[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 h.o.r.n.y 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 ill.u.s.tration, and one very much to the point at issue, the soft structures of the foot are to the h.o.r.n.y 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 papillae in certain positions (as, for example, the papillae of the coronary cushion, and those of the sensitive frog and sole), has also most p.r.o.nounced ridges (laminae) which run down the whole depth of the os pedis. Each lamina again carries ridges (laminellae) on its lateral aspects, giving a section of a lamina the appearance of being studded with papillae. 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 p.r.o.nounced, it is an exact prototype of the laminal formation of the corium of the horse's foot.

The distribution of the laminae over the foot we have discussed in the chapter devoted to the grosser anatomy. In a macerated foot the sensitive laminae of the corium interdigitate with the h.o.r.n.y laminae 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 papillae of the corium does interlocking of the h.o.r.n.y and sensitive laminae occur. It is only apparent. The h.o.r.n.y laminae 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 Laminae_, 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 papillae 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 papillae, the tips of the fingers the summits of the papillae, and the folds of skin pa.s.sing from finger to finger in the metacarpo-phalangeal region the depressions between the papillae. 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 pa.s.sing from off all the fingers or papillae."

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 ill.u.s.tration, 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 papillae, 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 papillae sunk in the corium, but from papillae projecting from its surface.

Although this disposes of the wall proper, there still confronts us the question of the development of the h.o.r.n.y laminae. 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 bas.e.m.e.nt 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.]

[Ill.u.s.tration: 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).]

[Ill.u.s.tration: FIG. 27.--SECTION FROM FOOT OF SHEEP EMBRYO. It shows a p.r.o.nounced epithelial ingrowth into the corium (Mettam).]

The next stage will demonstrate the first step in the formation of the sensitive laminae.[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 echelon_ into its connective tissue. Each point of the ingrowing lines of the _echelon_ has usually one cell further advanced into the corium than its neighbours, and may be termed the _apical cell_. The fine bas.e.m.e.nt 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 _h.o.r.n.y_ laminae, and cut up the corium into _sensitive_ laminae) are free from irregularities or secondary laminae. Before these are to be observed other changes in connection with the ingrowths are to be noticed.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 h.o.r.n.y elements to form the horn core, or 'h.o.r.n.y laminae' (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 laminae. The laminae 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 pa.s.sing their daughters into the middle of the process, and these pa.s.s through similar stages as those derived from the ensheathing epidermis. In other words, the daughter cells of the const.i.tuents of the rete Malpighii which have grown into the corium pa.s.s 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 h.o.r.n.y laminae, 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 h.o.r.n.y 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 h.o.r.n.y 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 papillae 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.'

[Ill.u.s.tration: FIG. 30.--SECTION THROUGH HOOF AND SOFT TISSUES OF A FOAL AT TERM. The horn of the wall is shown, and the horn-core ('h.o.r.n.y laminae') 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 h.o.r.n.y laminae. We see that the h.o.r.n.y laminae are ingrowths of the rete Malpighii, ploughing into and excavating the corium into the shape of leaves--the sensitive laminae. 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 h.o.r.n.y and the sensitive laminae are both portions of the same thing--namely, a modified skin, in which the epidermis is represented by the h.o.r.n.y laminae, and the corium by the sensitive--it is clear to see that the cells covering the inspreading h.o.r.n.y laminae are dependent for their growth and reproduction upon the cells with which they are in immediate contact--namely, those of the sensitive laminae--and that therefore the sensitive laminae are responsible for the growth of the h.o.r.n.y.

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 alb.u.min, its a.n.a.lysis 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:

Wall. Sole. Frog.

Water 16.12 36.0 42.0 Fatty matter 0.95 0.25 0.50 Matters soluble in water 1.04 1.50 1.50 Insoluble salts 0.26 0.25 0.22 Animal matter 81.63 62.0 55.78

Horn appears to be identical with epidermis, hair, wool, feathers, and whalebone, in yielding 'keratin,' a substance intermediate between alb.u.min 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.

[Ill.u.s.tration: 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 striae are the horn tubules in longitudinal section. The lighter striae 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 h.o.r.n.y laminae, with their laminellae, and the sensitive laminae. The right portion of the figure pictures the corium.

[Ill.u.s.tration: 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 h.o.r.n.y laminae. In this position, however, the structures interdigitating with the h.o.r.n.y laminae are not sensitive, but are themselves h.o.r.n.y. 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.

[Ill.u.s.tration: 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_, h.o.r.n.y laminae.]

[Ill.u.s.tration: FIG. 34.--SECTION OF FROG THROUGH CORIUM AND HORN. The long finger-like projections of corium into epidermis are sections of the long papillae 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 striae (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 papillae 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.