Part 229

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.

FIG. 577.—HOOF OF HORSE, RIGHT FORE FOOT, GROUND SURFACE.

37, Inner quarter; 38, outer quarter; 39, ground border of wall (toe); 40, white line; 41, sole; 42, apex of frog; 43, lateral ridge of frog; 44, central furrow of frog; 45, lateral furrow between frog and bar; 46, bar. (After Ellenberger-Baum, Anat. für Künstler.) ]

FIG. 578.—HORIZONTAL SECTION OF HOOF OF HORSE.

The section is cut just above the ridges of the frog and bars and parallel with the ground surface. 1, Wall; 2, sole; 3, median ridge of frog; 4, lateral ridge formed by junction of frog and bar; 5, median furrow over apex of frog; 6, laminæ of wall; 7, laminæ of bar. ]

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.

FIG. 579.—FRONTAL SECTION OF HOOF OF HORSE, POSTERIOR PART VIEWED FROM IN FRONT.

1, Wall; 2, sole; 3, bar; 4, frog; 5, median ridge of frog; 6, lateral ridge formed by junction of frog and bar; 7, laminæ of bar; 8, laminæ of wall; 9, coronary groove; 10, periople of heel. ]

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.

FIG. 580.—LATERAL VIEW OF FOOT OF HORSE AFTER REMOVAL OF HOOF AND PART OF SKIN. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

FIG. 581.—GROUND SURFACE OF FOOT OF HORSE AFTER REMOVAL OF HALF OF HOOF. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ]

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.

FIG. 582.—CROSS-SECTION OF DIGIT OF HORSE, THROUGH DISTAL END OF SECOND PHALANX.

a, Second phalanx; b, deep flexor tendon; b′, digital synovial sheath; c, plantar cushion; d, lateral cartilage; e, skin; f, hoof; g, anterior extensor tendon; h, cavity of coffin joint; i, suspensory ligament of navicular bone; k, lateral ligament of coffin joint; l, digital artery; m, digital nerve. (After Ellenberger, in Leisering’s Atlas.) ]

=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.