Navicular disease, however, affects only the lower surface of the bone; the upper surface shares another synovial sac, which lubricates the articulation of the coffin-bone with the lower bone of the pastern. This upper surface is never affected; the navicular bone may diminish or wither through disease, still the affection remains confined to its original situation; disease may lead to fracture of the bone or to rupture of the perforans tendon, still the superior portion of the navicular bone to the last exhibits a healthful condition.
This most annoying and terrible disorder springs from two causes. The first was a very favorite crotchet of the late Professor Coleman, who was always theorizing to the injury of the animal it was his office to cure. The disease is now largely distributed through that gentleman's favorite maxim concerning the absolute necessity that there should be pressure upon the frog. Every smith thus instructed tried to bring the frog as near the ground as possible, and the consequence was the spread of navicular disease. It is true, the frog, in a state of nature, was designed to bear pressure; but surely it is folly to talk about the natural condition of the horse when nothing like a wild horse exists. Here was Coleman's error; he legislated for the most artificial of living creatures, which consumes only prepared food, and which moves only over laboriously manufactured roads, as if it had been in an undomesticated condition, gamboling upon the unfilled earth.
The second cause is, the parsimony of most horse proprietors. Would these gentlemen have their favorites shod with leather, the smith would be obliged to slightly raise the frog; while the leather—if good, stout, sole leather—and the stopping would protect the seat of navicular disease from injury. With regard to the first cause, it was recognized by the late W. Percival, one of Coleman's most enthusiastic pupils; and, as concerns the last, its efficacy as a preventive needs no pleading nor any reference to establish its merits.
The horse, when attacked, commonly has a good open foot—in fact, before disease commences, the foot is healthy. An animal in this condition is being ridden or slowly led out of the stable. In the last case it, being fresh, may rejoice to feel and sniff the cool air of heaven. It may prance about, and we may admire its attitudes; but in an instant it becomes dead lame. So a horse may be mounted by a kind master; the creature may be going its own pace, when, of a sudden, the movement shall change, and the rider will be made conscious that his steed is lame.
In either case the foot is examined. It is cool, quite cool; no stone appears to have injured it—nor is any pebble sticking between the web of the shoe and the sole. Yet the lameness is acute and does not pass off. Now, to explain this, let the reader turn to the illustration which was last presented.
The portion of the foot, immediately under the navicular bone, has been placed upon a stone; the stone has been forced against the foot by the immense weight of the horse imposed upon it. The stone, under this impulse, has bruised the navicular bone. But the fleshy frog and the perforans tendon would have to be passed before this effect could reach the bone. Are neither of these also hurt? Doubtless they are. But the fleshy frog is a highly organized, secretive organ, and probably, by its innate energy, soon recovers from the effect. The tendon is, on the contrary, too soft and yielding to retain any harsh impression. The bone is firm and solid; and thus that which failed to act upon either of the intervening parts, leaves a lasting injury upon the osseous structure, which, moreover, is held stationary by the coronary bone, and which is disposed to display injury, being covered by synovial membrane.
The navicular bone belongs to a peculiar class called "sesimoid, or floating bones." These are more highly organized than the generality of osseous structures—in short, quite as much, or rather more, than the human tooth. Everybody must be acquainted with the anguish occasioned by unexpectedly biting upon a hard substance. The tooth, however, is coated with crystalline enamel. The bone is covered by delicate synovial membrane. The impression is, therefore, more likely to be lasting with the last than the first.
After the expiration of a week, however, the lameness disappears, and the proprietor fondly hopes all is over. The animal may work soundly for months—sometimes it never fails again. Generally, however, after some period, extending from six to nine months, the lameness reappears. This time the treatment occupies a longer space; and the subsequent soundness is of shorter duration. Thus the malady progresses; the period occupied in curative measures lengthens, while the season of usefulness diminishes; till, in the end, the horse becomes lame for life.
The worst of it is, that the pain in the lame foot occasions greater stress to be thrown upon the sound member; the result generally is that both legs ultimately become affected with the like disease: such is ordinarily the case. The horse with a tender foot will always bring it gently to the earth; but this circumstance obliges the animal to cast the other foot to the ground with heedless impetuosity. The consequence is, the sound foot is sooner or later forced upon some stone or other inequality; from the law of sympathy, the disease subsequently makes rapid strides; for at death both feet are usually found in a similar condition.
