Considered as predisposing causes, hereditary influences play an important role and may, owing to faulty conformation, subject an animal to affections of this kind because of disproportionate development of parts (weak and small joints and heavy muscular hips); or as a consequence of inherited traits, a subject may manifest susceptibility to degenerative bone changes which are signalized by the formation of exostoses of different parts on one or more of the legs.  Hereditary predispositions make for the presence of spavin in a large percentage of the progeny of sires so affected.  This fact has been repeatedly demonstrated in this country as well as elsewhere according to Quitman, Dalrymple and Merillat.[51] A number of states have passed stallion inspection laws stipulating that animals having such exostoses as spavin and ringbone cannot be registered except as “unsound.”

Asymmetrical conformation, particularly where the hock is obviously small and weak as compared with other parts of the leg, constitutes a noteworthy predisposing cause.

Peters’ theory is plausible that the screw-like joint between the tibia and the tibial tarsal (astragulus) bones causes these structures to functionate in a manner not in harmony with the provisions allowed by the collateral ligaments of the tarsus, permitting movement only in a direction parallel with the long axis of the body.

Because of the quality of their temperaments, nervous animals possessing no particular congenital structural defects of the hock and having no history of spavined progenitors, are subject to spavin when kept at work likely to produce tarsal sprain.  Spavin usually develops early in such subjects and examples of this kind may be frequently observed in agricultural sections of the country.  Where spavin develops in unshod colts at three and four years of age, shoeing is not an influencing agency when animals are not worked on pavements.

Exciting causes of spavin are sprain and concussion.  Various hypotheses are recorded as to how sprains are influenced and among others may be mentioned that of McDonough[52], which is that the foot is robbed of its normal manner of support by the ordinary three-calked shoe.  With such a shoe, little support is given the sides of the foot; hence, undue strain is put upon the collateral ligaments of the tarsus.  Moreover, the shoe with its calks increases the length of the leg and adds to the leverage on the hock, by virtue of such added length.  This makes for greater strain upon the mesial or lateral tarsal ligaments whenever the foot bears upon a sloping ground surface, so that one side (inner or outer) is higher or lower than the other.  But according to McDonough’s theory (a good one concerning horses that work on pavements), the chief error in shoeing lies in that the foot is deprived of its normal base or support on the sides—­the three-calked shoe being an unstable support—­and that this manner of shoeing city horses working on pavements is an “inhumane” practice, a “diabolical method.”