It has been remarked how strange it is that nails should so readily penetrate the comparatively hard covering of the foot.  The matter, however, admits of explanation.  One knows from common observation how easy it is to tilt a nail with its point upwards by exerting a pressure in a more or less slanting direction upon its head.  This is exactly the form of pressure that is no doubt put upon the nail if the animal treads upon it when moving at any pace out of a walk.  The foot in its movement forward tilts the nail up, and almost simultaneously puts weight upon it.  The great weight of the animal is then quite sufficient to account for its ready penetration.

In purely country districts cases of punctured foot are of far less frequent occurrence than in large towns.  In the latter, animals labouring in yards where a quantity of packing is done, or engaged in carting refuse containing such objects as we have mentioned, or broken pieces of earthenware or glass bottles, meet with it constantly.

For the manner of causation of those wounds to the foot occurring in the forge the reader may be referred to the matter under the heading of ‘nail-bound.’  As in that case so in this the nail may be wrongly directed by improper fitting of the shoe, by the ‘pitch’ of the hole, or by the position of the hole.  The nails may also be wrongly directed as a result of faulty pointing, or by meeting with the stump of a nail that has carelessly been allowed to remain in the substance of the horn.

Often pricking is a result of carelessness engendered by a rush of work.  Often it is almost unavoidable on account of the character of the foot that is brought to be shod.  Feet with thin horn, especially a thin sole, feet with horn shelly and brittle, each in their way are difficult to shoe.

Sometimes pricking is purely accidental, as in the case of a ‘split’ nail.  The nail as it is driven splits at its point, and continues to split down its centre, one half emerging at the correct spot on the wall, the other half bending inwards, and penetrating the sensitive structures.

Common Situations of the Wound.—­In a case of picked-up nail the common seat of puncture is about the point of the frog, either in one of the lateral lacunae, in the median lacuna, or the apex of the frog itself.  In comparison with this puncture of the sole is rare.

Prick sustained at the hands of the smith may, of course, run in either of the following directions:  (1) Directly into the position where the horny and sensitive laminae interleave; (2) between the sensitive laminae and the os pedis; (3) into the os pedis itself; (4) the nail may bend excessively immediately after entering the horn, and so pass either between the horny and sensitive sole; or (5) between the sensitive sole and the bone.

Classification.—­Punctured wounds of the foot may be classified as follows: 

Simple or superficial when penetrating no structure of great importance.  For instance, a prick that penetrates to the sensitive sole and is not driven with sufficient force to seriously injure the os pedis we may regard as simple.  In the same manner a prick to the frog that, although deep, is mainly concerned with penetrating the plantar cushion may also be classed as simple.