two portions corresponding to the two digits (the 3rd and 

 ith, the horse having only one digit, the 3rd). The cleft between 

 is called the inter-digital space. 



Exostoses. 



When the fore leg of a horse descends upon the ground it is 

 necessarily straight; otherwise the foot could not be put down 

 flat, or heel first, as happens in fast pacers. Since the limb is 

 rigid (the knee being quite straight) the force of concussion is 

 greatest nearest the ground where the impact occurs, and 

 gradually diminishes as it ascends the leg. As has been shown 

 above, there are numerous devices present in the foot for reducing 

 the shock caused by the impact, and not the least of these are 

 the presence of the foot-pad or frog, the laminae of the hoof, and 

 the yielding articulations of the foot and fetlock joints. In 

 the case of the hind limb the shock of concussion is partly 

 provided against by other means. Here instead of the limb being 

 straight, it is bent at the hock, and the impact is felt more 

 especially at this point. 



Since these facts have a direct bearing on the causes of the 

 various sorts of exostoses or pathological outgrowths of bone 

 which are so common in horses under domestication, it is not 

 out of place to give some accoimt of them in dealing with the 

 physiology of locomotion. 



In view of the facts stated above it is not surprising that the 



hock is more frequently affected with disease than is the knee. 



In the case of bog spavin there is no bony outgrowth, but merely 



an enlarged condition due to the distention of the joint capsule 



by an abnormal quantity of synovial fluid which collects there 



in response to strain. It occurs most commonly in cart-horses, 



and especially in Clydesdales, whose hocks stand out far behind. 



It is generally the result of severe exertion or overstrain, but it 



does not as a rule cause lameness. In bone spavin or true spavin 



there is a genviine exostosis usually on the internal side of the 



hock joint. This growth of bone is the result of inflammation, 



but the precise point of origin of the inflammation (whether it is 



in the articular cartilage or in the membrane covering the ends 



of the bones or elsewhere) is not clear. Spavin nearly always 



causes lameness. It is frequently followed by anchylosis ^ of one 



1 Fusion of two or more bones which normally are capable of separate 

 mpvement. 



