HORSE'S LIMBS AND FEET. It has been said that however superior the horse's body, lungs, and digestion, that he becomes an ineffi cient servant without these are accompanied with the most excellent limbs, and good feet; limbs are useless if the feet are unsound. It is, therefore, of the first importance that the great est care be taken, not only of the limbs, but especially of the feet. since these latter are most liable to injury. In the article, Spavin, will be Fig. 3 shows a side view of hind leg; a, bones of the tarsus; b, splint bone; c, cannon bone; d, sesamoid bone; e, pastern bone; f, coronet bone; g, coffin bone. It will be seen that below the knee and hock, the names of the bones are sim ilar. Thus, with the cut of skeleton previously given, the reader will not only get a perfect idea of the proper location of the bones but their correct names. Following is a comprehensive statement found cuts of the upper or larger hones of the limbs, showing diseased bony growths. It will only be necessary here to show the limbs below the hock and knee, including the hoof, to give an idea of the importance of the limbs. Fig. 1 shows the bones of the fore leg, side view; a, bones of carpus; b, splent or splint bone ; e, cannon bone; d, sesamoid bone; e, pastern bone; f; coronet bone; g, coffin bone. At Fig. 2 is shown bones of hind leg, front view; a, b, c, d, e, bones of the tarsus; f, cannon bone; g, pastern bone; h, coronet bone; i, coffin bone.
relating to the foot of the horse, as found in the correspondence of the United States Department of Agriculture, which we illustrate with refer ences. Immediately below the carpus and tar sus is a single large metacarpal or metatarsal bone, called the cannon bone. Upon either side of this is a rudimentary bone, the splint bone. These two splint bones, expanded at their upper extremities, where they enter into the formation of the knee and hock joints, grow gradually smaller as they pass down by the side, and rather to the rear of the main bone, and terminate be -fore reaching the fetlock joint. Below the can non bo.ne,.talong an oblique direetion anteriorly from it, the pastern bone, long pastern, (os suffraginzs). In length it is from one-third to one half that of the cannon bone.
Below this is the coronet bone, short pas tern, lower pas tern, (os coronce) which is nearly square in form; its transverse diameter being however, greater than its vertical. The last bone terminating the extremity is the cof fin bone, (os pedis). This bone has been described as having a body and wings. Its general outline is semilunar anteriorly, supe riorly in its convex, and posteriorly and inferi orly it is concave. In texture it is light and spong.y, perforated throughout by canals, (Figs. 4 and 5,) through which blood-vessels and nerves are abundantly distributed to the soft and sensi tive tissues that eover it. The wings extend di rectly backward from the body, and support the lateral cartilages. Upon its superior aspect is a smooth aud concave surface, placed obliquely to the body of the bone for articulation with the middle phalanx or coronet-bone. Applied to the joint between the coronet and coffin bones, pos teriorly, and lying in the concavity of the coffin bone, is a small bone of peculiar shape--the navicular, (Fig. 6). This is a sesamoid bone, being contained in, or attached to, the tendon of the deep flexor. It is from two to two and one half inches long, three-fourths of an inch in width at its Widest part, and half an inch in thickness. Two surfaces of this bone, meeting in front at an acute angle, are covered with car tilage and synovial membrane. The posterior surface is rough for the attachment of the ten don of the deep flexor. Attached to the upper edge of the wings of the coffin-bone are the two lateral. cartilages. They are irregular in form, elastic, and extend backward, giving form, sub stance, and elasticity to the heel upward as high as the pastern joint, and forward, so that only the width of the great extensor tendon of the foot separates them. In faet, the n13rous investment of the tendon is attached to these cartilages. The Cycloptedia of Anatomy and Physiology says, this widely distributed cartilage may be observed passing downward and surrounding on every side the rough and knotty extremities of the heels of the coffin bone, entering and filling up its sinuosities and taking strong attachment to these processes. It the,n extends horizontally inward, passing over the horny sole and bars, and, meeting the sides of the sensitive frog, inti mately unites with it, forming one inseparable mass and filling together the whole intenor area described by the sides of the coffin bone. The upright or lateral portion of the cartilage forms with the horizontal portion passing inward a right angle, thus making together a hollow space or reeeptaele at the back of the coffin bone that contains the spongy, elastic stuffing of the heels, together with the tendons, vessels, and nerves passing through the sole of the foot. The upper surface of the horizontal process of cartilage is fun of scabrous elevations and depressions that defy dissection, among which is found a quan tity of gelatino-ligamentous tissue. Beneath, or to the under surface of this horizontal layer, the sensitive sole and bar are adherent. As it ap proaches the frog or center of the foot, it loses its cartilaginous nature and becomes coriaceaus, or rather ligamento-eoriaceous, in texture, agree in.; in this with the internal frog. The horizon tal portion or process of the cartilage, known by veterinary wfiters as the stratiform procees, is of greater thickness and substance than the other parts. It is also of coe,trser grain and more elas tic nature. Both portions together communicate the general boundary of form to the lateral, pos terior, and inferior parts of the foot. When the bars and the frog are thrust upward by pressure from without, they are acting against this same horizontal flooring formed by the cartilage and the frog, and are .met by the depression of the bones of the foot forced down by pressure of the weight of the animal. The whole can then di late exteriorly along with the posterior and more elastic parts of the hoof. Several important pur poses are answered by this extensive distribution of elastic fibro-eartilage--1, the interposition of a layer of elastie tissue between the hard hoof and the hard bone prevents shock and jar to the body as the foot strikes the ground in walking or running; 2, the coffin bone not extending pos teriorly much beyond the middle of the foot, except by its projecting wings, a large portion of the hinder part of the foot is made up of soft elastic cartilage instead of bone, breaking the force of the blow of the tread; 3, the distribution of elastic cartilage serves to equalize the pres sure of the bones of the foot upon the broader surface of the hoof; and, lastly, this arrangement of an elastic cushion, increasing in thickness toward the posterior aspect of the foot, affords an elastic support to the movements of the coffin bone. in the hoof, thereby aiding the elastic laminae upon the superior convex surface in support of the bone. While the toe of the coffin b.one is comparatively stationary, there is considerable motion of the heel upon the toe as a center, thereby contributing to the extent, freedom, and ease of movement of the foot. Molded upon the surface of the ooffin bone, over its entire extent, is a thick, villous, highly vascular, and sensitive membrane having the general name of the sensitive foot, besides having several local names derived from the part of the hoof under which it lies, as sen sitive laminm, sensitive sole, and sensitive frog. This tissue is derived mainly from the skin. It may be said to be a process from the slrin, cover ing the coffin bone, and altered in its structure to adapt it to its office as, an excretory membrane. It exactly corresponds to that portion of the human skin which produces the nails. The proper skin of the leg, as it arrives at the foot, becomes thickened and altered in its structure, constituting the mass around the summit of the boot to which veterinary writers have given the name of coronary band. This is lodged in a groove seen around the upper edge of the horny wall, and from this the straight fibers of the wall are secreted. From the coronary band there is a prolongation of the skin downward over the coffin bone. This tissue is thrown into perma nent folds or laminm, the sensitive laminae, between five and six hundred in number, arranged lengthwise of the foot. They secrete matter which enters into the formation of the horriy wall, to the laminm of which' they are very closely united. According to Virchow, each lamina corresponds to a single papilla, as seen on the surface of the skin. Similar tissue, thickly studded with secreting papillae, covers the inferior surface of the coffin bone, the fibro elastic frog, and the widely distributed cartila ges, already mentioned. This tissue is richly supplied with nerves and blood-vessels, the latter forming large plexuses which extend to and above the margin of the hoof. The elastic laminae are found around the whole convex surface of the coffin bone, having a breadth of about one tenth of an inch, and a length of about two inches in front, decreasing to an inch at the heels. These laminae, much reduced in size and importance, are continued over the bars into the center of the foot. Each laminae consists of a single plait or fold of two layers of membrane, which apply closely to, and are firmly attached to, two corresponding surfaces of the laminm of the horny hoof. Either in the laminae them selves, or in the fibrous membrane on which they rest, and which is the medium of their union with the bone, great elasticity resides, so that the coffin bone, with the weight it sustains, receives a large part of its support from the laminae, acting as hundreds of elastic springs It is this structure and arrangement that aid greatly in obviating shocks and preserving the integrity of the soft tissues at the bottom of the foot. By taking two strips of paper and folding them together in regular, even plaits, we have an illustration of the method of union between the soft and sensitive and the horny laminm. If, in addition, one of the layers be considered elastic, the philosophy of the elastic suspension of the foot would be obvious. The tissue constituting the sensitive sole, averaging, perhaps, one-eiglith of an inch, thinner over the frog and thicker over the heels, is even more fibrous, vascular, and sensitive than the larninm. It is closely con nected with the fibrous tissue of the sensitive laminae in front and the covering of the heels and frog behind. From it the horny sole and frog are secreted. The sensitive tissues that invest the bones of the foot are covered and protected by a thick, dense, horny cap or box, the hoof. The physiological relation of the hoof to the parts which it covers is essentially the same as that of the human nail to the parts covered by it. Functionally, its relations are more exten sive and complete, and whatever differences exist in structure, in form, or extent of develop ment, come from modifications for special use. The hoof consists of three portions. which are so closely united as to seem but one; yet, by maceration, or by boiling, they can be separated. These are the wall or crust, the sole and the frog. The wall (Fig 7, e, e, e) is all that part of the hoof that is visible below the hair when the foot is placed upon the ground. It is in the form of a cylinder, cut across obliquely at the top. It is deepest in front, from three to four inches, and grows gradually less in depth toward its posterior aspect. This wall, which is secreted mainly by
the coronary band, and partly by the sensitive wall beneath, is in front about half an inch in thickness, becoming thinner on the back side as it extends around the foot. It has an edge bear ing upon the ground of about half an inch around the outside of the bottorn of the foot, (Fig. 7, e, e). Upon the inner side of the foot the wall is thinner than upon the outside. The explanation will show the relative parts: Ground surface of hoof, rt, toe, ; a 1, inner toe; a 2, outer toe; b 1, inner quarter; b 2, outer quarter; el, inner heel; c 2, outer heel; d, d, d, sole; e, e, wall of the hoof ; f, f, the bars; g,g, the comrnissures; k,l, the frog; h, part under the navicular joint; k, boundary of the cleft; i, i, the bulbs of the heels. The wall is divided into toe, quarters, heels, and bars, superior or coronary border, inferior or solar border, and laminae. Passing any special description of the borders, the laminm deserve more particular attention. The lamina or ianiel/a. are the very numerous, narrow, and thin plates which cover the entire interior aspect of the horny wall. They are in length from two inches in front to less than an inch at the heels. They are also visible over the bars. They have a very constant width of about one-tenth of an inch, and extend from the lower to the upper border of the hoof, are essentially parallel to each other, and have a free edge and two free surfaces. Each lamella is received into, and is very closely united to two of the lamellm of the sensitive wall. By this arrangement the surface by which the horny wall is attached to the sensitive hoof is very largely increased, (by exact calculation of Dr. Evans, increased by twelve times), and this attachment, while possessing great strength, has great elasticity, and admits of considerable motion between the horny sole and the coffin bone contained in it. The toe (Fig. 7, a) consti tutes about two-thirds of the wall, and is some times subdivided, for minute description, into toe, inner toe, and outer toe, (Fig. 7, a, a 1, a 2). It is the deepest and thickest part of the wall, and stands at an angle, in the average of good feet, of about forty-five degrees. When the angle of inclination is much greater than this, the feet are designated as flat and weak. Flat and weak feet usually obtain in large and heavy animals, and it has been thought that as the foot is flat tened, the anterior wall will be drawn down, by the weight, at length becoming fixed. The quarters (Fig. 7, b 1, b 2) are the portions on each side, midway between the toe and the heels, and are designated as the inside and outside quarters. The fibers composing them run obliquely upward and backward, parallel to those of the toe. The quarters slope downward and backward, and become thinner as they approach the heels. The heels (Fig. 7, i, ) are the two protuberant por tions of the wall by which it is terminated pos teriorly. The wall here is shortest and thinnest, the fibers being only about an inch in length, and not exceeding the fourth of an inch in thickness. While, in its natural state, there is some degree of elasticity in the entire wall, there ifd much more in the portion that covers the beds. The bars (Fig. 7, f, f,) are reflections of the wall in toward the center of the foot, on its ground surface. They gradually approach each other, and come together a little in front of the center of the foot. The bars are usually regarded as parts of the sole, but maceration shows them to be separable from the sole, but inseparable from the wall. ln the natural, healthy foot, that has never been shod, the bars appear as sharpened prominences, like braces, between the center of the foot and the heels. The best writers agree that they are well adapted to keep the heels open, and prevent contraction of the hoof. In the unshod foot, the bars have a bearing upon the ground, second only to that of the edge of the wall. The sole (Fig. 7, d, d, d,) fills the space between the wall and the bars. It is in the form of an irregular arched plate, the concavity being toward the ground. It is firmly attached, by its outer convex edge, to the inner surface of the solar border of the wall, while its inner straight edges are attached to the bars. It has been described as joining the frog, but throughout its whole extent the bars intervene between the sole and the frog. The center of the sole is the thinnest portion of it, and it also constitutes the summit of the arch. The lower circumference of the arch, which is also the thick est and strongest, everywhere abuts against the sides of the wall. The result of this mechanism is, that at every step, as the weight is thrown upon the foot, the coffin bone descends, elongat ing the elastic fibrous tissue connected with the sensitive and, pressing upon the highly elastic tissue of the sensitive sole, which rests upon the arch of the horny sole, causes the latter to yield and descend. The wall being elastic, especially toward the heels, is readily pressed outward, so that the ground surface of the foot is larger while bearing the weight than it is when the pressure is removed. Whenever the weight is taken off, the wall springs back, and the sole recovers its arched form. By this means the step is rendered elastic, jarring is obviated, and injury to the sensitive sole and sensitive frog is prevented. The frog (Fig. 7, h ,k,1,) is a wedge like mass filling the angular space between the bars, and consists not of solid horn, as might at first seem, but of a series of elastic arches. It has been not inaptly compared to an elastic keystone received into an elastic arch, communicating. in some cases, and admitting in all, the springing movements of which such an arch is capable. The base of the frog lies between and connects the posterior curved portions of the hoof, limiting to some extent their action. The sides are con nected with the bars by their upper edges, leav ing upon the ground surface two deep channels between the lower border of the hars and frog, which have been termed the commissures of the frog (Fig. 7, g). The horny material arching over these channels is called the arch of the com missures. In the center of the frog, as we look upon its ground surface, is a deep, narrow depression, the cleft of the frog, (Fig. 7, k,) which extends further into the soft tissues of the foot than the commissures. This cleft is arched over in a similar manner, and, the cone-like mass, as viewed on its inner upper surface, has received the name of frog stay or bolt. Looking upon both the exterior and interior surface of the frog, we see that with the bars it forms three elastic foldings, which act as springs to keep the heels apart and the foot well spread. In the natural, unshod foot, the frog, though protected to some degree by the solar border of the wall and by the sharp prominences of the bars, must still receive pressure at each step. he order of force in which the different parts of the foot press the ground in walking, running, etc., has been stated to be as follows: First, the solar border of the wall; second, the bars; and third, the frog. In the foot that has never been shod the frog has nearly if not quite as much pressure in the full step as the wall. ln rapid stepping, the edge of the wall, which is nearest the point of the coffin bone, receives the first force of the blow, while the frog, which mainly rests upon the elastic heels, a much more yielding substance, receives the weight as the foot settles back to its level. The effect of pressure is to flatten the arches of the commissures and cleft, to widen the frog, throw out the heels, and keep the foot freely expanded. The elasticity of the step of the horse is the result of a highly compound arrangement—first, the elasticity of the sensitive laminw; second, the greater elasticity of the sen sitive sole; third, the elasticity of the horny wall ; fourth, the arch of the sole; and, fifth, the triple spring formed by the foldings of the frog and the manner of its union with the bars. The movements of the foot are produced by two sets of muscles, flexors and extensors, similar to the distribution of a single finger in man. The flexors are two in number, and are situated upon the posterior aspect of the leg. In the fore legs the.se muscles are the flexor sublimis pmforatus, (Fig, 6, 6,) and the flexor prof-Ulu-Ns perforans, (Fig. 6, 7,) also called in works on farriery flexor peells pelforatus and flexor pedis perfo rans; also familiarly designated as the common and deep flexor muscles. These muscles take their origin in common from the internal protu berance of the humerus, and are united for a considerahle distance dowu the arm, when they separate to form two distinct tendons. Of these, that belonging to the perforatus runs beneath the annular ligament of the carpus, to be inserted into the upper and back part of the lower pastern or coronet bone. Just before reaching the pastern joint, this tendon divides, to allow the tendon of the perforans to pass through it. Each division where it plays over the joint has in it a sesamoid bone. The ten don of the perforans, lying deeper above, passes between the divided tendon of the first-named muscle, to be inserted into the posterior concav ity of the coffin bone. Attached to this tendon, as it passes over the joint formed by the coronet and coffin bones, is the navicular bone, consid ered as a sesamoid bone in this tendon. Two supplementary flexors, the accessores, are de scribed as arising from the posterior and inferior aspect of the ulna and the radius, and uniting below with the two main flexors. These mus cles flex the foot upon the leg. As antagonists to these, there are three extensor muscles situated upon the front of the leg, corresponding to the extensor communis digitarum and exten,sor digiti of the human arm. The extensor com munis, otherwis3 called in hippotomy extensor pedis, arises from the external condyle of the humerus, and from contiguous fascia, and from the upper and lateral part of the radius, and has a strong, fleshy belly, which terminates in a single tendon which passes down over the front of the leg to be inserted into the coronal process of the last phalanx—the coffin bone. It unites by a slip with the tendon of the next muscle. The extensor proprius minimi digitiis represented in the horse by two muscles, One of these, called the extensor of the pastern, is inserted by a strong tendon into the side of the first phalanx, the pas tern bone. The second muscle, placed between the two preceding muscles, furnishes a strong tendon which passes down in front of the carpus and becomes united with the communis at an acute angle. The united tendon (Fig. 6, 5) passes behind the c Jronary border of the hoof to its insertion in the coffin bone. The office of these muicles is to extend the foot upon the leg. Another muscle, the adductor longus c,alled in hippotomy the oblique extensor of the cannon, by its insertion into the base of the can non bone, acts as an extensor of the foot. A similar arrangement exists in the muscles of the hind leg. The tendon of the plantaris, of great strength, has a divided insertion corresponding to that of the flexor subtimis perforatus of the fore leg, while the flixor perforans sends. its single strong tendon between the two divisions of the preceding muscle to be inserted into the terminal phalanx. These flexors of the foot are assisted by the tendon of the flexor hallucis, which unites with the tendon of the perforans. The extensor tendon of the hind leg, of great strength, is fur nished mainly by the extensor communis muscle.