Definition. - In veterinary surgery the term 'corn' is used to indicate the changes following upon a bruise to that portion of the sensitive sole between the wall and the bar. Usually they occur in the fore-feet, and are there found more often in the inner than in the outer heel.

The changes are those depending upon the amount of haemorrhage and the accompanying inflammatory phenomena occasioned by the injury.

Thus, with the haemorrhage we get ecchymosis, and consequent red staining of the surrounding structures. As is the case with extravasations of blood elsewhere, the haemoglobin of the escaped corpuscles later undergoes a series of changes, giving rise to a succession of brown, blue, greenish and yellowish coloration.

With the inflammation thereby set up we get swelling of the surrounding bloodvessels, pain from the compression of the swollen structures within the non-yielding hoof, and moistness as a result of the inflammatory exudate.

In a severe case the inflammation is complicated by the presence of pus.

Classification. - Putting on one side the classification of Lafosse (natural and accidental), as perhaps wanting in correctness, seeing that all are accidental, and disregarding the suggested divisions of Zundel (corn of the sole and corn of the wall) as serving no practical use, we believe, with Girard, that it is better to classify corns according to the changes just described.

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 prone 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 horny 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 prone 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.