Chapter I: The Gross Anatomy of the Horse (2)

3. _The fleshy wall_, or _podophyllous tissue_ (Figs. 27, 28, _d_, and 29, _a_), is all that portion of the pododerm on which there are fleshy leaves. This leafy tissue covers the anterior surface of the os pedis and the lower portion of the external surface of the lateral cartilages. At the bulbs of the heels it turns inward at a sharp angle and extends forward and inward, between the bar portion of the coronary band and the posterior part of the velvety tissue of the sole, nearly to the middle of the solar surface of the foot, to form the _laminæ of the bars_ (Fig. 29, _a_). The fleshy wall and fleshy bars are not covered with villi, but with numerous prominent, parallel, _fleshy leaves_ placed close together, each of which runs in a straight line downward and forward from the coronary band to the lower border of the os pedis. Between the fleshy leaves are deep furrows in which, in a foot which has not been deprived of its horny capsule, lie the horny or insensitive leaves of the wall. The fleshy leaves (podophyllous laminæ) are related to one another somewhat as the leaves of a book; their posterior borders are attached to the body or basement membrane of the fleshy wall, while their anterior borders and sides are free. At their upper ends immediately below the coronary band the leaves are quite narrow, but they gradually increase in width down to the middle, and thereafter maintain that breadth to the lower border of the os pedis, where they terminate in free, fleshy villi, which differ in no respect from those of the fleshy sole. The number and length of the fleshy leaves vary; in a medium-sized foot there are about five hundred, while in a large foot there may be as many as six hundred. On the anterior surface of the os pedis the leaves are thickest and longest; on the sides and quarters they gradually decrease in length, while in the bar region they are the shortest and gradually disappear near the anterior ends of the bars. The width of the leaves decreases as they become shorter. Viewed with the naked eye the leaves appear flat and smooth, but under the microscope one can see on both sides of a fleshy leaf numerous small, fleshy leaflets parallel to one another and extending lengthwise with the larger leaf. The large ones are called _principal leaves_, and the small ones are known as _collateral leaves_, or simply as _leaflets_.

=The fleshy leaves (podophyllous tissue) secrete
the horny leaves (keraphyllous tissue) and serve to
bind the horny wall to the pododerm. The strength
of this union is due largely to the dovetailing of
the horny leaves and their leaflets with the fleshy
leaves and their leaflets.=

4. _The fleshy sole_ or _velvety tissue of the sole_ (Fig. 29, _b_) is that part of the pododerm which covers all the under surface of the foot except the plantar cushion, the bar laminæ, and the bar portion of the coronary band. It is sometimes slate-colored or studded with black spots, but is usually dark-red. It is thickly set with villi, which are especially long and strong[1] near its periphery. =The fleshy sole= covers the solar plexus, or net-work of veins, and =secretes the horny sole=.

[1] In order to see the length, thickness, and abundance of the villi of the pododerm, place the foot deprived of its hoof in a clear glass jar and cover it with water, renewing the latter until it is no longer tinged with blood.

5. _The velvety tissue of the frog_ (Fig. 29, _c_) covers the lower surface of the plantar cushion, and in the region of the bulbs (_e_) passes insensibly into the perioplic band. In comparison with the fleshy sole, it has much finer and shorter villi and contains fewer blood-vessels. =It secretes the soft, horny frog.=

Side view of hoof recently removed: _a_, the perioplic horn-band; it is swollen from prolonged maceration in water; the upper border shows adhering hairs; the inner surface (perioplic groove) presents many minute openings; _a′_, the perioplic horn-band broadens in passing over the bulb or glome of the heel, and is finally lost in the horny frog; _a″_, section of wall removed. That part of hoof on the right of _b_ is called the toe; between _b_ and _c_ is the side wall or “mamma,” and between _c_ and _d_ the “quarter;” _e_, projecting horny frog; _f_, coronary groove with numerous minute openings; _g_, keraphyllous layer of the wall (horny leaves).]

(_c_) =The horn capsule= or =hoof= (Fig. 30) is the entire mass made up of the horn-cells secreted from the whole surface of the pododerm, and next to the shoe is the organ with which the horseshoer has most to do. The horn capsule or hoof is nothing more than a very thick epidermis that protects the horse’s foot, just as a well-fitting shoe protects the human foot. The hoof of a sound foot is so firmly united with the underlying pododerm that only an extraordinary force can separate them. In its normal condition the hoof exactly fits the soft structures within it; hence it is evident that local or general contraction of the hoof must produce pressure on the blood-vessels and nerve-endings of the pododerm, disturb the circulation of the blood and the nutrition of the foot, and cause pain.

Plantar surface of right fore-hoof: _a_, _a_, bearing-surface of the toe; _a_, _b_, bearing-surface of the side walls or mammæ; _b, c_, bearing-surface of the quarters; _d_, buttress, or angle formed by wall and bar; _e_, bar; _f_, sole; _f′_, branches of the sole; _g_, white line; it passes between the sole and bars and ends at _g′_; _h_, horny frog; _i_, branches of the frog; _k_, heels, bulbs, or glomes of the hoof; _l_, median lacuna of horny frog. Between the bars and the horny frog lie the lateral lacunæ of the frog.]

The hoof is divided into three principal parts, which are solidly united in the healthy foot,—namely, the =wall=, the =sole=, and the =frog=. That part of the hoof which is almost wholly visible when the foot is on the ground (Fig. 30, _b_, _c_), and which protects the foot in front and upon the sides, is known as the =wall=. In position, course, direction, and arrangement of its parts it simulates the different parts of the pododerm from which it is developed. It extends from the edge of the hair just above the coronary band to the ground; backward it gradually decreases in height (length), passes around the bulbs of the heels, and turns forward and inward (Fig. 32, _d_, _e_, and 34, _a_, _b_) to form the =bars=, which are finally lost in the edge of the sole near the summit of the frog. It thus forms at each heel an angle (Fig. 31, _d_, and 32, _d_) known as a buttress, which encloses a branch of the horny sole. Externally the wall is smooth, covered with the varnish-like periople, and presents indistinct ring-like markings (Fig. 30). Its inner surface, on the contrary, presents a great number of horn-leaves which are spoken of collectively as the _keraphyllous tissue_ (Figs. 32, _g_, and 35, _f_). The upper or =coronary border= of the wall is thin and flexible, and on its inner aspect is the =coronary groove=, into which fits the =coronary band= (Fig. 30, _f_). The lower border of the wall, called the “=bearing-edge=” or _plantar border_ (Fig. 31, _a_), is the one to which the horseshoe is fastened. By dividing a hoof from before to behind along its median line, _outer_ and _inner_ halves or _walls_ are produced, and by dividing the entire lower circumference of the wall into five equal parts or sections, a =toe=, two =side walls or mammæ=, and two =quarters= will be exhibited (Figs. 32 and 33). In order to designate these regions of the hoof still more accurately, they are spoken of as outer and inner toes, quarters, and heels.

Wall and bars seen from below: _a_, toe; _b_, side wall, or mamma; _c_, quarter; _d_, buttress; _e_, bar; _g_, horn-leaves; _h_, space occupied by the frog.]

_The direction_ (slant) _and length of the wall_ vary in one and the same hoof, as well as between fore and hind hoofs. The portion of the wall of fore-hoofs is the most slanting,—that is, forms the most acute angle with the surface of the ground,—and is also the longest. Towards the quarters the wall gradually becomes very nearly vertical; in almost all hoofs the posterior part of the quarters slants downward and inward towards the median vertical antero-posterior plane of the foot. At the same time the wall, in passing back from the toe to the heel, becomes gradually shorter in such a manner that the heights of the toe, side walls, and quarters are related to one another about as 3: 2: 1 in front hoofs and as 4: 3: 2 in hind hoofs. The outer wall is, as a rule, somewhat more slanting than the inner. Viewing a foot in profile, the toe and heel should be parallel; that is, the line from the hair to the ground at the toe should be parallel to the line from the hair to the ground at the buttress. =All deviations of the wall from a straight line= (outward or inward bendings) =are to be regarded as faults or defects=.

A hoof in profile; _a_, toe (one half); _b_, side wall; _c_, quarter.]

Vertical section through the middle of a hoof, with horny frog removed, to show the position of the bar: _a_, _b_, marks the line at which the wall bends forward and inward towards the median line of the foot to become the bar. Bar runs forward and passes imperceptibly into the sole _c_; _a_, _a′_, the light shading shows the part of the bar that was in contact with the horny frog.]

The _thickness of the wall_ is also variable. In front hoofs the wall is thickest at the toe, and becomes gradually thinner towards the quarters, while in hind hoofs, there is very little difference in the thickness of the wall of the toe, sides, and quarters. _The more slanting half of the hoof is always the thicker_; thus, for example, the outer wall of a base-wide foot is always longer and more oblique than the inner wall, and is also thicker. According to Mayer, the thickness of the wall at the toe varies from three- to five-eighths of an inch, and at the quarters from two to three-eighths of an inch. These measurements are dependent upon the size and breeding of the horse.

The outer wall of the hoof has been removed by cutting vertically through the middle of the toe, down to the upper surface of the sole, then horizontally backward into the quarter, and, finally, upward through the quarter: _a_, perioplic horn-band; _b_, coronary groove; it turns inward and forward at _c_ to form the upper border of the bar; _d_, surface of section of the wall at the toe; _d′_, at the quarter; _e_, surface of horizontal section of the wall near its lower border; _f_, keraphyllous layer of the wall; at _f′_ it turns forward and inward to cover the bar; _f″_, horny leaves standing free and passing insensibly into the white horn of the middle layer or true wall; _g_, horny sole; _h_, white line; _i_, small horn-spur in middle of toe; _k_, part of horny frog which is in intimate union with the upper edge of the bar; _l_, frog-stay of horny frog; it divides the trough-like depression of the upper surface of the frog into _m_, the two upper channels of the frog.]

The horn wall is composed of _three superposed layers_. These from without to within are: (1) the =periople=, secreted by the perioplic band. It is very thin, glistening, and varnish-like in appearance, and covers the entire outer surface of the wall, except where it has been removed by the rasp, and prevents rapid evaporation of moisture from the horn. (2) The middle or =protective layer= (Fig. 35, _d_) is the thickest, strongest, and most important of the three layers; it forms the principal mass of the wall, and is developed or secreted by the coronary band, which fits into the coronary groove. There are in the coronary groove a great number of small, funnel-shaped openings into which project the horn-producing villi or papillæ of the coronary band. (3) The =inner layer= or =keraphyllous layer= (Fig. 35, _f_) consists of prominent, parallel horn-leaves lying side by side over the entire inner surface of the middle layer of the wall, and continuing beyond the buttresses to the ends of the bars (Fig. 35, _f′_). This layer of horn-leaves (keraphyllous layer) has in a general way about the same shape and arrangement as the layer of fleshy leaves (podophyllous layer) which secretes it; for the horn-leaves fit in with the fleshy leaves in such a way that every fleshy leaf is embraced by two horn-leaves, and every horn-leaf by two fleshy leaves (Fig. 36). The keraphyllous layer and the horn of the inmost part of the middle or protective layer are always white, even in pigmented (colored) hoofs.

Cross-section of keraphyllous and podophyllous laminæ (horny and fleshy leaves): _a_, inmost part of the solid wall; the horn-tubes approach very close to the horny leaves; _b_, body of the podophyllous membrane; _c_, horny portion of a horn-leaf directly continuous with the middle or principal layer of the wall; _c′_, a rudimentary horn-leaf that does not reach the body of the podophyllous membrane; _c″_, cross-section of horny leaves from the sides of which branch many secondary leaves (leaflets) composed of soft (young) horn-cells. These soft cellular horn-leaflets dovetail with the podophyllous or fleshy leaflets; _d_, podophyllous laminæ extending from the body of the podophyllous membrane; _d′_, podophyllous laminæ which have branched in their course to the wall, and thus given rise to _c′_, rudimentary horn-leaves; _d″_, cross-section of podophyllous leaflets extending from the sides of the podophyllous leaves; each two such leaflets secrete a keraphyllous leaflet between them; _e_, injected arterial vessels.]

Vertical section of the horny sole magnified: _a_, funnel-shaped openings which contain the horn-producing villi of the fleshy sole; they are of various sizes; _b_, horn-tubes; _c_, intertubular horn.]

Horny frog, with the posterior portion of the perioplic horn-band and the periople which covers the quarters removed from the hoof as one piece by maceration: _a_, trough-shaped depression of upper surface, which is divided posteriorly into the two upper channels of the frog by _b_, the frog-stay; _c_, part of the frog that is joined to the bar and forms the lateral wall of the depression (channels) on upper surface of frog; _d_, lateral surface of horny frog which, in its upper part, adheres to the bar, but below, at _d′_, lies free; _e_, point or summit of the frog; _f_, perioplic horn-band; _f′_, periople of the quarters.]

The =horn sole= (Fig. 31, _f_, and Fig. 35, _g_) is secreted by the velvety tissue of the sole. A sole from which the loose flakes of old horn have been removed is about as thick as the wall. It covers the under surface of the foot, and presents upon its upper surface a convexity which exactly fits into the concavity on the under surface of the os pedis. This upper surface is thickly covered by a multitude of minute funnel-shaped openings for the reception of the villi of the velvety tissue of the sole (Fig. 37). The lower surface of the sole is more or less concave, rough, uneven, and often covered by loose scales of dead horn. Behind, the sole presents a triangular opening whose borders lie partly in contact with the horny frog and partly with the bars. This opening or re-entering angle divides the sole into a _body_ (Fig. 31, _f_) and two wings or _branches_ (Fig. 31, _f′_). The outer border of the sole unites through the medium of the =white line= with the lower part of the inner surface of the wall,—that is, with the keraphyllous layer of the wall. This =white line= (Figs. 31, _g_, and 35, _h_), of so much importance to the horseshoer, is formed by the horn-leaves, and by those short plugs of tubular horn which are secreted by the villi that are always found at the lower ends of the fleshy leaves. The white line may be said to exist wherever the horn-leaves can be discerned upon the plantar surface of the hoof. It not only passes around the circumference of the sole from heel to heel, but may be followed forward from the buttresses along the bars almost to the summit of the frog. The horn of the white line is soft, unpigmented (white), and possesses so very little resistance (strength) that it is often found crumbling or even absent in places. The visible part of the white line is usually of a grayish-black color, owing to the working in from below of dirt and liquid manure, and to staining by rust from the nails. =The white line is very important, since it serves as the point from which we judge of the thickness of the wall, and because the horseshoe nail should penetrate it.=

A horny frog cut vertically and lengthwise through its middle: _a_, upper surface; _b_, frog-stay; _c_, median lacuna of frog, which at _c′_, is overlaid with superposed layers of horn.]

Longitudinal section of the wall magnified. The dark stripes parallel and close together are horn-tubes; the lighter surface between the tubes represents the intertubular horn. Notice that the horn-tubes are of various diameters. The space between _a_ and _b_ represents the small tubes of the outer, darker horn of the principal (middle) layer of the wall; the space between _b_ and _c_ the lighter, inner horn of the wall; _c_, _d_, the horn separating the wall proper from the horny leaves; _d_, _e_, the horny leaves (keraphyllous tissue), on which can be seen fine, parallel, vertical stripes; in the horn-leaf at _f_, _f′_, are seen fissures passing obliquely upward and outward towards the wall.]

=The Frog= (Figs. 31, _h_, 35, _k_, _l_, 38 and 39), secreted by the velvety tissue covering the plantar cushion and presenting almost the same form as the latter, lies as a wedge between the bars and between the edges of the sole just in front of the bars, with both of which structures it is intimately united. Its horn is _quite soft_ and _very elastic_. The median lacuna or cleft of the frog (Fig. 31, _l_) divides it into two branches (Fig. 31, _i_), which pass backward and outward into the horny bulbs (Fig. 31, _k_). In front of the median lacuna the two branches unite to form the _body_ of the frog (Fig. 31, _h_), which ends in a point, designated the _point_, apex, or summit of the frog. On the upper surface of the frog, directly over the median cleft of the lower surface, there is a small projection called the frog-stay (Figs. 35, _l_, 38 and 39, _b_), which fits into the median cleft of the plantar cushion. Besides, the upper surface of the frog shows many minute openings, similar to but smaller than those of the sole and coronary groove, for the reception of villi. In unshod hoofs the frog, sole, bars, and bearing-edge of the wall are on a level; that is, the plantar surface of such hoofs is perfectly flat.

Cross-section of the wall, magnified: _a_, horn-tubes; _b_, intertubular horn.]

The _minute structure of the horn_ can scarcely be considered in detail in an elementary treatise such as this is. However, a few of the most important facts are as follows:

If we carefully examine a transverse section of the horn of the wall (Fig. 41), sole, or frog, we will see with the naked eye, though much better with a magnifying glass, many minute points quite close to one another, and greatly resembling the small openings which we have seen in the coronary groove of the wall and on the upper surface of the horny sole and frog. If, now, we examine a longitudinal section of the wall (Fig. 40) or sole, we will see a number of fine, dark stripes which are straight, parallel, quite close to one another, of different widths, and which are separated by bands of lighter horn also of different widths. A thin section or slice of the wall taken at right angles to the direction of these dark lines (Fig. 41) shows us that the minute points that are visible to the naked eye, when held up to the light or moderately magnified, prove to be small openings (Fig. 41, _a_). Since these openings, shown in Fig. 41, represent the dark lines shown in Fig. 40, because an opening is found wherever there is a dark line, _we must regard all dark lines seen in longitudinal sections of wall, sole, and frog as hollow cylinders or tubes_, though they are not always hollow, but are often filled with loosely adjusted, crumbling, broken down horn-cells. The dark edges of the openings (_a_) consist of thick layers of horn-cells (tube-walls). The entire structure is called a =horn-tube=, and the lighter-colored masses of horn (Fig. 41, _b_) between the tubes are known as =intertubular horn=.

=With the exception of the horny leaves of the wall
and bars, all the horn of the hoof is composed of
horn-tubes and intertubular horn.=

The horn-tubes of the wall, sole, and frog always run downward and forward parallel to the direction of the wall at the toe,—that is, in a direction parallel with the inclination of the hoof as a whole. Although the wall, sole, and frog differ from one another considerably with respect to the size and number of the horn-tubes, the quality of the intertubular horn, and the thickness and strength of the horn-cells, these differences are only of subordinate interest or importance to the horseshoer; but he who desires to learn more of this matter is referred to the work of Leisering & Hartmann, “Der Fuss des Pferdes in Rücksicht auf Bau, Verrichtungen und Hufbeschlag,” eighth edition, Dresden, 1893. This book also treats of the variations in the quality of hoofs, which is very important for the practical horseshoer to know. It, furthermore, considers the solidity and strength of the horn of the different parts of the hoof.

With respect to solidity, two kinds of horn are distinguished,—namely, =hard= and =soft horn=. The periople, the white line, and the frog are soft horn structures; the middle layer of the wall and the sole are hard or solid horn. The wall, however, is somewhat harder and more tenacious than the sole, for the latter passes off in more or less large flakes (exfoliates) or crumbles away on its lower surface, at least in shod feet, while no such spontaneous shortening occurs in the wall.

Vertical section through middle of a forefoot, the skin and pododerm being in red. (In the figure the direction of both long and short pasterns, _B_ and _D_, is too nearly vertical—too steep). _A_, metacarpal bone (cannon); _B_, os suffraginis (long pastern); _C_, inner sesamoid bone (to render it visible a portion of the intersesamoid ligament was removed); _D_, os coronæ (short pastern); _E_, os pedis (foot-bone); _F_, navicular bone; _a_, extensor tendon; _b_, suspensory ligament of the fetlock; _b′_, superficial inferior sesamoid ligament; _c_, perforatus tendon or flexor of the os coronæ; _c′_, ring passing forward from this tendon and encircling the perforans tendon; _d_, perforans tendon; _e_, capsular ligament of fetlock-joint; _f_, capsular ligament of coronary joint; _g_, _g′_, capsular ligament of pedal joint; _h_, synovial sheath of the perforans tendon; _i_, plantar cushion and fleshy frog; _i′_, bulbs or glomes of plantar cushion; _i″_, indicates the lowest point reached by the plantar cushion, which in the figure is hidden below by the frog-stay of the horny frog; _k_, coronary band (red); _l_, podophyllous tissue (red); _m_, velvety tissue of the sole (red); _n_, velvety tissue of fleshy frog (red); _o_, wall; _p_, sole; _q_, frog; _q′_, the inner half of the frog-stay which reposes in the median lacuna of the fleshy frog; _s_, hair-skin (red).]

Soft horn differs from hard horn in that its horn-cells never become hard and horn-like. It is very elastic, absorbs water quickly, and as readily dries out and becomes very hard and brittle and easily fissured and chapped. With respect to _quality_, we distinguish good and bad horn; the former is fine and tenacious (tough), the latter coarse and either soft and crumbling or hard and brittle. If not dried out, all horn is elastic, though soft horn is more elastic than hard. All horn is a _poor conductor of heat_.

The relative positions of the various parts of the foot are shown in Fig. 42.

Fig. 43 represents the exterior of a well-formed foot.

Right forefoot viewed from the side: _A_, lower end of the cannon; _B_, fetlock-joint; _C_, long pastern; _D_, coronet; _E_, hoof; _F_, heel; _F′_ inner heel; _G_, foot-lock covering the ergot.]