[Ill.u.s.tration: FIG. 161.--NAVICULAR BONE (POSTERO-INFERIOR SURFACE) SHOWING THE 'WORM-EATEN' APPEARANCE CAUSED BY EROSION OF THE HYALINE CARTILAGE, AND COMMENCING RAREFACTIVE ARTHRITIS.]

'At this stage, or much earlier'--we are quoting Colonel Smith, A.V.D.--'may be found calcareous deposits in the fibro-cartilage and the bone. They are scattered like fine sand here and there, generally across the inferior half of the face of the bone; they are sometimes numerous, frequently scanty, occasionally entirely absent. The amount of calcareous degeneration depends upon the lesions present. If much destruction of bone exists, there will be but few calcareous deposits; whilst if there are many calcareous deposits, there may be but slight ulceration of bone tissue, and perhaps none at all. In fact, I have held the opinion, and see no reason to modify it, that calcareous deposits are safeguards against caries.'[A]

[Footnote A: _Journal of Comparative Pathology and Therapeutics_, vol. vi., p. 195.]

3. _Changes in the Tendon_.--The effect of these calcareous deposits on the under surface of the bone is to produce a certain amount of roughness.

Seeing that with every movement of the foot the perforans tendon is called upon to glide over this surface, it is clear that a secondary effect must be that of inducing erosion and destruction of the tendon. The point at which this usually commences is at the bottom of the depression that accommodates the ridge on the bone. With erosion of the cartilage and of the tendon at points exactly opposite each other, we have two surfaces come together that are p.r.o.ne to readily unite, and fibrous tissue adhesions often take place between the bone and the tendon. In some measure this accounts for the torn and ragged appearance of the tendon. Adhesions take place, and, under some small strain, are broken down. This may happen more than once or twice, and with each breaking of the adhesion between the bone and tendon, fibres from the latter are lacerated and torn from their place (see Fig. 162).

4. _Changes in the Bone_.--The changes occurring in the bone are essentially those of a rarefactive ost.i.tis. These changes are described by many writers, and, whether originating primarily in the bone or not, it seems certain that extensive changes may have occurred within the bone, with but little or nothing to be noted on its outer surface. It would seem that the first change is one of congestion of the vessels of the bone's cancellous tissue. With the cause, whatever it may be, in constant operation, the congestion persists until a low type of inflammation is set up, interfering, not only with the flow of synovia in the adjoining bursa, but with the nutrition of the bone itself. As the disease progresses, there is softening and enlarging of the cancellated tissue towards the centre of the bone. The cells break up, and absorption takes place. This goes on until a large portion of the interior of the bone is in a state of dry necrosis, with, in many cases, but slight signs of mischief on the exterior of the bone.

In other cases, however, the changes in the interior of the bone are accompanied by well-marked lesions on its gliding or postero-inferior surface, and by evidences of an osteoplastic periost.i.tis along its edges.

That an osteoplastic periost.i.tis has been in existence is witnessed by the appearance along the edges of the bone of numerous outgrowths of bone, termed osteophytes (see Fig. 163).

[Ill.u.s.tration: FIG. 162.--A FOOT WITH THE SEAT OF NAVICULAR DISEASE EXPOSED. On the anterior surface of the perforans fibres of the tendon are seen to be torn away from their abnormal adhesion with the navicular bone, while others are seen to be still attached thereto. The surface of the navicular bone itself exhibits small defects in the bony substance, which have been brought about by a rarefactive ost.i.tis. _a_, The perforans tendon cut through and reflected; _b_, the sole.]

The interosseous and postero-lateral ligaments of the articulation often partic.i.p.ate in the inflammatory changes, and in many cases become completely ossified. The true articulatory surface of the bone, that articulating with the os pedis and with the os coronae, is never affected.

_Causes_.--In enumerating the causes of navicular disease, we shall follow the example of Colonel Smith and cla.s.sify them under certain headings--namely, (1) _Hereditary Predisposition_; (2) _Compression_; (3) _Concussion_; (4) _A Weak Navicular Bone_; (5) _A Defective or Irregular Blood-supply to the Bone_; and (6) _Senile Decay_.

[Ill.u.s.tration: FIG. 163.--THE NAVICULAR BONE FROM A CASE OF LONG-STANDING NAVICULAR DISEASE. The erosion of the cartilage on its central ridge is most marked, and the porous appearance of the bone thus uncovered points to the existence within it of a rarefactive ost.i.tis. Along its edges large osteophytic outgrowths speak of the effects of an osteoplastic periost.i.tis.]

1. _Hereditary Predisposition_.--That navicular disease is hereditary is a fact that has for a long time been insisted on, and has come to be so generally admitted that we do not intend to dwell on it here. As we have said before, it is found in the lighter breeds of horses (and, according to Zundel, especially in the English breeds), and is there seen to be frequently transmitted from parent to offspring.

2. _Compression_.--By this is meant the compression of the navicular bone between the os pedis and the os coronae in front, and the perforans tendon behind.

In order to appreciate this explanation of the causation of navicular disease at its true value, it will be well to consider briefly the physiology of the parts in question.

The navicular bone is what we may term a complement of the os pedis. It exists, in fact, simply in order that the os coronae may have a sufficiently large articulatory surface to play upon. One wonders at first that Nature did not arrive at this by originally placing a larger bone below. Colonel Smith explains this by suggesting that this would in all probability have meant its fracture. In progression the hind part of the foot comes to the ground first, and upon the hinder portion of the articulation would fall the first effects of concussion, together with the greater part of the body-weight. A yielding joint was in this position necessary, and that formed by the navicular bone fills all requirements.

In this connection one next considers the part played by the front limbs during progression. As Zundel expresses it, they are columns of support rather than of impulsion, and, as the body-weight is thrown forward by the hind-limbs, it is the duty of the fore-limbs to receive it. The shock or concussion of the body-weight thus thrown forwards is first received by the muscles uniting the limb to the trunk, and a great part of it there minimized by their sling-like attachment. It is further absorbed by the shoulder-joint, and from there pa.s.sed on to the almost vertical bony column represented by the radius and ulna, the knee, and the metacarpus. On reaching the first phalanx, a portion of the remaining force is pa.s.sed on to the front of the phalanges and loses itself in front of the hoof, while the other portion is transmitted to the flexor tendons, finally to the perforans, and to the posterior parts of the foot. During progression, therefore, the navicular bone is constantly pushed downwards and backwards by the bony column, and is just as constantly pushed forwards and upwards by the resistance of the perforans tendon. This means, of course, that the navicular bone is more or less constantly subject to compression, and constant pressure, as we know full well, is a pretty sure factor in bringing about malnutrition of the parts, with atrophy or chronic inflammatory changes as an end result.

Even with the limb at rest the pressure on both sides of the navicular bone is still constant. The only circ.u.mstances under which we can conceive of it being entirely absent, in fact, are when the tension on the tendon is relaxed, and the body-weight altogether removed by the animal adopting the rec.u.mbent position.

The compression theory as to the causation of navicular disease was, we believe, first originated by Colonel Smith. He, at any rate, has laid much stress on it in his writings. If we accept it, and we see every reason that we should, then we must, with the author, admit the possibility of navicular disease arising from long standing in one position.

3. _Concussion_.--This we are bound to admit as a cause, and in so doing partly explain the comparative, almost total, immunity of the hind-feet from the disease. The fore-limbs, as we have already pointed out, are little more than props of support, and the force of the propelled body-weight is transmitted largely down their almost vertical lines, to end largely in concussion in the foot. With the hind-limbs matters are different. 'These,' as Percival explains it, 'have their bones obliquely placed, so as to const.i.tute, one with the other, so many obtuse angles, to the end, that by forming powerful levers, and affording every advantage for action to the muscles attached to them, they may be fitted for the purpose of propulsion of the body onward.'

The effect of these several obtuse-angled joints in the limb is to absorb the greater part of the force exerted by the body-weight before it reaches the foot. When with this we take the facts that the fore-limbs have to carry the head and neck, and that they have to bear this added weight, plus a propelling force from behind, we see why it is that they should be so subject to the disease, and the hind-limbs so exempt.

As pointing out the part that concussion plays in its causation, we may mention that navicular disease is a disease of the middle-aged and the worked animal. It is interesting to note, too, that it occurs in animals with well developed frogs--in feet in which frog-pressure with the ground is most marked. This at first sight appears to flatly contradict what we have said with regard to frog-pressure in other portions of this work. With this, however, must be reckoned other predisposing causes. In this case it is not to frog-pressure alone we must look, but to the condition of the frog itself, and that of the neighbouring parts. It is when we have a frog which, though well developed and apparently satisfying all demands as to size and build, is at the same time composed of a hard, dry, and non-yielding horn that we must look for trouble.

The foot predisposed to navicular disease is the strong, round, short-toed or clubby foot, open at the heels, with a sound frog jutting prominently out between them. Here is a frog exposed to all the pressure that might be desired for it, bounded at its sides by heels thick and strong, and indisposed to yield, and itself liable, from its very exposure, to become, in the warm stable, hard and dry, and incompressible' (Percival).

Here, instead of acting, as normally it should, as a resilient body, and an aid to the absorption of concussion, it seems rather to play the part of a foreign body, and to bring concussion about. Seeing, then, that the navicular bursa is in very near contact with it, it is conceivable that this joint-like apparatus should suffer, and the pedal articulation be left unaffected, the more so when we take into consideration the compression theory just described.

4. _A Weak Navicular Bone_.--When the disease commences first in the bone--and there is no denying the fact that sometimes, although not invariably, it does--it may be explained by attributing to the structure of the bone an abnormal weakness in build.

The navicular bone consists normally of compact and cancellated tissue arranged in certain proportions, the compact tissue without, and the cancellated within. These proportions can only be judged of by the examinations of sections of the bone, and when it is found in any case that the cancellated tissue bulks more largely in the formation of the bone than normally it should, we have what we may term a weak navicular bone. In this connection Colonel Smith says: 'Though it is far from present in every case of the disease, still I consider it a factor of great importance.'

5. _A Defective or Irregular Blood-supply to the Bone_.--This, Colonel Smith considers, is brought about by excessive and irregular work, and by the opposite condition--rest. The author points out that the bloodvessels pa.s.sing to and from the navicular bone run in the substance of the interosseous ligaments, or in such proximity to them that it is conceivable that under certain circ.u.mstances mechanical interference may occur to the navicular circulation. He further points out a fact that is, of course, well known to every veterinarian, that in periods of work the circulation of the foot is hurried, and that in rest there is always a tendency to congestion; and he says in conclusion: 'I cannot help thinking that irregularities in the blood-supply in a naturally weak bone must be a factor of some importance, especially when the kind of work the horse is performing is a series of vigorous efforts followed by rest.'

6. _Senile Decay_.--With approaching age the various tissues lose their vigour, and are p.r.o.ne to disease. The navicular bone and surrounding structures are not exempt. With the other and more active causes we have described acting at the same time it is not surprising that navicular disease is seen as a result.

In conclusion, it is well, perhaps, to say that, no matter to which particular theory of causation we may lean, we should make up our minds to consider them as a whole. While one cause may be exciting, the other may be predisposing, and the two must act together before evil results are noticed. It may be that even more than two are concerned in bringing on the disease, and to each the careful veterinarian will give due consideration.

_Symptoms and Diagnosis_.--In the early stages of navicular disease the symptoms are obscure. Pointing of the affected limb is the first evidence the animal gives. This, however, more often than not, goes unnoticed, and the first symptom usually observed by the owner or attendant is the lameness. Even this is such as to at first occasion no alarm, being intermittent and slight, and only very gradually becoming marked. In a few cases, however, lameness will come on suddenly, and is excessive from the commencement. It is the lameness, slow in its onset, intermittent in its character, and gradual in its progress, however, that is ordinarily characteristic of navicular disease.

The animal is taken out from the stable sound, with just a vague suspicion, perhaps, that he moved a bit stiffly. While out he is thought by his driver or rider to be going feelingly with one foot or with both. Even this is not marked, and the driver has some difficulty in a.s.suring himself whether or no he really observed it, or whether it was but imagination.

On the return home the limb is examined, and nothing abnormal is to be found. The leg is of its normal appearance, and neither heat nor tenderness is to be observed in it or in the foot. On the following day the animal again is sound, and the lameness of the previous day is put down to a slight strain or something equally simple. The patient is then, perhaps, rested for a day or two. When next he is worked he again moves out from the stable sound, but again during the going gives the driver the unpleasant impression that something is amiss; and so the case goes on. One day the owner fears the animal is becoming seriously enough affected to warrant him in calling in his veterinary surgeon; the next he is confidently a.s.suring himself that nothing is wrong.

Perhaps the animal is now rested for a week or two, or even for a month or two, hoping that this will put him sound. Immediately on commencing work, however, the same symptoms as before a.s.sert themselves, and the veterinary surgeon is called in.

With a history such as we have given the veterinarian's suspicions are aroused. He has the animal trotted, and may notice at this stage that there is an inclination to go on the toes, that the lame limb or limbs are not put forward freely, and that progression is stilty and uncertain; it is such, in fact, as to at once suggest the possibility of corns being present.

In some cases there is just the suspicion of a limp with one limb, and this only at intervals during the trot. At one moment the veterinarian is positive that he sees the animal going lame; at another he is just as confident he sees him coming towards him sound.

Nothing is found in the limb--neither heat, tenderness, nor swelling. There is nothing in the gait (either a limited movement of the radius, or a circular sweep with the leg) to indicate shoulder or other lameness, and the veterinary surgeon, by eliminative evidence, is bound to conclude that the trouble is in the foot.

The foot is then examined--pared, percussed, pinched, and in other ways manipulated--but nothing further is forthcoming. In such a case the veterinary surgeon is wise to declare the abortive result of his examination, to hint darkly of his suspicions, and to suggest a second examination at some future date. It may be that two, three, four, or even more, such examinations are necessary before he can justly p.r.o.nounce a positive verdict.

Later he is enabled to do this by an increase in the severity of the symptoms, and by the changes that take place in the form of the foot.

The lameness is now more marked, and the 'pointing' in the stable more frequent. With regard to the latter symptom, it has been seriously discussed whether the horse with navicular disease points with the heel elevated or with it pressed to the ground. In either case, of course, the limb is advanced; but while some hold that the phalangeal articulations are flexed and the heel slightly raised, in order to relieve the pressure of the perforans tendon on the affected area, and so obtain ease, there are others who hold that the heel is pressed firmly to the ground in order to deaden the pain. It may be, and most probably is, that both are right; but, in our opinion, there is no doubt whatever that pointing with the heel elevated is by far the most common.

The lameness is now excessive, and is especially noticeable when the animal is put to work on a rough or on a hard ground. Even now, however, heat of the foot or tenderness is so slight as to be out of all proportion to the alteration in gait.

With the case thus far advanced, evidence of pain may be obtained by pressing with the thumb in the hollow of the heel. Evidence of pain may also be obtained by using the farrier's pincers on the frog. These methods, however, are never wholly satisfactory, as a horse with the soundest of feet will sometimes flinch under these manipulations.

Extreme and forcible flexion of the corono-pedal articulation also sometimes gives evidence of tenderness. In this case the foot is held up, the animal's metacarpus resting on the operator's knee, and the toe of the hoof pushed downwards with some degree of force.

The same movement of the joint is given by causing the animal to put full weight upon the diseased limb, a small wedge of wood being first placed under the toe. In this manner the pressure of the perforans tendon upon the bursa is greatly increased, and the animal is caused to show symptoms of distress.

The lameness may also be increased, and diagnosis helped, by paring the heels, so as to leave the frog prominent and take the whole of the body-weight. The same end is also obtained by applying a bar shoe. This was originally pointed out by Brauell, and is quoted by Zundel and by Moller.

The changes in the form of the hoof may now be noticed. These are largely dependent on the fact that more or less constantly the patient saves the heel. The horn of the walls in this region, and the horn of the frog, is thereby put out of action and induced to atrophy. The hoof gradually a.s.sumes a more upright shape, and the heels contract. We thus get a hoof which is visibly narrowed from side to side, with a frog that is atrophied and often thrushy, and with a sole that is abnormally concave, hard, and affected with corns.

When occurring in the hind-feet--a condition that is rare, but which has been noticed by Loiset, and quoted by Zundel--the animal is stiff behind, walks on his toes, and gives one the impression that he is suffering from some affection in the region of the loins.

One such case is reported by an English veterinary surgeon, and we quote it here:

'A gray gelding, and a capital hunter, the property of a gentleman in this neighbourhood, became lame in the near fore-foot after the hunting season of 1859. The lameness was believed to be due to navicular disease.

The operation of neurectomy was ultimately had recourse to. The horse subsequently did his work as well as ever, and was ridden to hounds regularly till the end of the year 1861, when he went lame of the off fore-foot. From this date he also showed very peculiar action behind, and was at times lame of both hind-limbs without any apparent cause.

'In the year 1862, from the groom's indiscreet use of physic, superpurgation was brought on which caused the animal's death. On a post-mortem examination being made, the horse was found to have _navicular disease of all four feet_. It is worthy of note that this horse had always "extravagant" action behind, but was a remarkably quick and good jumper.'[A]

[Footnote A: F. Blakeway, M.R.C.V.S., _Veterinarian_, vol. ii., p. 21.]