206 DISEASES OF THE FEET. 



advances, the Haversian canals increase at the expense of the bone surround- 

 ing them, and when they reach a sufficient size they can be seen to contain 

 a small quantity of pink granulation tissue surrounding the vessel. If a 

 portion of the bone at this stage be macerated, it presents a porous, spongy 

 appearance. Here we have a case of what we may call caries. If pus bs 

 present, we shall have ulceration of bone. Owing to the presence of in- 

 flammation in the substance of the bone or in the cartilage, small nodules, 

 with or without ulceration, form on the lower surface of the navicular bone, 

 over which the perforans tendon plays, and there is destruction of the 

 cartilage of the joint. Apparently owing to the roughness of the once 

 smooth gliding surface, inflammation is set up in the opposing portion of 

 the perforans tendon, and in the synovial bursa, which lies between the 

 tendon and the affected surface of bone, so that the tendon, in old cases, 

 becomes more or less worn through. In the further progress of the disease, 

 the weakened navicular bone may become fractured by the pressure of the 

 tendon on its lower surface ; the tendon may fray out to such an extent 

 that it will break in two ; it may become adherent to the navicular bone ; 

 or the abraded portion of tendon may continue to work over its rough 

 puUey with great discomfort to the animal, which in any case will suffer 

 more or less pain from movement. Naturally, a long rest will favour union 

 between the diseased bone and the abraded tendon. 



' The (lower) surface over which the perforans tendon plays, is the only 

 surface of the navicular bone wHoli becomes affected in this disease. The 

 other two surfaces, wliich form a joint with the pedal bone and short 

 pastern bone (Figs. 61 and 70) always remain free from morbid change 

 brought on by this malady. 



On referring to Fig. 70, we see that the perforans tendon is attached to 

 the base of the pedal bone, and passes behind the navicular bone, which 

 forms a pulley for it. On leaving the navicular bone, this tendon passes 

 behind the pastern, fetlock joint, suspensory ligament, and knee, and is, 

 tinally, united to the muscle, by the contraction of which the foot is flexed. 

 A horse affected with navicular disease is said, in stable parlance, to be 

 " groggy." 



PREDISPOSING INFLUENCES.— 1. A naturally weah or de- 

 fective condition of hone, in the production of which the influence 

 of heredity is well marked. 



2. Altered nutrition of the bone due to chill from the practice 

 of standing on cold stones. This supposition seems to be borne 

 out by the fact that a comparatively large number of troop horses 

 in England are annually " cast " for this complaint, which, in these 

 instances, is apparently brought on by the not uncommon practice, 

 in cavalry stables, of keeping the horses standing, the greater part 

 of the day, on, bare stones, with not much more than an hour's 

 exercise in the twenty-four ; while the ground upon which they 

 parade and drill -is usually hard. The effect of this continued 

 standing on a cold surface which is a good conductor of heat, is to 

 lower the vitality of the living structures of the feet, and to pre- 

 dispose them to become injuriously affected by subsequent con- 

 cussion. In India, on the contrary, where troop horses are accus- 

 tomed to stand on earth, and not on stone, and the groimd is much 

 warmer, navicular disease is far more rare. Cab horses, which are 

 particularly liable to this malady, have, as a rule, to do all their 



