DISEASES OP THE FETLOGK, ANKLE, AND FOOT. 419 



a healthy horn by which the injury may be repaired. Heavy cart 

 horses are more often affected than those of any other class. 



Causes. — ^The essential element in the production of canker is the 

 parasite; consequently the disease may be called contagious. As in 

 all other diseases due to specific causes, however, the seeds of the dis- 

 order must find a suitable soil in which to grow before they are 

 reproduced. It may be said, then, that the conditions which favor 

 the preparation of the tissues for a reception of the seeds of this 

 disease are simply predisposing causes. 



The condition most favorable to the development of canker is 

 dampness — in fact, dampness seems indispensable to the existence 

 and growth of the parasite; the disease is rarely, if ever, seen in 

 high, dry' districts, and is much more common in rainy than in dry 

 seasons. Filthy stables and muddy roads have been classed among 

 the causes of canker ; but it is very doubtful whether these conditions 

 can do more than favor a preparation of the foot for the reception 

 of the disease germ. 



All injuries to the feet, by exposing the soft tissues, may render the 

 animal susceptible to infection ; but neither the injury nor the irrita- 

 tion and inflammation of the tissues which follow are sufficient to 

 induce the disease. 



For some unknown reason horses with lymphatic temperaments — 

 thick skins, 'flat feet, fleshy frogs, heavy hair, and particularly with 

 white feet and legs — are especially liable to canker. 



Symptoms. — ^Usually, canker is confined to one foot; but it may 

 attack two, three, or all of the feet at once ; or, as is more commonly 

 seen, the disease attacks first one then another, until all may have 

 been successively affected. When the disease follows an injury. which 

 has exposed the soft tissues of the foot, the wound shows no tendency 

 to heal, but instead there is secreted from the inflamed parts a pro- 

 fuse, thin, fetid, watery discharge, which gradually undermines and 

 destroys the surrounding horn, until a large part of the sole and frog 

 is diseased. The living tissues are swollen, dark colored, and covered 

 at certain points with particles of new, soft, yellowish, thready horn, 

 which are constantly undergoing maceration in the abundant liquid 

 secretion by which they are immersed. As this secretion escapes to 

 the surrounding parts, it dries and forms small, cheesy masses com- 

 posed of partly dried horny matter, exceedingly offensive to the sense 

 of smell. When the disease originates independently of an injury, 

 the first evidences of the trouble are the offensive odor of the foot, 

 the liquid secretion from the cleft and sides of the frog, and the 

 rotting away of the horn of the frog and sole. 



In the earlier stages there is no interference with locomotion, but 

 later the foot becomes sensitive, particularly if the animal is used on 



