Following his system, we shall recognise three forms: (1) _Dry_, (2) _moist_, (3) _suppurating_.

The _dry_ corn is one in which the injury has fortunately been unattended with excessive inflammatory changes, and where nothing but the coloration imparted to the horn by the extravasated blood remains to indicate what has happened.

The _moist_ corn is that in which a great amount of inflammatory exudate is the most prominent symptom. It indicates an injury of comparatively recent infliction.

The _suppurating_ corn, as the name indicates, is a corn in which the inflammatory changes are complicated by the presence of pus.

_Causes_.--The causes of corns we may consider under two headings--namely, _predisposing_ and _exciting_.

_Predisposing Causes_.--By the heading of this chapter we have already intimated that corns are due to faulty conformation of the foot. It is, therefore, merely a description of such shapes of foot as favour their formation that will need mention here.

The wide, flat foot, with low heels, may be first considered. Here the posterior portions of the sole, those portions between the wall and the bars, fall very largely in the same plane as the wearing surface of the bars and the wall. As a consequence, these portions of the sole are more p.r.o.ne to receive injury from stones and rough roads and from the pressure of the shoe.

The low heels, too, favour a more than due proportion of the body-weight being thrown on to the posterior parts of the foot. Two evils, both inclining to the production of corn, result from this. In the first place, the sensitive structures of the posterior portions of the foot are subjected to undue pressure from above; secondly, the posterior half of the foot, by reason of the extra weight thrown upon it, is exposed also to greater effects of concussion than normally it should meet. Added to this we find that the abnormally flat condition of the sole has resulted in a great loss of resiliency. With undue pressure above, and a loss of resiliency and added effects of concussion below, the sensitive structures included between the opposing pedal-bone and the h.o.r.n.y sole are bound to suffer more or less bruising each time the foot comes to the ground, especially if the animal is moved at a rapid pace.

Writing here of the effects of pressure and concussion affords a fitting occasion to mention the fact that corns occurring in feet affected with side-bones are always worse than in feet with normal elastic cartilages.

The explanation of this is simple, for there can be no doubt that the loss of resiliency in the diseased cartilage is only another aid to undue pressure and concussion. The sensitive structures are pinched between unyielding bone above and practically unyielding horn below.

Feet with high and contracted heels are also predisposed to corn. The contraction in this case interferes with the downward movements of the os pedis during progression, while in a state of rest there is a more or less constant pressure upon the sensitive structures, due to the correct downward displacement of the pedal-bone being opposed by the amount of contraction present. In the contracted foot, too, the nutrition of the vessels supplying the secretory apparatus of the horn is largely interfered with. The horn loses its natural elasticity, fails to respond to the normal movements of the parts within, and aids in the compression and laceration of the sensitive structures.

Weak feet, with horn too thin to withstand the expansive movements continually going on--in other words, feet with weak, spreading heels--are also p.r.o.ne to suffer from corns. In this case the flatness induced by the spreading, and the insufficient protection afforded by the thin horn, both combine to lay the sole open to the effects of concussion and direct injury.

Brittle feet--feet with horn of undue dryness, by reason of the contraction thus brought about--are, again, particularly subject to corn.

So also with long feet. Whether occurring as a natural deformity, or as the result of insufficient paring, bruises of the sole in feet thus shaped are common. The reason for this will be better understood when we come to deal with the shoeing.

Other and minor predisposing causes are those mainly referring to an unnatural dryness of the hoof when animals reared in the country are put to work in large towns. We here really get several predisposing causes combining. A sudden change is made from a more or less moist condition underfoot to one excessively dry. The character of the travelling is wholly altered from occasional work upon soft lands to continual labour upon hard-paved roads. The horn is often exposed to the vicious influences of unsuitable litter, the application of unsuitable dressings, and the deleterious effects of the street mud of our cities. All these play their part in determining a condition of the horn, rendering it open to receive the effects of the more exciting causes which we shall next consider.

_Exciting Causes_.--Than the shoeing, no more frequent and exciting cause of corn exists. Whatever the predisposing influences may be, it is the shoeing that in nearly every case completes the list, and finally inflicts the injury.

The evils in this connection we shall consider under two headings--viz., (1) the manner in which the foot is pared; (2) the make and fitting of the shoe.

First among the faulty preparations of the foot comes that of excessive thinning of the sole, especially in the regions subject to corn. The farrier addicted to this is not as a rule content to confine his operations to the sole alone. In addition, the frog and the bars also suffer from the too lavish use of his knife. His main object is doubtless that of giving a broad and open appearance to the foot. It follows from this that his operations are confined more to the posterior than the anterior parts of the foot, and that the toe is therefore left too long. This gives us a combination of causes leading to pressure and bruises upon the sensitive structures at the seat of corn.

By this unequal paring of the toe and the heels greater weight is thrown upon the posterior half of the foot. What then happens to the structures thinned as we have described is this: the pared frog, lessened in volume, does not meet the ground. It therefore fails to expand laterally with weight, and cannot a.s.sist, as normally it should, in aiding the heels generally in their movements of expansion. The weakened bars and the thinned sole, meeting with no opposition from the frog, give downwards and inwards with the body-weight at the precise moment these movements should be directed mainly outwards. As a further result of non-resistance on the part of the frog, this time in a lateral direction, the bars, the sole, and the wall at the heels all contract at the exact time they should expand.

The end result must mean abnormal pressure and bruising of the sensitive structures in that particular region. Naturally, also, the excessive thinning of the horn renders direct injury to the sole from stones or other objects in the road far more probable.

For this one reason alone--the manner in which it favours the production of corn--too great a condemnation cannot be placed upon excessive paring of the sole, the bars, and the frog.

When corns are already present, as they may be from other causes, the same remarks will again apply to excessive paring. It is the custom with many smiths to carefully pare down the discoloured horn in every case of corn they meet with, and at the same time to again weaken the bars and even part of the wall at the heels, with the laudable idea of relieving pressure on the part diseased. After what has gone before, we need hardly say that their well-meant efforts have a precisely opposite effect to the one they intend.

The fitting of the shoe is, perhaps, to a greater extent responsible for the causation of corn than is the paring we have just described.

A few of the evils connected with the shoe may, however, be justly described as unavoidable. We _must_ shoe; we cannot shoe and leave a normal foot!

A shoe excessively seated, especially from the last nail-hole backwards, may be regarded as dangerous. In this case, with every application of the body-weight, there is given to the foot a tendency to contract, especially at its lower margin. Result: undue pressure upon the tissues around and the production of corn.

On the other hand, varying with the form of foot, the seating may be insufficient. In the case of flat-foot, or dropped sole, for instance, insufficient seating will lead to undue pressure of the web of the shoe upon the sole, and in that way bring about bruising of the sensitive sole beneath.

Shoes with heels or calks too high, by destroying the counter-pressure of the frog with the ground, serve to bring about a series of changes we have described under contraction, and again result in pinching and bruising of the sensitive structures.

The opposite excess--a shoe thick at the toe and thin at the heels--is blamed by Zundel for causing a like injury. In our opinion, the reason this author gives--namely, that the throwing of greater weight upon the heels leads to bruising of the sensitive structures--can only correctly apply to a _wrongly-applied_ shoe of this type, and not to the shoe itself. True, a shoe with a thick toe and thinned heels will throw an undue proportion of the body-weight upon the heels if the foot is not properly prepared for it. A wise man, however, will most certainly so cut down the toe for the reception of this shoe that, with the shoe in position, there will still be maintained a tread that is normal. To our minds harm is far more likely to arise from a shoe of this cla.s.s through the thinned iron heels of the shoe becoming attenuated under wear to the point of bending, and so inflicting an injury upon the adjoining sole.

Similarly, this last remark with regard to the thinning of the heels of the shoe will apply to a shoe with too broad a web. As the thinning of the shoe proceeds with wear, the inner portion of the thinned branch is bent up on to the sole, and again inflicts the injury.

The matter of bearing is also of importance when considering the causation of corn. In a previous chapter we have already described the correct bearing as that which includes the whole of the lower margin of the wall and the white line, and just impinges on the sole. Any marked deviation from that will, if long continued, be followed by injury to the foot.

With the bearing surface of the shoe too narrow--in contact with the wall solely, or perhaps only a portion of it--it is evident that a large proportion of the foot that should properly bear weight is thrown out of action. A heavy strain is imposed on the white line, and undue descent of the sole and contraction of the heels brought about. Again the result of this is compression and bruising of the tissues around the seat of corn.

With its bearing surface too wide, the shoe immediately exerts direct pressure upon the sole with every movement of the animal. The sole normally is not made to receive this, and harm is bound to result.

Among other ill-fitting shoes we may mention the one with branches too short, and the one with the extremities of the branches too pointed. In the first case, as wear of the shoe proceeds, the thinned end is far more likely to turn in under the seat of corn than is a shoe with branches of ordinarily correct length. It is evident in the second case that the pointed branch, when thinned, is a more dangerous agent than the branch which is nearer the square at its end.

The matter contained in the first half of the foregoing paragraph explains in a large measure the rarity of corns in the hind-feet. Here there is nothing to prevent a shoe with branches of full length being used. The correct bearing is thus maintained, even with a shoe excessively thinned with wear, and the liability to injury from it decreased. An exception is to be found in the case of a feather-edged shoe, such as is used to prevent cutting or brushing. The thinning by wear from above to below of the branch already purposely thinned from side to side leads to the formation of a thin and narrow piece of iron admirably calculated to bend over and injure the sole.

Even with a shoe of correct length, with a flat-bearing surface at the heels, and other conditions favourable to correct application, evil may still result from the shoe itself being made too narrow. As a result of this, the branch of _each_ side is set too far under the foot, with consequent injury to the sole. This is, of course, sheer carelessness on the part of the smith. When practised, however, it is not easy of detection, as in all cases the foot is rasped down to cover what has been done. In other words, the foot is made to fit the shoe and not the shoe the foot.

Recognising this close fitting of the shoe as a cause, we are able to explain in some measure how it is that corns should occur with greater frequency in the inner than in the outer heel. There is no doubt that the inner branch of the shoe is nearly always fitted closer than is the outer.

In the fore-foot it is also often shorter. Take these two evils and add to them the fact that the inner heel is called upon to bear more of the body-weight than is the outer, and the frequency of corns in the inner heel will no longer be wondered at.

Indirectly, the shoe may still be a cause of corn by reason of the irritation set up by gravel and small pieces of flint becoming firmly fixed between the sole and the web of the shoe. In nearly every case of this description the part to be injured is the white line.

Corns may also result from the animal picking up a stone. The stone becomes firmly wedged in between the inner border of the branch of the shoe and the bar or the frog. With every step the animal takes it becomes wedged more tightly into position. Projecting below the level of the lower surface of the shoe, it imparts the concussion it thus obtains directly to the sole. A bruise--and a bad bruise--is the result.

Finally, it cannot be denied that the work the horse is put to is largely responsible for the causation of corn. In country animals corns are comparatively rare, while in animals in town, almost constantly upon hard paving, they are common. This seems to point strongly to the fact that concussion through constant work upon unyielding roads is a great factor in their production.

_Symptoms_.--Unless the discoloration of the horn is accidentally discovered by the smith, the simple, dry corn may go undetected. The disturbance excited by it is so small, and the pain occasioned so slight, that the patient may offer no indication of its existence.

Ordinarily, however, the first symptom is that of pain. The animal goes feelingly with one or both feet, in some cases even showing decided lameness. The lameness, however, is in no way diagnostic, and the lesion itself must be discovered before an exact opinion can be p.r.o.nounced.

As an aside, it is well to observe in this connection that a negative opinion as to the existence of corn should never be given unless the superficial layers of horn have first been removed with the knife.

When standing at rest the animal exhibits signs more or less common to all foot lamenesses. He 'points' the foot--in other words, the limb is slightly advanced, the fetlock partly flexed, and the heels from off the ground.

When both feet are affected they are pointed alternately, and the animal often manifests his uneasiness by repeated pawing movements, and by sc.r.a.ping his bedding behind him.

Should the injury run on to suppuration, the lameness becomes most acute. The pawing movements become more p.r.o.nounced, and there is evident disinclination on the part of the animal to place the foot squarely on the ground. One is then led to manipulate the foot. The hoof is hot to the touch. Percussion causes the animal to flinch, and to flinch particularly when that portion of the wall adjoining the corn is struck. Finally, exploration with the knife reveals the serious extent to which the injury has developed. In a neglected case of this description it is even possible to detect the presence of pus by the amount of swelling and fluctuating condition of the coronet. The suppurative process has advanced in the direction of least resistance, and is on the point of breaking through the tissues immediately above the horn.

Lameness due to corn is oftentimes intermittent. With a simple corn, dry or moist, this intermission is largely dependent on the degree of dryness of the hoof or the road, and also on the character of the road surface. With a neglected, suppurating corn, on the other hand, variation in the degree of lameness, in addition to depending on circ.u.mstances such as these, is dependent to a larger extent upon the changes occurring with the suppuration. In this case the time of greatest lameness is immediately before the pus gains outlet. Immediately after its exit at the coronet the animal will go almost sound. Soundness continues so long as the opening at the coronet remains clear. The tendency, however, is for the opening thus made to quickly close again. Pus again acc.u.mulates, lameness arises as before, and disappears again with the second discharge of the contents of the sinus now formed.

_Pathological Anatomy_.--When dealing with their cla.s.sification we gave in outline the main pathological changes to be met with in corns. It now only remains to give the same matter in slightly greater detail.

_In dry corn_ the changes we meet with are those accompanying blood extravasation. From excessive compression of the parts, or from the effects of direct injury, a portion of the sensitive sole has become lacerated.

The escaping blood stains the surrounding soft tissues after the manner of blood extravasation elsewhere. If the escape of blood is sufficiently large, the horn fibres in the immediate vicinity also are stained. It is this stain in the horn that is the direct evidence of the injury, and is itself popularly known as the corn. It may vary in size from quite a small spot to a broad patch as large as half a crown, while its colour may be a uniform red, or a mottled red and white. The microscopic changes in this connection are ill.u.s.trated in Fig. 99.

[Ill.u.s.tration: FIG. 99.--HORIZONTAL SECTION OF A CORN. The section cut at about the base of the papillae of the sensitive sole. _a_, papillae, with horn-cells surrounding them; _b_, interpapillary or intertubular horn; _c_, hollow s.p.a.ces in the intertubular material filled with blood; _d_, a papilla and its surrounding horn-cells filled with blood.]

Ordinarily, this ecchymosis of the h.o.r.n.y sole is due to injury of the sensitive sole _immediately beneath_ it. It may, however, proceed from injury to the vessels of the laminae either of the bars or of the wall. In this case the ecchymosis of the h.o.r.n.y sole may be explained by the fact that the escaped blood tends to _gravitate_ to that position.

When the corn is of long standing, or is due to _repeated_ injuries on the same spot, the horn adjacent to the lesion becomes hard and dry, and often abnormally brittle, simply on account of the inflammatory changes thus kept in continuation. This is often seen when attempts are made to _pare out_ the corn with the knife.