Among other questions productive of heated argument come those relating to expansion of the horse's hoof. In the past many observers have strenuously insisted on the fact that expansion and contraction regularly occur during progression. Opposed to them have been others equally firm in the belief that neither took place. Quite within recent times this question also has been settled once and for all by the experiments of A. Lungwitz, of Dresden. His conclusions were published in an article entitled 'Changes in Form of the Hoof under the Action of the Body-weight.'[A]
[Footnote A: Journal of Comparative Pathology and Therapeutics, vol. iv., p. 191. The whole of the matter in this article, from which we have borrowed Figs. 35 and 36, is too long for reproduction here. It forms, however, most instructive reading, and its careful perusal will well repay everyone interested in this most important question (H.C.R.).]
In connection with this it is interesting to note how, all unconsciously, two separate observers were simultaneously arriving by almost identical means at an equally satisfactory answer to the question. Prior to the publication of Lungwitz's article on the subject, Colonel F. Smith, A.V.D., had arrived at similar conclusions by working on the same methods.
Fig. 35. I. Electric Bell with Dry Element. a, Under part, with box, for the dry element; 6, roller for winding up the conducting-wires; c, dry element, with screw-clamp for attachment of the conducting-wires; c', conducting-wire leading to the screw-clamp, with contact-spring in c', Fig. 2, or to the wall in Fig. 3; d, upper part, with bell; d', conducting-wire to the shoe d' in Figs. 2 and 3; e, strap for slinging the apparatus around the body of the assistant or rider; f, connecting-wire between bell and dry element.
Fig. 35. II. Hoof Shod with Shoe provided with Toe-piece and Calkins; Wall of the Hoof covered with Tinfoil. a, Heel angle, with b, the contact-screws; c, screw-clamp, with contact-spring (isolated from the shoe); c' conducting-wire from the same; d, screw-clamp, with conducting-wire (d') screwed into the edge of the shoe; e, nails isolated by cutting a small window in the tinfoil.
Fig. 35. III. Hoof Shod with Plain Shoe; Horny Wall covered with Tinfoil. a, Toe and heel angle, with b, the contact-screws; c, conducting-wire passing from the tinfoil on the wall; d, conducting-wire passing from the shoe; c', d', ends of the conducting-wires, which must be imagined connected with the ends c', d', passing from the apparatus.
It is unnecessary for our purpose here to minutely describe the exact modus operandi of these two experimenters. Briefly, the method of inquiry adopted in each case was the 'push and contact principle' of the ordinary electric bell, and the close attention which was paid to detail will be sufficiently gathered from Figs. 35 and 36.
Fig. 36. I. Left Fore-Foot Shod And Mounted To Recognise The Sinking Of The Sole. a, Iron plate covering the inner half of the horny sole; b, openings in the same, with screw-holes for the reception of the contact-screw c (the part of the sole under the plate is covered with tinfoil, which at d passes out under the outer branch of the shoe, and becomes connected with the tinfoil of the wall; in order to give the freshly applied tinfoil a better hold, copying-tacks are at e passed through it into the horn, and one is similarly used to protect the tinfoil at the place where the contact-screw touches the latter); f, holes with screw thread for the fastening of the angle required to measure the movement of the wall, and also for the fastening of the conducting-wire, g; h, conducting-wire passing from the tinfoil; i, isolated nails.
Fig. 36. II. Bar-Shoe With Openings. a, Near the inner margin and in the longitudinal bar; b, for the reception of the contact-screw c; d, openings for fastening the angle and the conducting-wires.
After numerous experiments with the depicted contact-screws, moved to the various positions indicated in the drawings, the following conclusions were arrived at:
1. Behaviour Of The Coronary Edge. - During uniform weighting of all four hoofs the coronary edge shows a tendency to contraction in the anterior and lateral regions of the hoof, and a tendency to expansion posteriorly. With heavy weighting of the hoof, which is shown by a backward inclination of the fetlock, contraction in the anterior and lateral regions is slight, but the expansion behind, in the region of the heels, is distinct, commencing gradually in front, becoming stronger, and diminishing again posteriorly. The coronary edge of the heels becomes slightly bulged outwards. The bulbs of the heels swell up and incline a little backwards and downwards.
When the fetlock is raised the expansion of the coronary edge of the heels disappears from behind forwards, passing forwards like a fluid wave. In the lateral and anterior regions of the coronary edge the contraction disappears; and when the weight is thrown off the foot it passes into a gentle expansion of the coronary edge of the toe. During the opposite movement of the fetlock, that of sinking backwards, this change of form is executed in the converse manner.
In short, the coronary edge resembles a closed elastic ring, which yields to pressure, even the most gentle, of the body-weight, in such a way that a bulging out of any one part is manifested by an inward movement of another part.
In Fig. 37, b, the dotted line represents the changes of form in comparatively well-formed and sound hoofs at the moment of strongest over-extension[A] of the fetlock-joint.
[Footnote A: The term 'over-extension,' as employed by Lungwitz, is intended to indicate that position assumed by the fetlock-joint when the opposite foot is raised from the ground.]
2. Behaviour Of The Solar Edge. - Under the action of the body-weight this is somewhat different from that of the coronary edge. Anteriorly, and at the sides, as far as the wall forms an acute angle with the ground, the tendency to expansion exists, but the change of form first becomes measurable in the region where the lateral cartilages begin. Quite posteriorly the expansion again diminishes.
Fig. 37, a, by the dotted line represents the expansion at the moment of over-extension of the fetlock-joint. This expansion is itself rather less than at the coronary edge, and it shows itself distinctly only when the weighted hoof is exposed to a counter-pressure on the sole and frog, no matter whether the counter-pressure is produced naturally or artificially. Thus anything tending to the removal of the pressure from below, such as a decayed condition of the frog or excessive paring in the forge, will diminish the extent of expansion of the solar edge.
Contraction of the solar edge of the heels occurs at the moment of greatest over-extension of the fetlock-joint - that is, in a foot with pressure from below absent. On the face of it, this appears impossible. Lungwitz, however, has perfectly demonstrated it; and, when dealing with the functions of the lateral cartilages in a later paragraph, we shall show reason for why it is but a simple and natural result of the foot dynamics.
3. Behaviour Of The Sole. - The horny sole becomes flattened under the action of the body-weight. This is most distinct at the solar branches, and gradually shades off anteriorly and towards the circumference. As might be supposed, width of hoof and thickness of the solar horn exert an influence on the extent of this movement. The sinking of the horny sole is most marked in flat hoofs.