G. THE COMPLEMENTARY APPARATUS OF THE OS PEDIS.

This consists of two lateral pieces, the LATERAL CARTILAGES or _Fibro-cartilages_ of the pedal bone, united behind and below by the _Plantar Cushion_.

1. THE LATERAL CARTILAGES.--Each is a flattened plate of cartilage, possessing two faces and four borders separated by four angles.

The external face is convex, covered by a plexus of veins, and slightly overhangs the pedal bone. The internal face is concave, and covers in front the pedal articulation and the synovial sac, already mentioned as protruding between the antero- and postero-lateral ligaments of that joint.

We have already remarked that this is a point of interest to be remembered in connection with the operation for quittor. Below and behind, the internal face of the cartilage is united to the plantar cushion.

[Ill.u.s.tration: FIG. 16.--EXTERNAL FACE OF THE OUTER LATERAL CARTILAGE. 1, External face of cartilage--(_a_) its upper border, (_b_) its posterior border, (_c_) its anterior border, (_d_) its inferior border; 2, the os pedis; 3, wing of os pedis.]

The upper border, sometimes convex, sometimes straight, is thin and bevelled, and may easily be felt in the living animal. It is this border that the digital vessels cross to gain the foot, and the border is often broken by a deep notch to accommodate them. The inferior border is attached in front to the basilar and retrossal processes, behind which it blends with the plantar cushion. The posterior border is oblique from before to behind, and above to below, and joins the preceding two. The anterior border is oblique in the same direction, and is intimately attached to the antero-lateral ligament of the pedal articulation. The cartilages of the fore-feet are thicker and more extensive than those of the hind.

2. THE PLANTAR CUSHION on FIBRO-FATTY FROG.--Composed of a fibrous meshwork, in the interstices of which are lodged fine elastic and connective fibres and fat cells, this wedge-shaped body occupies the s.p.a.ce between the two lateral cartilages, the extremity of the perforans tendon, and the h.o.r.n.y frog. It offers for consideration an antero-superior and an infero-posterior face, a base, an apex, and two borders.

The antero-superior face is in contact with the terminal expansion of the perforans tendon. The infero-posterior face is covered by the keratogenous membrane, and follows closely the shape of the h.o.r.n.y frog, on whose inner surface it is moulded. It presents, therefore, at its centre a single conical prolongation, the _Pyramidal Body_, which is continued behind, as is the h.o.r.n.y frog, in the shape of two lateral ridges divided by a median cleft. The _base_ of the cushion lies behind, and consists of two lateral ma.s.ses, _the Bulbs of the Plantar Cushion_. In front these are continuous with the ridges of the pyramidal body, while behind they become confounded with the lateral cartilages and the coronary cushion. The _apex_ is fixed into the plantar surface of the os pedis, in front of its semilunar ridge.

The _borders_, right and left, are wider behind than before, and are in relation with the inner faces of the lateral cartilages.

H. THE KERATOGENOUS MEMBRANE.

THE KERATOGENOUS, OR HORN-PRODUCING MEMBRANE, is in reality an extension of the dermis of the digit. It covers the extremity of the digit as a sock covers the foot, spreading over the insertion of the extensor pedis, the lower half of the external face of the lateral cartilages, the bulbs of the plantar cushion, the pyramidal body, the anterior portion of the plantar surface of the os pedis, and over the anterior face of the same bone. In turn, as the human foot with its sock is covered by the boot, this is encased by the hoof, the formation of which we shall study later.

To expose the membrane for study the hoof must be removed. This may be done in two ways. By roasting in a fire, and afterwards dragging off the h.o.r.n.y structures with a pair of pincers, a knife having first been pa.s.sed round the superior edge of the h.o.r.n.y box. Or by maceration in water for several days, when the hoof will become loosened by the process of decomposition, and may be easily removed by the hands. The latter method is less likely to injure the sensitive structures, and will expose them with a fresh appearance for observation.

For purposes of description the keratogenous membrane is divided into three regions:

1. The Coronary Cushion.

2. The Velvety Tissue.

3. The Podophyllous Tissue, or the Sensitive laminae.

1. THE CORONARY CUSHION. In the foot stripped of the hoof the coronary cushion is seen as a rounded structure overhanging the sensitive laminae after the manner of a cornice. It extends from the inner to the outer bulbs of the plantar cushion, and is bounded above by the perioplic ring, and below by the laminae.

When _in situ_ it is accommodated by the _Cutigeral Groove_, a cavity produced by the bevelling out of the superior portion of the inner face of the wall of the hoof. Its superior surface is covered by numerous elongated papillae, set so closely as to give the appearance of the 'pile' of velvet.

This is observed to the best advantage with the foot immersed in water.

_The Superior Border_ of the cushion is bounded by the _Perioplic Ring_, the cells of which have as their function the secreting of the _Periople_, a layer of thin horn to be noted afterwards as covering the external face of the wall. From the perioplic ring the cushion is separated by a narrow and shallow, though well-marked, groove.

The inferior border is bounded by the sensitive laminae.

[Ill.u.s.tration: FIG. 17.--THE KERATOGENOUS MEMBRANE (VIEWED FROM THE SIDE).

(THE HOOF REMOVED BY MACERATION.) 1. The sensitive laminae, or podophyllous tissue; 2, the coronary cushion; 3, the perioplic ring; 4, portion of plantar cushion; 5, groove separating perioplic ring from coronary cushion; 6. the sensitive sole.]

The upper portions of the laminae, those in contact with the cushion, are pale in contrast with the portions immediately below, and thus there is given the appearance of a white zone adjoining the inferior border of the cushion.

Widest at its centre, the cushion narrows towards its extremities, which, arriving at the bulbs of the plantar cushion, bend downwards into the lateral lacunae of the pyramidal body, where they merge into the velvety tissue of the sole and frog.

The papillae of the coronary cushion secrete the horn tubules forming the wall, and the papillae of the perioplic ring secrete the varnish-like veneer of thin horn covering the outside surface of the hoof.

[Ill.u.s.tration: FIG. 18.--THE KERATOGENOUS MEMBRANE (VIEWED FROM BELOW).

(THE HOOF REMOVED BY MACERATION.) 1, The sensitive sole; 2, the sensitive frog[A]--(a) its median lacuna, (6) its lateral lacuna; 3. V-shaped depression accommodating the toe-stay; 4, the sensitive laminae which interleave with the h.o.r.n.y laminae of the bar.]

[Footnote A: The sensitive frog thinly invests the plantar cushion or fibre-fatty frog, the outline of which is here indicated.]

2. THE VELVETY TISSUE.--This is the portion of the keratogenous membrane covering the plantar surface of the os pedis and the plantar cushion. To the irregularities of the latter body--its bulbs, pyramidal body, and its lacunae--it is closely adapted. Its surface may, therefore, be divided into _(a) The Sensitive Frog_, and _(b) The Sensitive Sole_.

_(a) The Sensitive Frog_ is that part of the velvety tissue moulded on the lower surface of the plantar cushion. The shape of the plantar cushion has already been described as identical with that of the h.o.r.n.y frog. It only remains to state that, like the coronary cushion, the surface of the sensitive frog is closely studded with papillae. The cells clothing the papillae are instrumental in forming the h.o.r.n.y frog.

_(b) The Sensitive Sole_.--As its name indicates, this is the portion of the keratogenous membrane that covers the plantar surface of the os pedis.

It also is clothed with papillae, which again give rise to the formation of that part of the h.o.r.n.y box to which they are adapted--namely, the sole.

3. THE PODOPHYLLOUS TISSUE, OR SENSITIVE LAMINae.--This portion of the keratogenous membrane is spread over the anterior face and sides of the os pedis, limited above by the coronary cushion, and below by the inferior edge of the bone. It presents the appearance of fine longitudinal streaks, which, when closely examined with a needle, are found to consist of numerous fine leaves. These extend downwards from the lower border of the coronary cushion to the inferior margin of the os pedis. At this point each terminates in several large villous prolongations, which extend into the h.o.r.n.y tubes at the circ.u.mference of the sole. At the point of the toe this membrane sometimes shows a V-shaped depression, into which fits a inverted V-shaped prominence on the inner surface of the wall at this point.

The sensitive laminae increase in width from above to below. Their free margin is finely denticulated, while their sides are traversed from top to bottom by several folds (about sixty), which, examined microscopically, are seen to consist of secondary leaves, or _laminellae_.

Examined on the foot, deprived of its h.o.r.n.y covering, the sensitive laminae are, the majority of them, in close contact with each other. In the normal state this is not so. The interstices between the leaves are then occupied by the h.o.r.n.y leaves, to be afterwards described as existing on the inner surface of the wall.

Reaching and rounding the heels, the sensitive laminae extend forward for a short distance, where they interleave with the h.o.r.n.y laminae of the bars.

Much discussion has centred round the point as to whether or no the cells of the sensitive laminae take any share in the formation of the horn of the wall. This will be alluded to in a future chapter.

I. THE HOOF.

Removed from the foot by maceration a well-shaped hoof is cylindro-conical in form, and appears to the ordinary observer to consist of a box or case cast in one single piece of horn. Prolonged maceration, however, will show that the apparently single piece is divisible into three. These are known as (1) THE WALL, (2) THE SOLE, and (3) THE FROG. In addition to these, we have also an appendage or circular continuation of the frog named (4) THE PERIOPLE, or CORONARY FROG BAND. These various divisions we will study separately.

1. THE WALL is that portion of the hoof seen in front and laterally when the horse's foot is on the ground. Posteriorly, instead of being continued round the heels to complete the circle, its extremities become suddenly inflected downwards, forwards, and inwards. These inflections can only be seen with the foot lifted from the floor, and form the so-called _Bars_. It will be noticed, too, with the foot lifted, that the wall projects beyond the level of the other structures of the plantar surface, taking upon itself the bearing of the greatest part of the animal's weight.

The horn of the wall, viewed immediately from the front, is known as the _Toe_, which again is distinguished as _Outside Toe_ or _Inside Toe_, according as the horn to its inner or outer aspect is indicated. The remainder of the external face of the wall, that running back to the heels, is designated the _Quarters_.

In the middle region of the toe, the wall following the angle of the bones is greatly oblique. This obliquity decreases as the quarters are reached, until on reaching the heels the wall is nearly upright.

[Ill.u.s.tration: FIG. 19.--THE WALL OF THE HOOF. 1, The toe; 2, inner toe; 3, outside toe; 4, the quarter; 5, entigeral groove; 6, h.o.r.n.y laminae.]

For observation the wall offers two faces, two borders, and two extremities.

_The External Face_ is convex from side to side, but straight from the upper to the lower border. Examined closely, it is seen to be made up of closely-arranged parallel fibres running in a straight line from the upper to the lower border, and giving the surface of the foot a finely striated appearance. In addition to these lines, which are really the horn tubules, the external face is marked by a series of rings which run horizontally from heel to heel. These are due to varying influences of food, climate, and slight or severe disease. This will be noted again in a later page. In a young and healthy horse the whole of the external face of the wall is smooth and shining. This appearance is due to a thin layer of horn, secreted independently of the wall proper, termed the periople.

[Ill.u.s.tration: FIG. 20.--INTERNAL FEATURES OF THE WALL, FROG, AND SOLE (MESIAN SECTION OF HOOF). 1, h.o.r.n.y laminae covering internal face of wall; 2, superior border of wall; 3, junction of wall with h.o.r.n.y sole; 4, the cutigeral groove; 5, the h.o.r.n.y sole; 6, the h.o.r.n.y frog (that portion of it known as the 'frog-stay'); 7, inverted V-shaped ridge on wall and sole (known as the 'toe-stay'); 8, anterior face of wall; 9, inferior border of wall.]

_The Internal Face_ of the wall, that adapted to the sensitive laminae, is closely covered over its entire surface with white parallel leaves _(Keraphyllae_, or horn leaves, to distinguish them from the _Podophyllae_, or sensitive leaves). These keraphyllae dovetail intimately with the sensitive laminae, covering the os pedis. Running along the superior portion of the inner face is the _Cutigeral Groove_. This cavity has been mentioned before as accommodating the coronary cushion, whose shape and general contour it closely follows, being widest and deepest in front, and gradually decreasing as it proceeds backwards. It is hollowed out at the expense of the wall, and shows on its surface numberless minute openings which receive the papillae of the coronary cushion.

At the bottom of the internal face, at the point where the toe joins the sole, will be noted the before-mentioned inverted V-shaped prominence.

Its position will be clearly understood when we say that it gives the appearance of having been forced there by the pressure of the toe-clip of the shoe. This will be noted again when dealing with the sole.

_The Inferior Border_ of the wall offers little to note. It is that portion in contact with the ground, and subject to wear. A point of interest is its union with the sole. This will be noticed in a foot which has just been pared as a narrow white or faint yellow line on the inner or concave face of the wall at its lower portion. It marks the point where the h.o.r.n.y leaves of the wall terminate and become locked with corresponding leaves of the circ.u.mference of the sole.

_The Superior Border_ follows closely the line marked by the perioplic ring and the groove separating the latter from the coronary cushion.