Diseases of the Horse's Foot

by H. Caulton Reeks

A. DEVELOPMENT OF THE HOOF.

Starting with the statement that it is epidermal in origin, we will first consider the structure of the skin, and follow that with a brief description of the structure and mode of growth of the human nail, a short study of which will greatly assist us when we come to investigate the manner of growth of the horse's hoof.

THE SKIN is composed of two portions, the EPIDERMIS and the CORIUM.

THE EPIDERMIS is a stratified epithelium. The superficial layers of the cells composing it are hard and horny, while the deeper layers are soft and protoplasmic. These latter form the so-called Rete Mucosum of Malpighi.

FIG. 23.—VERTICAL SECTION OF EPIDERMIS (HUMAN). (AFTER RANVIER) A, The horny layer of the epidermis; B, the rete mucosum; a, the columnar pigment-containing cells of the rete; b, the polyhedral cells; c, the stratum granulosum; d, the stratum lucidum; e, swollen horny cells; f the stratum squamosum.

Commencing from below and proceeding upwards, we find that the lowermost cells of the rete mucosum, those that are set immediately on the corium, are columnar in shape. In animals that have a coloured skin these cells contain pigment granules. Directly superposed to these we find cells which in shape are polyhedral. Above them, and forming the most superficial layer of the rete mucosum, is a series of flattened, granular-looking cells known as the stratum granulosum.

Immediately above the stratum granulosum the horny portion of the epidermis commences. In the human skin this is formed of three distinct layers. Undermost a layer of clear compressed cells, the stratum lucidum. Next above it a layer of swollen cells, the nuclei of which are indistinguishable. Finally, a surface layer of thin, horny scales, the stratum squamosum, which become detached and thrown off in the form of scurf or dandruff. In the skin of the horse, except where it is thickest, these layers are not clearly defined.

It is the Malpighian layer of the epidermis that is most active in cell division. As they are formed the new cells push upwards those already there, and the latter in their progress to the surface undergo a chemical change in which their protoplasm is converted into horny material. This change, as we have already indicated, takes place above the stratum granulosum.

In addition to its constant formation of cells to replace those cast off from the surface, the active proliferation of the elements of the Malpighian layer is responsible for the development of the various appendages of the skin, the hairs with their sebaceous glands, the sweat glands, horny growths and the hoof, and, in the human subject, the nail. These occur as thickenings and down-growths of the epithelium into the corium.

The epidermis is devoid of bloodvessels, but is provided with fine nerve fibrils which ramify between the cells of the rete mucosum.

THE CORIUM is composed of dense connective tissue, the superficial layer of which bears minute papillæ. These project into the epidermis, which is moulded on them. For the most part the papillæ contain looped capillary vessels, rendering the superficial layer of the corium extremely vascular. Why this must be a moment's reflection will show. The epidermis, as we have already said, is devoid of bloodvessels. It therefore depends entirely for its nourishment upon the indirect supply it receives from the vessels of the corium. The need for extreme vascularity of the corium is further explained when we call to mind the constant proliferation and casting off of the cells of the epidermis, the growth of the hairs, the production of the horn of the hoof, and the work performed by the numerous sweat and other glands.

Others of the papillæ contain nerves, ending here in tactile corpuscles, or continuing, as we have mentioned before, to ramify as fine fibrils in the rete mucosum of the epidermis.

THE HAIRS are growths of the epidermis extending downwards into the deeper part of the corium. Each is developed in a small pit, the Hair Follicle, from the bottom of which it grows, the part lying within the follicle being known as the Root. It is important to note their structure, as it will be seen later that they bear an extremely close relation to the horn of the hoof.

Under a high power of the microscope, and in optical section, the central portion of a hair is tube-like. In some cases the cavity of the tube is occupied by a dark looking substance formed of angular cells, and known as the Medulla. The walls of the tube, or the main substance of the hair, is made up of a pigmented, horny, fibrous material. This fibrous structure is covered by a delicate layer of finely imbricated scales, and is termed the Hair Cuticle.

The root of the hair, that portion within the follicle, has exactly the same formation save at its extreme end. Here it becomes enlarged into a knob-like formation composed of soft, growing cells, which knob-like formation fits over a vascular papilla projecting up in the bottom of the follicle.

We have already stated that the hairs are down-growths of the epidermis. It follows, therefore, that the hair follicles, really depressions or cul-de-sacs of the skin itself, are lined by epithelial cells and connective tissue. So closely does the epidermal portion of the follicle invest the hair root that it is often dragged out with it, and is known as the Root Sheath. This is made up of an outer layer of columnar cells (the outer root sheath) corresponding to the Malpighian layer of the epidermis, and of an inner horny layer, next to the hair, corresponding to the more superficial layer of the epidermis, and known as the inner root sheath.

The hair grows from the bottom of the follicle by a multiplication of the cells covering the papilla upon which its root is moulded. When a hair is cast off a new one is produced from the cells covering the papilla, or, in case of the death or degeneration of the original papilla, the new hair is produced from a second papilla formed in place of the first at the bottom of the follicle.

FIG. 24.—SECTION OF SKIN WITH HAIR FOLLICLE AND HAIR. a, The hair follicle; b, the hair root; c, the medulla; d, the hair cuticle; e, the outer root sheath; f, the inner root sheath; g, the papilla from which the hair is growing; h, a sebaceous gland; i, a sudoriferous gland.

THE SEBACEOUS GLANDS are small saccular glands with their ducts opening into the mouths of the hair follicles. They furnish a natural lubricant to the hairs and the skin.

THE SUDORIFEROUS OR SWEAT GLANDS are composed of coiled tubes which lie in the deeper portion of the skin, and send up a corkscrew-like duct to open on the surface of the epidermis. They are numerous over the whole of the body.

FIG. 25.—LONGITUDINAL SECTION THROUGH NAIL AND NAIL-BED OF A HUMAN FOETAL FINGER.[A] a, The nail; b, the rete mucosum; c, the longitudinal ridges of the corium.

[Footnote A: Seeing that the section is a longitudinal one, it would appear from the way the ridges cut that they are running transversely beneath the nail. Their extreme delicacy, however, prevents a single one showing itself along the length of the section, and their constant accidental cutting makes them appear to run transversely (H.C.R.).]

THE HUMAN NAILS are thickenings of the lowermost layer of the horny portion of the epidermis, the stratum lucidum. They are developed over a modified portion of the corium known as the nail-bed. The horny substance of the nail is composed of clear horny cells, and rests immediately upon a Malpighian layer similar to that found in the epidermis generally. Instead of the papillæ present elsewhere in the skin, the corium of the nail-bed is marked by longitudinal ridges, a similar, though less distinct, arrangement to that found in the laminæ of the horse's foot.

Having thus paved the way, we are now in a better position to discuss our original question (Are the horny laminæ secreted by the sensitive?), and better able to appreciate the work that has been done towards the elucidation of the problem.

A most valuable contribution to this study is an article published in 1896 by Professor Mettam.[A] Here the question is dealt with in a manner that must effectually silence all other views save such as are based upon similar methods of investigation—namely, histological examination of sections of equine hoofs in various stages of foetal development.

[Footnote A: The Veterinarian, vol. lxix., p.1.]

Professor Mettam commences by drawing attention to the error that has been made in this connection by studying the soft structures of the foot separated by ordinary putrefactive changes from the horny covering. "In this way," the writer points out, "a wholly erroneous idea has crept in as to the relation of the one to the other, and the two parts have been treated as two anatomical items, when, indeed, they are portions of one and the same thing. As an illustration, and one very much to the point at issue, the soft structures of the foot are to the horny covering what the corium of the skin and the rete Malpighii are to the superficial portions of the epidermis. Indeed, the point where solution of continuity occurs in macerating is along the line of the soft protoplasmic cells of the rete."

In the foregoing description of the skin we have seen that the corium is not a plane surface, but that it is studded by numerous papillary projections, and that these projections, with the depressions between them, are covered by the cells of the epidermis.

The corium of the horse's foot, however, although possessed of papillæ in certain positions (as, for example, the papillæ of the coronary cushion, and those of the sensitive frog and sole), has also most pronounced ridges (laminæ) which run down the whole depth of the os pedis. Each lamina again carries ridges (laminellæ) on its lateral aspects, giving a section of a lamina the appearance of being studded with papillæ. We have already pointed out the ridge-like formation of the human nail-bed, and noted that, with the exception that the secondary ridges are not so pronounced, it is an exact prototype of the laminal formation of the corium of the horse's foot.

The distribution of the laminæ over the foot we have discussed in the chapter devoted to the grosser anatomy. In a macerated foot the sensitive laminæ of the corium interdigitate with the horny laminæ of the hoof; that is to say, there is no union between the two, for the simple reason that it has been destroyed; they simply interlock like the unglued junction of a finely dovetailed piece of joinery. But no further, however, than the irregularities of the underneath surface of the epidermis of the skin can be said to interlock with the papillæ of the corium does interlocking of the horny and sensitive laminæ occur. It is only apparent. The horny laminæ are simply beautifully regular epidermal ingrowths cutting up the corium into minute leaf-like projections.

In a macerated specimen, then, the exposed sensitive structures of the foot exhibit the corium as (1) the Coronary Cushion, fitting into the cutigeral groove; (2) the Sensitive Laminæ, clothing the outer surface of the terminal phalanx, and extending to the bars; (3) the Plantar Cushion, or sensitive frog; and (4) the Sensitive Sole.

The main portion of the wall is developed from the numerous papillæ covering the corium of the coronary cushion. We have in this way numberless down-growing tubes of horn. Professor Mettam describes their formation in a singularly happy fashion: "Let the human fingers represent the coronary papillæ, the tips of the fingers the summits of the papillæ, and the folds of skin passing from finger to finger in the metacarpo-phalangeal region the depressions between the papillæ. Imagine that all have a continuous covering of a proliferating epithelium. Then we shall have a more or less continuous column of cells growing from the tip of the finger or papilla (a hollow tube of cells gradually moving from off the surface of the finger or papilla like a cast), and similar casts are passing from off all the fingers or papillæ."

From this description it will be noticed that each down-growing tube of horn bears a striking resemblance to the growth of a hair, described on p. 47. In fact, the horn tube may be regarded as what it really is, a modified hair.

We next continue Professor Mettam's illustration, and note how the modified hairs or horn tubes become as it were matted together to form the hoof wall. The cells lining the depressions are also proliferating, and their progeny serve to cement together the hollow casts of the papillæ, thus giving the inter-tubular substance. We have thus produced hollow tubes, united together by cells, all arising from the rete Malpighii of the coronary corium. Section of the lower part of the horn tubes shows them to contain a cellular debris.

Thus, in all, in the horn of the wall we find a tubular, an intertubular, and intratubular substance. In fact, hairs matted together by intertubular material, and only differing from ordinary hairs in their development in that they arise, not from papillæ sunk in the corium, but from papillæ projecting from its surface.

Although this disposes of the wall proper, there still confronts us the question of the development of the horny laminæ. To accurately determine this point it is absolutely essential to examine, histologically, the feet from embryos.

In the foot of any young ungulate in the early stages of intra-uterine life horizontal sections will show a covering of epidermis of varying thickness.[A] This may be only two or three cells thick, or may consist of several layers. Lowermost we find the cells of the rete Malpighii. As some criterion of the activity with which these are acting, it may be noted that with the ordinary stains their nuclei take the dye intensely. The cells of this layer rest upon a basement membrane separating the epidermis from the corium. At this stage the corium has a perfectly plane surface.

[Footnote A: 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).

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.

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.

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.'

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.