The foot-axis is an imaginary line pa.s.sing from the fetlock joint through the long axes of the two pasterns and coffin bone. This imaginary line, which shows the direction of the pasterns and coffin bone, should always be straight--that is, never broken, either forward or backward when viewed from the side, or inward or outward when observed from in front. Viewed from one side, the long axis of the long pastern, when prolonged to the ground, should be parallel to the line of the toe. Viewed from in front, the long axis of the long pastern, when prolonged to the ground, should cut the hoof exactly at the middle of the toe.

Raising the heel or shortening the toe not only tilts the coffin bone forward and makes the hoof stand steeper at the toe, but slackens the tendon that attaches to the under surface of the coffin bone (perforans tendon), and therefore allows the fetlock joint to sink downward and backward and the long pastern to a.s.sume a more nearly horizontal position. The foot-axis, viewed from one side, is now broken forward; that is, the long pastern is less steep than the toe, and the heels are either too long or the toe is too short. On the other hand, raising the toe or lowering the heels of a foot with a straight foot-axis not only tilts the coffin bone backward and renders the toe more nearly horizontal, but tenses the perforans tendon, which then forces the fetlock joint forward, causing the long pastern to stand steeper. The foot-axis, seen from one side, is now broken backward--an indication that the toe is relatively too long or that the heels are relatively too low.

The elastic tissues of the foot are preeminently the lateral cartilages and the plantar cushion. The lateral cartilages are two irregularly four-sided plates of gristle, one on either side of the foot, extending from the wings of the coffin bone backward to the heels and upward to a distance of an inch or more above the edge of the hair, where they may be felt by the fingers. When sound, these plates are elastic and yield readily to moderate finger pressure, but from various causes may undergo ossification, in which condition they are hard and unyielding. The plantar cushion is a wedge-shaped ma.s.s of tough, elastic, fibro-fatty tissue filling all the s.p.a.ce between the lateral cartilages, forming the fleshy heels and the fleshy frog, and serving as a buffer to disperse shock when the foot is set to the ground. It extends forward underneath the navicular bone and perforans tendon, and protects these structures from injurious pressure from below. Instantaneous photographs show that at speed the horse sets the heels to the ground before other parts of the foot--conclusive proof that the function of this tough, elastic structure is to dissipate and render harmless violent impact of the foot with the ground.

The horn-producing membrane, or "quick," as it is commonly termed, is merely a downward prolongation of the "derm," or true skin, and may be conveniently called the pododerm (foot skin). The pododerm closely invests the coffin bone, lateral cartilages, and plantar cushion, much as a sock covers the human foot, and is itself covered by the h.o.r.n.y capsule, or hoof. It differs from the external skin, or hair skin, in having no sweat or oil glands, but, like it, is richly supplied with blood vessels and sensitive nerves. And, just as the derm of the hair skin produces upon its outer surface layer upon layer of h.o.r.n.y cells (epiderm), which protect the sensitive and vascular derm, so, likewise, in the foot the pododerm produces over its entire surface soft cells, which, pushed away by more recent cells forming beneath, lose moisture by evaporation and are rapidly transformed into the corneous material which we call the hoof. It is proper to regard the hoof as a greatly thickened epiderm having many of the qualities possessed by such epidermal structures as hair, feathers, nails, claws, etc.

The functions of the pododerm are to produce the hoof and to unite it firmly to the foot.

There are five parts of the pododerm, easily distinguishable when the hoof has been removed, namely: (1) The perioplic band, a narrow ridge from one-sixteenth to one-eighth of an inch wide, running along the edge of the hair from one heel around the toe to the other. This band produces the perioplic horn, the thin varnishlike layer of glistening horn, which forms the surface of the wall or "crust," and whose purpose seems to be to r.e.t.a.r.d evaporation of moisture from the wall. (2) The coronary band, a prominent fleshy cornice encircling the foot just below and parallel to the perioplic band. At the heels it is reflected forward along the sides of the fleshy frog, to become lost near the apex of this latter structure. The coronet produces the middle layer of the wall, and the reflexed portions produce the "bars," which are, therefore, to be regarded merely as a turning forward of the wall. (3) The fleshy leaves, 500 to 600 in number, parallel to one another, running downward and forward from the lower edge of the coronary band to the margin of the fleshy sole. They produce the soft, light-colored h.o.r.n.y leaves which form the deepest layer of the wall, and serve as a strong bond of union between the middle layer of the wall and the fleshy leaves with which they dovetail. (4) The fleshy sole, which covers the entire under surface of the foot, excepting the fleshy frog and bars. The h.o.r.n.y sole is produced by the fleshy sole. (5) The fleshy frog, which covers the under surface of the plantar cushion and produces the h.o.r.n.y frog.

The h.o.r.n.y box or hoof consists of wall and bars, sole and frog. The wall is all that part of the hoof which is visible when the foot is on the ground (see fig. 8). As already stated, it consists of three layers--the periople, the middle layer, and the leafy layer.

The bars (see fig. 1c) are forward prolongations of the wall, and are gradually lost near the point of the frog. The angle between the wall and a bar is called the "b.u.t.tress." Each bar lies against the h.o.r.n.y frog on one side and incloses a wing of the sole on the other, so that the least expansion or contraction of the h.o.r.n.y frog separates or approximates the bars, and through them the lateral cartilages and the walls of the quarters. The lower border of the wall is called the "bearing edge," and is the surface against which the shoe bears. By dividing the entire lower circ.u.mference of the wall into five equal parts, a toe, two side walls, and two quarters will be exhibited. The "heels," strictly speaking, are the two rounded soft prominences of the plantar cushion, lying one above each quarter. The outer wall is usually more slanting than the inner, and _the more slanting half of a hoof is always the thicker_. In front hoofs the wall is thickest at the toe and gradually thins out toward the quarters, where in some horses it may not exceed one-fourth of an inch. In hind hoofs there is much less difference in thickness between the toe, side walls, and quarters. The h.o.r.n.y sole, from which the flakes of old horn have been removed, is concave and about as thick as the wall at the toe. It is rough, uneven and often covered by flakes of dead horn in process of being loosened and cast off. Behind the sole presents an opening into which are received the bars and h.o.r.n.y frog. This opening divides the sole into a body and two wings.

The periphery of the sole unites with the lower border of the wall and bars through the medium of the white line, which is the cross section of the leafy horn layer of the wall and of short plugs of horn which grow down from the lower ends of the fleshy leaves. This white line is of much importance to the sh.o.e.r, since its distance from the outer border of the hoof is the thickness of the wall, and in the white line all nails should be driven.

The frog, secreted by the pododerm covering the plantar cushion or fatty frog, and presenting almost the same form as the latter, lies as a soft and very elastic wedge between the bars and between the edges of the sole just in front of the bars. A broad and shallow depression in its center divides it into two branches, which diverge as they pa.s.s backward into the h.o.r.n.y bulbs of the heel. In front of the middle cleft the two branches unite to form the body of the frog, which ends in the point of the frog. The bar of a bar shoe should rest on the branches of the frog.

In unshod hoofs the bearing edge of the wall, the sole, frog, and bars are all on a level; that is, the under surface of the hoof is perfectly flat, and each of these structures a.s.sists in bearing the body weight.

With respect to solidity, the different parts of the hoof vary widely.

The middle layer of the wall is harder and more tenacious than the sole, for the latter crumbles away or pa.s.ses off in larger or smaller flakes on its under surface, while no such spontaneous shortening of the wall occurs. The white line and the frog are soft-horn structures, and differ from hard horn in that their horn cells do not under natural conditions become hard and hornlike. They are very elastic, absorb moisture rapidly, and as readily dry out and become hard, brittle, and easily fissured. Horn of good quality is fine grained and tough, while bad horn is coa.r.s.e grained and either mellow and friable or hard and brittle. All horn is a poor conductor of heat, and the harder (drier) the horn the more slowly does it transmit extremes of temperature.

THE PHYSIOLOGICAL MOVEMENTS OF THE HOOF.

A hoof while supporting the body weight has a different form, and the structures inclosed within the hoof have a different position than when not bearing weight. Since the amount of weight borne by a foot is continually changing, and the relations of internal pressure are continuously varying, a foot is, from a physiological viewpoint, never at rest. The most marked changes of form of the hoof occur when the foot bears the greatest weight, namely, at the time of the greatest descent of the fetlock. Briefly, these changes of form are: (1) An expansion or widening of the whole back half of the foot from the coronet to the lower edge of the quarters. This expansion varies between one-fiftieth and one-twelfth of an inch. (2) A narrowing of the front half of the foot, measured at the coronet. (3) A sinking of the heels and a flattening of the wings of the sole. These changes are more marked in the half of the foot that bears the greater weight.

The changes of form occur in the following order. When the foot is set to the ground the body weight is transmitted through the bones and sensitive and h.o.r.n.y leaves to the wall. The coffin bone and navicular bone sink a little and rotate backward. At the same time the short pastern sinks backward and downward between the lateral cartilages and presses the perforans tendon upon the plantar cushion. This cushion being compressed from above and being unable to expand downward by reason of the resistance of the ground acting against the h.o.r.n.y frog, acts like any other elastic ma.s.s and expands toward the sides, pushing before it the yielding lateral cartilages and the wall of the quarters.

This expansion of the heels is a.s.sisted and increased by the simultaneous flattening and lateral expansion of the resilient h.o.r.n.y frog, which crowds the bars apart. Of course, when the lateral cartilages are ossified, not only is no expansion of the quarters possible, but frog pressure often leads to painful compression of the plantar cushion and to increase of lameness. Frog pressure is therefore contraindicated in lameness due to sidebones (ossified cartilages).

Under the descent of the coffin bone the h.o.r.n.y sole sinks a little; that is, the arch of the sole around the point of the frog and the wings of the sole become somewhat flattened. All these changes of form are most marked in sound unshod hoofs, because in them ground pressure on the frog and sole is p.r.o.nounced; they are more marked in fore hoofs than in hind hoofs.

The movement of the different structures within the foot and the changes of form that occur at every step are indispensable to the health of the hoof, so that these elastic tissues must be kept active by regular exercise, with protection against drying out of the hoof. Long-continued rest in the stable, drying out of the hoof, and shoeing decrease or alter the physiological movements of the hoof and sometimes lead to foot diseases. Since these movements are complete and spontaneous only in unshod feet, shoeing must be regarded as an evil, albeit a necessary one, and indispensable if we wish to keep horses continuously serviceable on hard, artificial roads. However, if in shoeing we bear in mind the structure and functions of the hoof and apply a shoe whose branches have a wide and level bearing surface, so as to interfere as little as may be with the expansion and contraction of the quarters, in so far as this is not hindered by the nails, we need not be apprehensive of trouble, provided the horse has reasonable work and his hoofs proper care.

GROWTH OF THE HOOF.

All parts of the hoof grow downward and forward with equal rapidity, the rate of growth being largely dependent upon the amount of blood supplied to the pododerm, or "quick." Abundant and regular exercise, good grooming, moistness and suppleness of the hoof, going barefoot, plenty of good feed, and at proper intervals removing the overgrowth of hoof and regulating the bearing surface, by increasing the volume and improving the quality of the blood flowing into the pododerm, favor the rapid growth of horn of good quality; while lack of exercise, dryness of the horn, and excessive length of the hoof hinder growth.

The average rate of growth is about one-third of an inch a month. Hind hoofs grow faster than fore hoofs and unshod ones faster than shod ones.

The time required for the horn to grow from the coronet to the ground, though influenced to a slight degree by the precited conditions, varies in proportion to the distance of the coronet from the ground. At the toe, depending on its height, the horn grows down in 11 to 13 months, at the side wall in 6 to 8 months, and at the heels in 3 to 5 months. We can thus estimate with tolerable accuracy the time required for the disappearance of such defects in the hoof as cracks, clefts, etc.

Irregular growth is not infrequent. The almost invariable cause of this is an improper distribution of the body weight over the hoof--that is, an unbalanced foot. Colts running in soft pasture or confined for long periods in the stable are frequently allowed to grow hoofs of excessive length. The long toe becomes "dished"--that is, concave from the coronet to the ground--the long quarters curl forward and inward and often completely cover the frog and lead to contraction of the heels, or the whole hoof bends outward or inward, and a crooked foot, or, even worse, a crooked leg, is the result if the long hoof be allowed to exert its powerful and abnormally directed leverage for but a few months upon young plastic bones and tender and lax articular ligaments. All colts are not foaled with straight legs, but failure to regulate the length and bearing of the hoof may make a straight leg crooked and a crooked leg worse, just as intelligent care during the growing period can greatly improve a congenitally crooked limb. If breeders were more generally cognizant of the power of overgrown and unbalanced hoofs to divert the lower bones of young legs from their proper direction, and, therefore, to cause them to be moved improperly, with loss of speed and often with injury to the limbs, we might hope to see fewer knock-kneed, bow-legged, "splay-footed," "pigeon-toed," cow-hocked, interfering, and paddling horses.

If in shortening the hoof one side wall is, from ignorance, left too long or cut down too low with relation to the other, the foot will be unbalanced, and in traveling the long section will touch the ground first and will continue to do so till it has been reduced to its proper level (length) by the increased wear which will take place at this point. While this occurs rapidly in unshod hoofs, the shoe prevents wear of the hoof, though it is itself more rapidly worn away beneath the high (long) side than elsewhere, so that by the time the shoe is worn out the tread of the shoe may be flat. If this mistake be repeated from month to month, the part of the wall left too high will grow more rapidly than the low side whose pododerm is relatively anemic as a result of the greater weight falling into this half of the hoof, and the ultimate result will be a "wry," or crooked foot.

THE CARE OF UNSHOD HOOFS.

The colt should have abundant exercise on dry ground. The hoofs will then wear gradually, and it will only be necessary from time to time to regulate any uneven wear with the rasp and to round off the sharp edge about the toe in order to prevent breaking away of the wall.

Colts in the stable can not wear down their hoofs, so that every four to six weeks they should be rasped down and the lower edge of the wall well rounded to prevent chipping. The soles and clefts of the frog should be picked out every few days and the entire hoof washed clean. Plenty of clean straw litter should be provided. Hoofs that are becoming "awry"

should have the wall shortened in such a manner as to straighten the foot-axis. This will ultimately produce a good hoof and will improve the position of the limb.

[Ill.u.s.tration: FIG. 1.--Ground surface of a right fore hoof of the regular form: a, a, wall; a-a, the toe; a-b, the side walls; b-d, the quarters; c, c, the bars; d, d, the b.u.t.tress; e, lateral cleft of the frog; f, body of the sole; g, g', g", leafy layer (white line) of the toe and bars; h, body of the frog; i, i, branches of the frog; k, k, h.o.r.n.y bulbs of the heels; l, middle cleft of the frog.]

CHARACTERISTICS OF A HEALTHY HOOF.

A healthy hoof (figs. 1 and 8) is equally warm at all parts, and is not tender under pressure with the hands or moderate compression with pincers. The coronet is soft and elastic at all points and does not project beyond the surface of the wall. The wall (fig. 8) is straight from coronet to ground, so that a straightedge laid against the wall from coronet to ground parallel to the direction of the horn tubes will touch at every point. The wall should be covered with the outer varnishlike layer (periople) and should show no cracks or clefts. Every hoof shows "ring formation," but the rings should not be strongly marked and should always run parallel to the coronary band. Strongly marked ring-formation over the entire wall is an evidence of a weak hoof, but when limited to a part of the wall is evidence of previous local inflammation. The bulbs of the heels should be full, rounded, and of equal height. The sole (fig. 1) should be well hollowed out, the white line solid, the frog well developed, the middle cleft of the frog broad and shallow, the s.p.a.ces between the bars and the frog wide and shallow, the bars straight from the b.u.t.tress toward the point of the frog, and the b.u.t.tresses themselves so far apart as not to press against the branches of the frog. A hoof can not be considered healthy if it presents reddish discolored horn, cracks in the wall, white line, bars, or frog, thrush of the frog, contraction or displacement of the heels.

The lateral cartilages should yield readily to finger pressure.

[Ill.u.s.tration: FIG. 2.--Pair of fore feet of regular form in regular standing position.]

VARIOUS FORMS OF HOOFS.

As among a thousand human faces no two are alike, so among an equal number of horses no two have hoofs exactly alike. A little study of different forms soon shows us, however, that the form of every hoof is dependent in great measure on the direction of the two pastern bones as viewed from in front or behind, or from one side; and that all hoofs fall into three cla.s.ses when we view them from in front and three cla.s.ses when we observe them in profile. Inasmuch as the form of every foot determines the peculiarities of the shoe that is best adapted to it, no one who is ignorant of, or who disregards the natural form of, a hoof can hope to understand physiological shoeing.

[Ill.u.s.tration: FIG. 3.--Pair of fore feet of base-wide form in toe-wide standing position.]

FORMS OF FEET VIEWED FROM IN FRONT AND IN PROFILE.

Whether a horse's feet be observed from in front or from behind, their form corresponds to, or at least resembles, either that of the regular position (fig. 2), the base-wide or too-wide position (fig. 3), or the base-narrow position (fig. 4).

By the direction of the imaginary line pa.s.sing through the long axes of the two pasterns (figs. 2, 4, 5) we determine whether or not the hoof and pasterns stand in proper mutual relation.

In the regular standing position (fig. 2) the foot-axis runs straight downward and forward; in the base-wide position (fig. 3) it runs obliquely downward and outward, and in the base-narrow position (fig. 4) it runs obliquely downward and inward.

[Ill.u.s.tration: FIG. 4.--Pair of fore feet of base-narrow form in toe-narrow standing position.]

Viewing the foot in profile, we distinguish the regular position (fig.

5b) and designate all forward deviations as acute-angled (long toe and low heel, fig. 5a), and all deviations backward from the regular (steep toe and high heel, fig. 5c) as steep-toed, or stumpy. When the body weight is evenly distributed over all four limbs, the foot-axis should be straight; the long pastern, short pastern, and wall at the toe should have the same slant.

[Ill.u.s.tration: FIG. 5.--a, Side view of an acute-angled fore foot (shod); b, side view of a regular fore foot, showing the most desirable degree of obliquity (34); c, side view of a stumpy, or "upright," fore foot; obliquity above 50. In a, b, c, note particularly the relation between the length of the slope and the overhanging of the heels. Note also the toe roll of the shoes.]

_A front hoof of the regular standing position._--The outer wall is a little more slanting and somewhat thicker than the inner. The lower border of the outer quarter describes the arc of a smaller circle--that is, is more sharply bent than the inner quarter. The weight falls near the center of the foot and is evenly distributed over the whole bottom of the hoof. The toe forms an angle with the ground of 45 to 50 and is parallel to the direction of the long pastern. The toe points straight ahead, and when the horse is moving forward in a straight line the hoofs are picked up and carried forward in a line parallel to the middle line of the body, and are set down flat. Coming straight toward the observer the hoofs seem to rise and fall perpendicularly.

_A hoof of the base-wide position_ is always awry. The outer wall is more slanting, longer, and thicker than the inner, the outer quarter more curved than the inner, and the outer half of the sole wider than the inner. The weight falls largely into the inner half of the hoof. In motion the hoof is moved in a circle. From its position on the ground it breaks over the inner toe, is carried forward and inward close to the supporting leg, thence forward and outward to the ground, which the hoof meets first with the outer toe. Horses that are toe-wide ("splay-footed"--toes turned outward) show all these peculiarities of hoof-form and hoof-flight to a still more marked degree and are therefore more p.r.o.ne to "interfere" when in motion.

_A hoof of the base-narrow position_ is awry, but not to so marked a degree as the base-wide hoof. The inner wall is usually a little more slanting than the outer, the inner half of the sole wider than the outer, and the inner quarter more curved than the outer. The outer quarter is often flattened and drawn in at the bottom. The weight falls largely into the outer half of the hoof. In motion the hoof breaks over the outer toe, is carried forward and outward at some distance from the supporting leg, thence forward and inward to the ground, which it generally meets with the outer toe. The foot thus moves in a circle, whose convexity is outward, a manner of flight called "paddling." A base-narrow horse, whose toes point straight ahead, frequently "interferes," while a toe-narrow (pigeon-toed) animal seldom does.

_A regular hoof_ (fig. 5b), viewed from one side, has a straight foot-axis inclined to the horizon at an angle of 45 to 50. The weight falls near the center of the foot and there is moderate expansion of the quarters.

_An acute-angled hoof_ (fig. 5a) has a straight foot-axis inclined at an angle less than 45 to the horizon. The weight falls more largely in the back half of the hoof and there is greater length of hoof in contact with the ground and greater expansion of the heels than in the regular hoof.

_Upright or stumpy hoof._--In the upright or stumpy hoof (fig. 5c) the foot-axis is straight and more than 55 steep. The hoof is relatively short from toe to heel, the weight falls farther forward, and there is less expansion of the heels than in the regular hoof.