Part 228

THE SKIN OF THE HORSE

The =thickness= of the skin of the horse varies from 1 to 5 mm. in different regions, and is greatest at the attachment of the mane and the ventral surface of the tail.

The =glands= are numerous and are larger than those of the other domesticated animals. The sebaceous glands are specially developed on the lips, the prepuce, mammary glands, perineum, and labia of the vulva. The sweat glands are yellow or brown in color. They occur in almost all parts of the skin, but are largest and most numerous on the outer wing of the nostril, the flank, mammary glands, and free part of the penis.

In addition to the ordinary and tactile =hairs= certain regions present coarse hairs of great length. The mane (Juba) springs from the dorsal border of the neck and the adjacent part of the withers; its anterior part, which covers the forehead to a variable extent, is termed the foretop (Cirrus capitis). The tail, with the exception of its ventral surface, bears very large and long hairs (Cirrus caudæ). The tuft of long hairs on the flexion surface of the fetlock (Cirrus pedis) gave rise to the popular name of this region.

The development of these special hairs varies widely, and is in general much greater in the draft breeds than in others. In Shire and Clydesdale horses, for instance, the hair on the volar aspect of the metacarpus and metatarsus and fetlock is often so long and abundant as to account for the term “feather,” which is commonly applied to it by horsemen.

THE HOOF

The =hoof= (Unguis) is the horny covering of the distal end of the digit. It is convenient to divide it for description into three parts, termed the wall, sole, and frog.

1. The =wall= is defined as the part of the hoof which is visible when the foot is placed on the ground. It covers the front and sides of the foot, and is reflected posteriorly at an acute angle to form the =bars=. The latter (Pars inflexa medialis, lateralis) appear on the ground surface of the hoof as convergent ridges, which subside in front and are fused with the sole; they are united with each other by the frog. For topographic purposes the wall may be divided into an anterior part or “toe” (Pars ungulæ dorsalis), lateral parts or “quarters” (Pars ungulæ medialis, lateralis), and the angles or “heels” (Pars ungularis medialis, laterales). It presents two surfaces and two borders.

The outer surface is convex from side to side and slopes obliquely from edge to edge. In front the angle of inclination on the ground plane is about 50° for the fore limb, 55° for the hind limb; on the sides the angle gradually increases and is 100° at the angles. The curve of the wall is wider on the external than on the internal side, and the external quarter is more oblique than the internal one. The surface is smooth and is crossed by more or less distinct ridges, which are parallel with the coronary border and indicate variations in the activity of the growth of the hoof. It is also marked by fine parallel striæ, which extend from border to border in an almost rectilinear manner.

The =slope= of the wall varies considerably in apparently normal hoofs. Lungwitz found by careful measurements of 56 fore and 36 hind feet the following average angles:

FORE FOOT HIND FOOT Toe 47.26° 54.1° Inner angle 101.57° 96.50° Outer angle 101.37° 96.1°

The wall of the fore foot may even be more upright than that of the hind, and may have an angle of 60°. The =length= of the wall at the toe, quarters, and heels is in the ratio of about 3 ∶ 2 ∶ 1 in the fore foot and about 2 ∶ 1½ ∶ 1 in the hind hoof.

The inner surface is concave from side to side, and bears about six hundred thin =primary laminæ= (Cristæ ungulæ), which extend from the coronary groove to the junction of wall and sole. Each bears a hundred or more =secondary laminæ= on its surfaces, so that the arrangement is pennate on cross-section. These horny laminæ are continued on the inner surface of the bars, and alternate with corresponding laminæ of the matrix.

The =proximal= or =coronary border= (Margo occultus) is thin. Its outer aspect is covered by a layer of soft horn known as the =periople=; this appears as a ring-like prominence above and gradually fades out below; at the angle it forms a wide cap or bulb and blends centrally with the frog. The inner aspect of the border is excavated to form the =coronary groove= (Sulcus coronalis ungulæ), which contains the thick coronary matrix. The groove is deepest in front, narrows on the sides, and is wide and shallow at the heels. It is perforated by innumerable small, funnel-like openings which are occupied by the papillæ of the coronary matrix in the natural state.

FIG. 573.—DIGIT OF HORSE, SHOWING SURFACE RELATIONS OF BONES AND JOINTS. THE LATERAL CARTILAGE IS EXPOSED.

a, First phalanx; b, second phalanx; c, third phalanx; d, lateral cartilage; e, third sesamoid or navicular bone; f, pastern joint; g, coffin joint; h′, cut edge of wall of hoof (h); i, laminar matrix. (After Ellenberger, in Leisering’s Atlas.) ]

The =distal= or =ground border= (Margo liber) of the unshod hoof comes in contact with the ground. Its thickness is greatest in front and decreases considerably from before backward on the sides, but there is a slight increase at the angles. Its inner face is united with the periphery of the sole by horn of lighter color and softer texture, which appears on the ground surface of the hoof as the =white line=.

In the case of horses at liberty the wall is usually worn off to the level of the adjacent sole, but if the ground is too soft the wall is likely to become unduly long and split or break or undergo deformation. On very hard or rough ground, on the other hand, the wear may be in excess of the growth. In the case of shod horses it is necessary to remove the excess of growth of the wall at each shoeing. The =thickness= of the wall at the toe, quarters, and heels is about in the ratio of 4 ∶ 3 ∶ 2 for the fore foot and about 3 ∶ 2½ ∶ 2 for the hind foot.

2. The =sole= (Solea ungulæ) forms the greater part of the ground surface of the hoof. It is somewhat crescentic in outline, and presents two surfaces and two borders.

The =superior= or =inner surface= is convex, and slopes with a varying degree of obliquity downward to the convex border. It presents numerous small funnel-like openings which contain the papillæ of the sole matrix in the natural state.

FIG. 574.—CROSS-SECTION OF FOOT OF HORSE, CUT PARALLEL WITH THE CORONARY BORDER. ]

The =inferior= or =ground surface= is the converse of the preceding. It is normally arched—and more strongly in the hind than in the fore foot—but the curvature is subject to wide variation; in heavy draft horses the sole is commonly less curved than in the lighter breeds and may even be flat. The surface is usually rough, since the horn exfoliates here in irregular flakes.

FIG. 575.—HALF OF HOOF OF HORSE, INTERNAL SURFACE ]

The =convex border= is joined to the wall by relatively soft horn, previously referred to as forming the white line on the ground surface of the hoof. The angle of junction is rounded internally and presents a number of low ridges and specially large openings for the papillæ of the matrix. There is frequently a ridge of larger size at the toe.

The =concave border= has the form of a deep angle which is occupied by the bars and frog. By its junction with these it forms two pronounced ridges in the interior of the foot. The parts of the sole between the wall and bars may be termed its angles.

3. The =frog= (Furca ungulæ) is a wedge-shaped mass which occupies the angle between the bars and sole, and extends considerably below these on the ground surface of the foot. It may be described as having four surfaces, a base, and an apex.

FIG. 576.—INTERNAL SURFACE OF PART OF HOOF OF HORSE. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

The =superior= or =internal surface= is deeply concave from side to side. It bears a central ridge, the =spine= or “=frog-stay=” (Spina furcæ ungulæ), which is high posteriorly and subsides abruptly in front. On either side of this is a deep depression, bounded outwardly by the rounded ridge formed by the junction of the frog with the bar and sole. This surface presents fine striæ and openings for the papillæ of the frog matrix.