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.

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.

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.

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.

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.

The inferior or ground surface presents a central furrow (Sulcus intercruralis), which is bounded by two ridges or crura (Crura furcæ ungulæ); these converge and form the apex.

The sides (Facies medialis et lateralis) are united at the upper part with the bars and sole, but are free below and form the central wall of the deep lateral furrows (Sulci cruroparietales), which are bounded outwardly by the bars.

The base is wide and high. It is depressed centrally and prominent at the sides, where it unites with the angles of the walls; the junction here is covered by the expanded periople and constitutes the bulb of the hoof.

The apex occupies the central angle of the concave border of the sole, and forms a blunt, round prominence a little in front of the middle of the ground surface of the hoof.

Structure of the Hoof.—The hoof is composed of epithelial cells which are more or less completely keratinized except in its deepest part, the stratum germinativum; here the cells have not undergone cornification, and by their proliferation maintain the growth of the hoof. The cells are in part arranged to form tubes which are united by intertubular epithelium, and inclose medullary cells and air-spaces.

The wall may be regarded as consisting of three layers. The external layer consists of the periople and the stratum vitreum. The periople is composed of soft, non-pigmented tubular horn. It is continuous with the epidermis above, and extends downward a variable distance. Usually it forms a distinct band somewhat less than an inch wide, except at the heels, where it is much wider, and caps the angle of inflection of the wall proper. The stratum vitreum is a thin layer of horny scales which gives the wall below the periople its smooth, glossy appearance. The middle or tubular layer forms the bulk of the wall, and is the densest part of the hoof. Its horn tubes run in a straight direction from the coronary to the ground border. In dark hoofs it is pigmented except in its deep part. The internal or laminar layer consists of the primary and secondary laminæ and is non-pigmented. The laminæ are narrow and thin at their origin at the lower margin of the coronary groove, but become wider and thicker below. At the junction of the wall and sole they are united by interlaminar horn to form the white line. Only the central part of the laminæ becomes fully keratinized. They are composed of non-tubular horn in the normal state.

The sole consists of tubular and intertubular horn. The tubes run parallel with those of the wall and vary much in size.

The frog is composed of relatively soft horn, which is much more elastic than that of the wall or sole, and is not fully keratinized. The horn tubes in it are slightly flexuous.

The hoof is non-vascular and receives its nutrition from the matrix. It is also destitute of nerves.

The Matrix of the Hoof

The matrix of the hoof (Matrix ungulæ) is the specially modified and highly vascular part of the corium of the common integument which furnishes nutrition to the hoof. It is convenient to divide it into five parts which nourish corresponding parts of the hoof.

1. The perioplic matrix or ring (Margo matricis ungulæ) is a band 5 to 6 mm. in width which lies in a groove between the periople and the coronary border of the wall. It is continuous above with the corium of the skin, and is marked off by a groove from the coronary matrix. At the heels it widens and blends with the matrix of the frog. It bears very fine, short papillæ which curve downward and are received in depressions of the periople, to which it supplies nutrition.

2. The coronary matrix or cushion (Corona matricis ungulæ) is the thick part of the corium which occupies the coronary groove, and furnishes nutrition to the bulk of the wall. It diminishes in width and thickness posteriorly, and along the upper border of the bar it is not very clearly defined from the matrix of the frog. The convex superficial surface is thickly covered with papillæ 4 to 6 mm. in length, which are received into the funnel-like openings of the coronary groove. At the heels and along the bars the papillæ are arranged in rows, separated by fine furrows. The deep surface is attached to the extensor tendon and the lateral cartilages by an abundant subcutis which contains many elastic fibers and a rich venous plexus.

3. The laminar matrix (Latus matricis ungulæ)—also termed the sensitive laminæ—bears primary and secondary laminæ which are interleaved with the horny laminæ of the wall and bars in the natural state. It is attached to the wall surface of the third phalanx by a modified periosteum (Stratum periosteale) which contains a close-meshed network of vessels, and to the lower part of the lateral cartilages by a subcutis which contains a rich venous plexus. The laminæ are small at their origin above, become wider below, and end in several papillæ 4 to 5 mm. in length. The sensitive laminæ supply nutrition to the horny laminæ and to the interlaminar horn of the white line.

4. The sole matrix (Latus volare matricis ungulæ)—also termed the sensitive sole—corresponds to the horny sole, to which it supplies nutrition. It is often more or less pigmented and bears long papillæ, which are specially large along the convex border and at the angles. Centrally it is continuous with the matrix of the frog and bars. The deep surface is attached to the sole surface of the third phalanx by a modified and highly vascular periosteum.

5. The frog matrix (Furca matricis ungulæ)—also called the sensitive frog—is moulded on the deep surface of the frog and bears small papillæ. Its deep face is blended with the plantar cushion. The germinal cells of the frog derive their nutrition from this part of the matrix.

The digital or plantar cushion (Torus digitalis) is an important part of the elastic apparatus of the foot. It is a wedge-shaped mass which overlies the frog. It presents for description four surfaces, a base, and an apex. The superior surface faces upward and forward and is connected with the distal fibrous sheath of the deep flexor tendon. The inferior surface, covered by the matrix of the frog, is moulded on the upper face of the frog. The lateral surfaces are related chiefly to the lateral cartilages; inferiorly the cushion is closely attached to the cartilages, but higher up a rich venous plexus intervenes. The base, situated posteriorly, is partly subcutaneous, and is divided by a central depression into two rounded prominences termed the bulbs of the plantar cushion. The apex lies under the terminal part of the deep flexor tendon. The cushion is poorly supplied with vessels. It consists of a feltwork of fibrous trabeculæ and elastic fibers, in the meshes of which are masses of fat. The bulbs are soft and loose in texture and contain a relatively large amount of fat, but toward the apex the cushion becomes denser and more purely white fibrous in structure. Branched coil glands occur chiefly in the part of the cushion which overlies the central ridge of the frog. Their ducts pursue a slightly flexuous course through the matrix and pass in a spiral manner through the frog. Their secretion contains fat.

Vessels and Nerves.—The matrix is richly supplied with blood by the digital arteries. The veins are valveless, and form remarkable plexuses which communicate freely with each other and are drained by the digital veins. The lymph vessels form subpapillary plexuses in the matrix of the sole and frog, and a wider-meshed plexus at the base of the plantar cushion. A considerable lymph vessel lies in the attached edge of each of the laminæ. The nerves are branches of the digital nerves; some fibers end in lamellar corpuscles and end-bulbs.

The ergot is a small mass of horn which is situated in the tuft of hair at the flexion surface of the fetlock. It is the vestige of the second and fourth digits of extinct equidæ and hence is absent in cases in which these digits are developed. A small tendinous band, 3 to 5 mm. in width, extends downward and slightly forward from the fibrous basis of the ergot on each side, crosses over the digital vessels and nerves very obliquely, and blends below with the digital fascia and the plantar cushion. It is known as the ligament or tendon of the ergot.

The term chestnut is applied to the masses of horn which occur on the inner surface of the forearm about a handbreadth above the carpus, and on the lower part of the inner face of the tarsus. They have an elongated oval form and are flattened. They are regarded usually as vestiges of the first digit. That of the hind limb is absent in the donkey and very small in the mule.

These horny vestiges are quite variable in form and size and are correlated with the fineness or coarseness of the integument in general. The supracarpal chestnut is about 1½ to 2½ inches long, oval in outline, the proximal end being pointed; it overlies the flexor carpi internus at a quite variable distance above the carpus, and hence should not be used as a surgical landmark. The tarsal chestnut lies at a point behind the lower part of the internal lateral ligament of the hock. When well developed it is about 2 to 2½ inches long, broad below and produced above to form a long pointed end, with a short blunt anterior process. They are composed of horn somewhat like that of the frog.

Organ of Smell

The peripheral part of the olfactory apparatus or organ of smell (Organon olfactus) is that part of the nasal mucous membrane which was referred to in the description of the nasal cavity as the olfactory region; this (Regio olfactoria) is limited to the ethmoturbinals and the adjacent part of the superior turbinal and the septum nasi, in which the fibers of the olfactory nerve ramify. It is distinguished by its yellow-brown color, thickness, and softness. It contains characteristic tubular olfactory glands (of Bowman), which are lined by a single layer of pigmented cells, and a neuro-epithelium, the olfactory cells, the central processes of which extend as non-medullated fibers to the olfactory bulb.

The epithelium is non-ciliated and is covered by a structureless limiting layer. It consists essentially of three kinds of cells, supporting, basal, and olfactory. The supporting cells are of long columnar form above and contain pigment granules; below they taper and often branch, and their central processes unite with those of adjacent cells to form a protoplasmic network. The basal cells are branched and lie on a basement membrane. The olfactory cells are situated between the supporting cells; they have the form of long narrow rods, with an enlarged lower part which is occupied by the nucleus. The peripheral end pierces the limiting membrane and bears a tuft of fine hair-like cilia (olfactory hairs). A central process extends from the nucleated pole of the cell to the olfactory bulb as a non-medullated olfactory nerve-fiber.

The vomero-nasal organ (of Jacobson) lies along each side of the anterior part of the lower border of the septum nasi. It communicates with the nasal cavity through the naso-palatine canal. It consists of a tube of hyaline cartilage lined with mucous membrane; a small part of the latter along the inner side is olfactory in character.

The Organ of Taste

The peripheral part of the gustatory apparatus (Organon gustus) is formed by the microscopic taste buds (Calyculi gustatorii), which occur especially in the foliate, fungiform, and vallate papillæ, in the free edge and anterior pillars of the soft palate, and the oral surface of the epiglottis. The taste buds are ovoid masses, which occupy corresponding recesses in the ordinary epithelium; each presents a minute opening, the gustatory pore. The buds consist of fusiform supporting cells grouped around central gustatory cells. The latter are long and narrow; the peripheral end of each bears a small filament, the gustatory hair, which projects at the gustatory pore; the central end is produced to form a fine process which is often branched. The taste buds are innervated by fibers of the glosso-pharyngeal nerve and the lingual branch of the trigeminus.