76 THE SURGICAL ANATOMY OF THE HORSE 



invaded. He considers spavin to consist in an ostitis rarefaciens et 

 condensans." 



We have indicated what is the common, in fact the usual, seat of 

 spavin. But cases have been recorded where the seat has been the 

 external aspect of the hock, with a history exactly coinciding with that 

 of spavin in the usual situation. Several theories have been put forward 

 in explanation of the greater frequency of the occurrence of spavin in the 

 position stated. The one which appears to be the most satisfactory, and 

 is the most generally accepted, attributes it to the arrangement of the 

 bones of the limb, claiming that the oblique disposition of the long axis 

 of the tibia downwards, backw^ards, and inwards causes, during the 

 necessarily slight rotation of the tibio-astragaloid joint, greater pressure 

 to be thrown upon the inner side of the joint than the outer. 



Hence it is stated that spavin is found most frequently in animals in 

 which the pelvis is broad, in consequence of the greater degree of obliquity 

 taken by the shaft of the tibia. This applies to the effect on the rotation 

 in the hock. On the other hand, the articular surfaces of the bones 

 are more likely to be injured by concussion when the degree of inclination 

 of the long axis of the tibia to the horizontal axis of the tarsus approaches 

 too nearly the perpendicular. 



Regarding the conformation of the hock itself, there is considerable 

 divergence of opinion as to the particular kind of hock which is most 

 susceptible to the disease. Most maintain that animals with sickle or 

 cow hocks are very susceptible. In this connection it is interesting to 

 note the comparative shortness of the flexor metatarsi in relation to the 

 length of the limb. Even in animals with the tibia disposed less obliquely 

 than usual this muscle appears too short, and is generally in a condition 

 of tension, a point which becomes very evident in cases of rupture of this 

 muscle. In sickle-hocked animals, then, there must be considerable 

 pressure thrown upon the articular surfaces of the small bones when 

 either the flexor metatarsi or its antagonist, the gastrocnemius, is acting. 

 ^' Tied-in hocks," in which the heads of the metatarsal bones, and the 



