SECTION OF FOOT OF EQUINE FOETUS, SEVENTY-SEVEN DAYS OLD

FIG. 26.—SECTION OF FOOT OF EQUINE FOETUS, SEVENTY-SEVEN DAYS OLD. The rete Malpighii rests on a plane corium; the rent in the section is along the line of the cells of the rete (Mettam).

SECTION FROM FOOT OF SHEEP EMBRYO

FIG. 27.—SECTION FROM FOOT OF SHEEP EMBRYO. It shows a pronounced epithelial ingrowth into the corium (Mettam).

The next stage will demonstrate the first step in the formation of the sensitive laminæ.[A] The plain surface of the corium has now become broken up, and what is noticed is that the broken-up appearance is due to the epithelial cells irrupting and advancing en échelon into its connective tissue. Each point of the ingrowing lines of the échelon has usually one cell further advanced into the corium than its neighbours, and may be termed the apical cell. The fine basement membrane separating epithelium from corium is still clearly evident. This epidermal irruption of the corium takes place at definite points right round the foot. It is extremely probable, however, that it commences first at the toe and spreads laterally.

[Footnote A: Sheep embryo, exact age unknown.]

As yet, these cellular ingrowths (which are destined to be the horny laminæ, and cut up the corium into sensitive laminæ) are free from irregularities or secondary laminæ. Before these are to be observed other changes in connection with the ingrowths are to be noticed.

SECTION FROM CALF EMBRYO

FIG. 28.—SECTION FROM CALF EMBRYO. The epithelial ingrowths hang down from the epidermis into the corium like the teeth of a comb (Mettam).

The first is merely that of elongation of the epithelial processes into the connective tissue, until the rete Malpighii gives one the impression that it has hanging to its underneath surface and into the corium a number of thorn-like processes. These extend all round the front of the foot, and even in great part behind. Accompanying this elongation of the processes is a condensation of the epithelial cells immediately above the rete Malpighii, with a partial or total loss of their nuclei. This is the first appearance of true horn, and its commencement is almost coincident with the first stages of ossification of the os pedis.

SECTION OF AN EPITHELIAL INGROWTH FROM AN EQUINE FOETUS

FIG. 29.—SECTION OF AN EPITHELIAL INGROWTH FROM AN EQUINE FOETUS. It shows commencing secondary laminar ridges. In the centre are epithelial cells which are undergoing change into horny elements to form the horn core, or 'horny laminæ' (Mettam).

With the appearance of horn comes difficulty of sectioning. The last specimen that Professor Mettam was able to satisfactorily cut upon the microtome was from a foetus between three and four months old. In this the secondary laminar ridges were clearly indicated, and the active layer of the rete Malpighii could be traced without a break from one ingrowing epithelial process to the next, and around this, following all the irregularities of its outline, and covering the branches of the nascent laminæ. The laminæ mostly show this branching as if a number of different growing points had arisen, each to take on a function similar to the epithelial process as it at first appeared.

In the centre of the processes a few nuclei may be observed, but they are scarce, and stain only faintly; they have arisen from the cells of the rete Malpighii which have grown into the corium. In fact, the active cells are passing their daughters into the middle of the process, and these pass through similar stages as those derived from the ensheathing epidermis. In other words, the daughter cells of the constituents of the rete Malpighii which have grown into the corium pass through a degeneration precisely similar to that undergone by cells shed at desquamation, or those which eventually give rise by their agglutination to a hair.

This is the real origin of the horny laminæ, and the thickness of these is increased merely by an increase in the area covered by the cells of the rete Malpighii—i.e., by the development of secondary laminar ridges. If a section from a foal at term be examined, the processes will be found far advanced into the corium, and, occupying the axis of each process, will be seen a horny plate, continuous with the horn of the wall. No line of demarcation can be observed between the horn so formed and the intertubular material of the wall. They merge into and blend with each other, with no indication of their different origins. The cells that have invaded the corium have thus not lost their horn-forming function. There has merely been an increase in the area for horn-producing cells. The horny processes are continuous with the hoof proper at the point where the epithelial ingrowth first commenced to invade the corium, and fuses here with the horn derived from the cells of the rete Malpighii which have not grown inwards, and which are found between the processes in the intact foot. From this it is clear that some considerable portion of the horn of the wall is derived from the cells of the rete Malpighii covering the corium of the foot. It becomes even more clear when we remember the prompt appearance of horn in cases where a portion, or the whole, of the wall has been removed by operation or by accident (see reported cases in Chapter VII.).

The activity of the cells of the rete Malpighii of the corium covering the remainder of the foot will be quite as necessary as the activity of the cells of the coronary papillæ which form the horn tubes themselves. 'For,' in Professor Mettam's own words, 'I am inclined to believe that much of the "white line" which is found uniting the wall of the hoof to the sole has been derived from the horn formed from the rete of the foot corium. This origin will explain the absence of pigment from this thin uniting "line," as it does from the horn lining the interior of the wall. The cells of the rete are free of colouring matter.'

SECTION THROUGH HOOF AND SOFT TISSUES OF A FOAL AT TERM

FIG. 30.—SECTION THROUGH HOOF AND SOFT TISSUES OF A FOAL AT TERM. The horn of the wall is shown, and the horn-core ('horny laminæ') of the epithelial ingrowth. The latter has advanced far into the corium, and is now provided with abundant secondary laminar ridges (Mettam).

From the matter here given us it is easy to understand how, in a macerated foot, the appearance is given of interlocking of the sensitive and horny laminæ. We see that the horny laminæ are ingrowths of the rete Malpighii, ploughing into and excavating the corium into the shape of leaves—the sensitive laminæ. Putrefactive changes simply break into two separate portions what originally was one whole, by destroying the cells along its weakest part. This part is the line of soft protoplasmic cells of the rete Malpighii. Thus the more resistant parts (the horn on the one hand, and the corium covering the foot on the other) are easily torn asunder.

As a result of the evidence we have quoted, we are able to answer our original question in the affirmative. Seeing that the horny and the sensitive laminæ are both portions of the same thing—namely, a modified skin, in which the epidermis is represented by the horny laminæ, and the corium by the sensitive—it is clear to see that the cells covering the inspreading horny laminæ are dependent for their growth and reproduction upon the cells with which they are in immediate contact—namely, those of the sensitive laminæ—and that therefore the sensitive laminæ are responsible for the growth of the horny.

B. CHEMICAL PROPERTIES AND HISTOLOGY OF HORN.

Horn is a solid, tenacious, fibrous material, and its density in the hoof varies in different situations. It is softened by alkalies, such as caustic potash or soda and ammonia, the parts first attacked being the commissures, then the frog, and afterwards the sole and wall. Strong acids, such as sulphuric acid and nitric acid, also dissolve it.

The chemical composition of the hoof shows it to be a modification of albumin, its analysis yielding water, a large percentage of animal matter, and materials soluble and insoluble in water. The proportions of these, as existing in the various parts of the hoof, have been given by Professor Clement as follows:

Horn appears to be identical with epidermis, hair, wool, feathers, and whalebone, in yielding 'keratin,' a substance intermediate between albumin and gelatine, and containing from 60 to 80 per cent. of sulphur.

That horn is combustible everyone who has watched the fitting of a hot shoe knows. That it is a bad conductor of heat, the absence of bad after-effects on the foot testifies.

PERPENDICULAR SECTION OF HORN OF WALL.

FIG. 31.—PERPENDICULAR SECTION OF HORN OF WALL.

In a previous page we have described the manner of growth of the horn tubules, and noted the direction they took in the wall; also, we have noticed the existence between them of an intertubular horn or cement.

Those who wish to give this subject further study will find an excellent series of articles by Fleming in the Veterinarian for 1871. We shall content ourselves here with introducing one or two diagrams and photo-micrographs, and dealing with the histology very briefly.

Under the microscope the longitudinal striation of the wall is found to be due to the direction taken by the horn tubules.

Fig. 31 is a magnified perpendicular section of the wall. In it the parallel dark striæ are the horn tubules in longitudinal section. The lighter striæ represent the intertubular material.

Fig. 32 gives us the wall in horizontal section. To the left of this picture we find the horn tubules cut across, and standing out as so many concentrically ringed circles. In the centre of the figure are seen the horny laminæ, with their laminellæ, and the sensitive laminæ. The right portion of the figure pictures the corium.

HORIZONTAL SECTION OF HORN OF WALL.

FIG. 32.—HORIZONTAL SECTION OF HORN OF WALL.

Fig. 33 is, again, a horizontal section, cut this time at the junction of the wall with the sole. To the left are seen, again, the horn tubules of the wall, and to the centre the horny laminæ. In this position, however, the structures interdigitating with the horny laminæ are not sensitive, but are themselves horny. As the diagram shows, they contain regularly arranged horn tubules cut across obliquely. It is this horn which forms the 'white line.' To the extreme right of the figure are seen the horn tubules of the sole.

There remains now but to notice the arrangement of the horn tubules in the frog. The peculiar, indiarubber-like toughness of this organ is well known. Histological examination gives a reason for this.

HORIZONTAL SECTION OF HORN THROUGH THE JUNCTION OF THE WALL WITH THE SOLE.

FIG. 33.—HORIZONTAL SECTION OF HORN THROUGH THE JUNCTION OF THE WALL WITH THE SOLE. a, Horn tubule of the wall; b, horn tubule of the sole; c, d, horny laminæ.

SECTION OF FROG THROUGH CORIUM AND HORN.

FIG. 34.—SECTION OF FROG THROUGH CORIUM AND HORN. The long finger-like projections of corium into epidermis are sections of the long papillæ from which the horn-tubes of the sole grow. In the stainable portion of the epidermis are to be clearly seen light and dark streaks pointing out the alternate strata-like arrangement of cells mentioned in the text (Mettam).

The horn tubules of the frog are sinuous in their course. This is accounted for by the fact that in the horn of the frog there is a large amount of intertubular material, this having the effect of frequently turning the horn tubules from the straight. In addition to this, the intertubular material has a peculiar arrangement of the cells composing it. These are laid down in alternating striæ (1) of cells with their long axes longitudinal, and (2) of cells with their long axes horizontal. This is seen in Fig. 34, between the long papillæ of the corium, where the lines of longitudinally arranged cells in horizontal section stand out darker than the adjoining strata in which their arrangement is horizontal. The tortuous direction of the horn tubules, and the almost interlocking nature of the alternating strata of the intertubular material, together combine to give the frog its characteristic toughness and resiliency.

C. EXPANSION AND CONTRACTION OF THE HOOF.

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.

Electric Bell with Dry Element.

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.

LEFT FORE-FOOT SHOD AND MOUNTED TO RECOGNISE THE SINKING OF THE SOLE.

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.

D. THE FUNCTIONS OF THE LATERAL CARTILAGES.[A]

We have just referred to contraction of the heels as taking the place of a normal expansion in those cases where ground frog-pressure was absent. We shall readily understand this when we bear in mind the anatomy of the parts concerned, especially that of the plantar cushion. This wedge-shaped structure we have already described as occupying the irregular space between the two lateral cartilages, the extremity of the perforans tendon, and the horny frog.

Now, when weight or pressure is exerted from above on to this organ, and the frog is in contact with the ground below, it is clear from the position the cushion occupies that, whatever change of form pressure from above will cause it to take, it must certainly be limited in various directions.

expansion of the solar edge of the hoof

FIG. 37. a, The dotted lines in this diagram represent the expansion of the solar edge of the hoof at the moment of over-extension of the fetlock-joint; b, the dotted line represents the change in form of the coronary edge under similar circumstances.

Because of the shape of the cushion its change of form cannot be forwards (simultaneous pressure from above and below on to this wedge with its apex forwards must tend to give it a backward change of form). Because of the pastern being horizontal, and aiding in the downward pressure, its change of form cannot be upwards. And because of the ground it cannot be downwards. It follows, therefore, that the movement must be backwards and outwards, being especially directed outwards because of its shape and the median lacuna in its posterior half—this latter, the lacuna, accommodating as it does the frog-stay, preventing the tendency to backward movement becoming excessive, and directing the change of form to the sides. Where the greatest pressure is transmitted, then, is to the inner aspects of the flexible lateral cartilages. The coronary cushion being continuous with the plantar, the backward and outward movements of the latter will tend to pull upon and tighten the former, especially in front. This will account for the contraction noted by Lungwitz in the anterior half of the coronary edge of the hoof.

Remove the body-weight, and naturally the elastic nature of the lateral cartilages and the coronary and plantar cushions, with, in a less degree, that of the hoof, cause things to assume their normal position.

Repeat the weighting of the hoof, in this second case without frog-pressure, and we shall see at once that we have done away with one of the greatest factors in determining the outward and backward movements of the plantar cushion—namely, the pressure from below on its wedge-shaped mass. The movement of the plantar cushion will now be downwards as well as backwards; and, seeing that it is attached to the inner aspect of each lateral cartilage, we shall expect these latter, by the downward movement of the plantar cushion, to be drawn inwards. This Lungwitz has shown to occur.

The chief function of the lateral cartilages, therefore, is to receive the concussion engendered by locomotion, which concussion is directed backwards and outwards by the pad-like plantar cushion.

In addition to this, the lateral cartilages, together with the plantar and coronary cushions, play the part of a valve to the whole of the veins of the foot.

It is in this way: We have only to refer to the chapter on anatomy to see that the whole of the foot is covered with a tissue of extreme vascularity. Thus we find papillæ—the over the coronary cushion; enlarged and modified papillæ sensitive laminæ—covering the anterior face of the os pedis; and numberless papillæ again covering the sole. There can be no doubt that the quantity of fluid brought by the bloodvessels of these papillæ to the foot acts largely as a means of hydraulic protection to the soft structures.[A] In like manner as that delicate organ, the brain, is best protected by being floated upon the cerebro-spinal fluid and bloodvessels (which fluids transmit waves of concussion or pressure through the organ without injury to the delicate cells forming it), so, in like manner, does the extreme vascularity of the foot protect the cells of its softer structures from the effects of pressure and concussion.

[Footnote A: The Veterinary Record, vol. iii., p. 518.]

That this law of hydraulics may operate in the horse's foot to the best

advantage, the veins must be provided with valves, and valves of no mean strength. These we know to be absent. It is here that the lateral cartilages and the elastic substances of the coronary and plantar cushions step in to supply the deficiency.

At the time when weight is placed upon the foot (with, of course, a tendency to drive the blood upwards in the limb), and, therefore, the time when a valvular apparatus is needed to retain the fluid in the foot, we find the wanting conditions supplied by the pressure outwards of the plantar cushion compressing the large plexuses of veins on each side of the lateral cartilages, to which plexuses, it will be remembered, the bulk of the venous blood from the foot was directed. A more perfect valvular apparatus, automatic and powerful, it would be difficult to imagine.

E. GROWTH OF THE HOOF.

We will conclude this chapter with a few brief remarks on the growth of the hoof. That the rate of growth is slow is a well-known fact to every veterinarian, and it will serve for all practical purposes when we state that, roughly, the growth of the wall is about 1/4 inch per month. This rate is regular all round the coronet, from which it follows that the time taken for horn to grow from the coronary edge to the inferior margin will vary according as the toe, the quarters, or the heels are under consideration.

As might naturally be expected, the rate of growth will depend on various influences. Any stimulus to the secreting structures of the coronet, such as a blister, the application of the hot iron, or any other irritant, results in an increased growth. Growth is favoured by moisture and by the animal going unshod, as witness the effects of turning out to grass. Exercise, a state of good health, stimulating diets—in fact, anything tending to an increased circulation of healthy blood—all lead to increased production of horn. With the effects of bodily disease and of ill-formed legs and feet on the wear of the hoof, and the growth of horn, we shall be concerned in a future chapter.

CHAPTER IV

METHOD OF EXAMINING THE FOOT

As a general rule, it may be taken that most diseases of the foot are comparatively easy of diagnosis. When, however, the condition is one which commences simply with an initial lameness, the greatest care will have to be exercised by the practitioner.

What remarks follow here should rightly be confined to a treatise on lameness. This much, however, we may state: As compared with lameness arising from abnormal conditions in other parts of the limb, that emanating from abnormalities of the foot is easy of detection. With a case of lameness before him, concerning which he is in doubt, the practitioner remembers that a very large percentage may safely be referred to the foot, and, if wise, subjects the foot to a rigorous examination.

Much may be gathered by first putting the animal through his paces. When at a trot, notice the peculiarity of the 'drop,' whether any alteration in going on hard or soft ground, and watch for any special characteristic in gait. At the same time inquiry should be made as to the history of the case; its duration; whether pain, as evidenced by lameness, is constant or periodic; the effect of exercise on the lameness; and the length of time elapsed since the last shoeing.

This failing to reveal adequate cause for the lameness in any higher part of the limb, one is led, by a process of negative deduction, to suspect the foot. If 'pointing' is a symptom, its manner is noticed. The foot is compared with the other for any deviation from the normal. In some cases the two fore or the two hind feet may differ in size. Though this may not necessarily indicate disease, it may, nevertheless, be taken into account if the lameness is not easily referable to any other member. Measurement with calipers will then be of help, and a pronounced increase in size, especially if marked in one position only, given due consideration. The hand is used upon each foot alternately to look for change of temperature, to detect the presence of growths small enough to escape the eye, and to discover evidence of painful spots along the coronet.

At this stage the method of percussion recommends itself, and in many cases no more useful diagnostic agent is to be found than the ordinary hammer. As a preliminary, the foot of the sound limb should be always tapped first. This precaution will serve to bring to light what is frequently met with—the aversion nervous animals sometimes exhibit to this manner of manipulation of the hoof. Unless this is done, the ordinary objection to interference is apt to be read as evidence of pain. No aversion to the method being shown, the suspected foot is gently tapped in various places round the wall, a keen look-out being kept for any manifestation of tenderness. This may vary from a slight resentment to each tap, indicated by a sudden lifting and setting down again of the foot, to a complete removal of the foot from the ground, and a characteristic pawing of the air that points out clearly enough the seat of pain.

Evidence of pain once given, the tapping is persisted in until, in some cases, the exact position of the tender spot is definitely located.

Failing evidence obtained from percussion, attention should next be given to the shoeing. We may add here that, even when difficulties have to be encountered in doing it, it is always a wise plan to have the shoe removed.

The nails should be removed one by one, the course they have taken, their point of emergence on the wall, and the condition of their broken ends all being carefully noted as they are withdrawn.

The removed shoe should next be examined as to the coarseness or fineness of its punching and the 'pitch' of its nail-holes, and close attention given to the shape of its bearing surface.

From that we may pass to a consideration of the underneath surface of the foot. The drawing-knife should be run lightly over the whole of its surface, the first thing to be noticed being the point of entrance of the nails as compared with the coarseness or fineness of the punching, and the staining or otherwise of the horn immediately around. We may thus be guided towards mischief arising from tight nailing apart from actual prick of the foot.

This done, more than usual care should be taken in following up any other small prick or dark spot that may show itself upon the white surface of the cleaned sole. In any case, a suspicious-looking speck should be followed up with the searcher until it is either cut out or is traced to the sensitive structures.

While this is done, we should also have noticed the condition of the horn at the seat of corn; should have noticed the shape of the heels, contracted or otherwise; and the appearance of the frog, clean or discharging.

A point to be remembered in making this exploratory paring of the foot is the peculiar consistency of the horn of the frog, and its tendency to hide the existence of punctures. In like manner, as a pin pierces a piece of indiarubber, and leaves no clearly visible trace of the hole it has made, so does a nail or other sharp object penetrate the frog, leaving but little to show for the mischief that has been done.

After all, even though we may have fully decided the foot is at fault, our case of lameness may remain obscure so far as a cause is concerned. Nothing remains, then, but to acknowledge the inability to discover it, to advocate poulticing, or some other expectant palliative measure, and to bring the case up for further examination at no distant date. Where, though we may have suspected the foot, we have not been able to definitely assure ourselves that there the mischief is to be found, a further method of examination presents itself—namely, subcutaneous injections of cocaine along the course of the plantar nerves.

The salt of cocaine used is the hydrochlorate, 2-1/2 grains for a pony, 4 grains for a medium-sized animal, and 6 grains for a large horse. A solution of this is made in boiled water (about 3 drams), and injected at the seat of the lower operation of neurectomy.

It is advisable to first render aseptic the seat of operation, and to sterilize both the needle and the syringe by boiling. A suitable point to choose for the injection is exactly over the upper border of the lateral faces of the two sesamoids, the needle being introduced behind the cord formed by the nerve and accompanying vessels, and parallel with it.

It is possible that the vein or the artery may be wounded, but such accident is of little importance. All that is necessary in that case is to partly withdraw the needle and again insert it. It is advisable to use a twitch.

When the needle is in position, the injection should be made slowly, and at the same time the point of the needle should be made to describe a semicircular sweep, so as to spread the solution over as wide an area as is possible.

Anæsthesia ensues in from six to twenty minutes, and if the cause of the lameness is below the point of injection the animal moves sound.

Regarding this method of diagnosis, Professor Udriski of Bucharest, after a series of trials, sums up as follows:

1. For the diagnosis of lameness cocaine injections are of very considerable value.

2. These injections should be made along the course of the nerves.

3. Solutions heated to 40° or 50° C. produced quicker, deeper, and longer anæsthesia than equally strong cold solutions.

4. In the sale of horses cocaine injections conceal fraud.

Cocaine being an irritant, it must be remembered that after the anæsthesia the lameness is somewhat more marked than before.

To the cocaine other practitioners add morphia in the following proportions:

As a diagnostic this mixture of the two is said to be far superior to either cocaine or morphia alone.

In connection with this subject, Professor Hobday has published, among others, the following cases illustrating the practical value of this method of diagnosis:[A]

[Footnote A: The Journal of Comparative Pathology and Therapeutics vol. viii., pp. 27, 43.]

CASE I.—Cab gelding. Seat of lameness somewhat obscure; navicular disease suspected. Injected 2 grains of cocaine in aqueous solution on either side of the limb, immediately over the metacarpal nerves.

Five Minutes.—Lameness perceptibly diminished.

Ten Minutes.—Lameness scarcely perceptible.

CASE II.—Mare. Obscure lameness; foot suspected. Injected 30 minims of a 5 per cent. solution on either side of the leg just above the fetlock.

Ten Minutes.—No lameness, thus proving that the seat of lameness was below the point of injection.

CASE III.—Cab gelding, aged, free clinique; Messrs. Elme's and Moffat's case. Obscure lameness; foot suspected of navicular disease; very lame. Injected 30 minims of a 5 per cent. solution of cocaine on either side of the leg over the metacarpal nerves.

Six Minutes.—Lameness perceptibly less; there was no response whatever on the inside of the leg to the prick of a pin. On the outside, which had not been injected so thoroughly, there was sensation, although not so much as in a healthy foot.

Ten Minutes.—Lameness had almost disappeared; so much so, that the opinion as to navicular disease was confirmed, and neurectomy was performed. Immediately after this operation there was no lameness whatever.

The same author also reports numerous cases among horses and cattle, dogs and cats, pointing out the toxic properties of the drug. The symptoms following an overdose are interesting enough to relate here, and I select the following case of Professor Hobday's as being fairly typical:[A]

[Footnote A: Loc. cit.]

CASE IV.—Cart gelding. Free clinique; navicular disease. Injected subcutaneously over the metacarpal nerves on each side 6 grains of cocaine in aqueous solution. During the operation the animal manifested no signs of pain whatever, not even when the nerve was cut. This animal received altogether 12 grains of cocaine (3 grains were given on either side first, then fifteen minutes afterwards the same dose repeated). The effect was manifested on the system in ten minutes after the second injection by clonic spasms of the muscles of the limbs (the legs being involuntarily jerked backwards and forwards at intervals of about twenty seconds), which materially interfered with the performance of the operation. The animal was also continually moving the jaws, and was very sensitive to sounds, moving the ears backwards and forwards. This hyperæsthesia, as evinced by the movement of the ears, lasted for some considerable time after the animal had been allowed to get up.

Cocaine hydrochlorate solutions, if intended to be kept for any length of time, should have added to them when freshly made 1/200 part of boric acid in order to preserve them. Even then they are liable to spoil, and should, for subcutaneous injection, be made up just before needed for use.

CHAPTER V

GENERAL REMARKS ON OPERATIONS ON THE FOOT

A. METHODS OF RESTRAINT.

Many of the simple operations on the foot, such as the probing of a sinus, the paring out of corns, or the searching of pricks, may most suitably be performed with the animal's leg held by the operator as a smith holds it for shoeing. According to the temperament of the animal, even the operation for the removal of a portion of the sole, or the injection of sinuses with caustics, may be carried out with the animal simply twitched.

When the operation is still a simple one, casting inconvenient or impossible, and the animal restive, the twitch must be supplemented by some other method. The most simple and one of the most effective is the blind, cap, or bluff (Fig. 38). With it the most vicious animal or the most nervous is in many instances either cowed into submission or soothed into quietness.

At the same time, more forcible means than the operator's own strength must be taken to hold the animal's foot from the ground. If the foot is a fore-foot, and the point desired to be operated on is to the outside, the pastern should be firmly lashed to the forearm by means of a thin, short cord, or a leather strap and buckle. Much may then be done in the way of paring and probing that would otherwise be impossible.