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Horse AnatomyCHAPTER IITHE HORSE  HORSES, SHOWING THE RELATIVE PROPORTIONS OF A HUNTER AND A HEAVY DRAUGHT HORSE WHEN dealing with all the domesticated animals, consideration should be given to their present- day artificial conditions of life and interbreeding for special requirements, such as flesh for food, speed, or strength, quantity of coat, etc. In other words, a knowledge of the common ancestor is essential, and provided one can visualise this correctly, the divergence from type can be easily traced, and the character at once seized upon. For example, the bones of a horse's leg become simple to understand if one realises that originally it was of a five-toed animal, about the size of a fox, and known as the Hyracotherium (Fig. z). During the progress of time he grew in size, but lost the use of two of his toes, and the next link comes in the shape of Mesohippus (Fig. 3), a three-toed horse with first and fifth toes present, but barely reaching the ground. From this we come down to the comparatively modern Hipparion (Fig. 4), which inhabited these islands previous to man, and excellent fossils are to be found in the Natural History Museum at South Kensington of the bones of the leg and head of this animal. Living types are found in the Tapir and the Rhinoceros, both closely allied to the Horse tribe. Observe the identical action of their muzzles with that of the upper lip of a Horse when searching for small particles of foods. Both eye and ear are again similar, and the feet of the Tapir show four distinct toes to each front leg and three to each hindleg (Fig. 6), which it has retained in its environment of marshy swamps. Scientific theory is that some forms of this animal left the low-lying swamps and gained an existence in higher and firmer grounds, wherein the soft splay foot not only ceased to become a support, but actually a hindrance to sustained or quick movement. Figs. 2 to 6 show plainly the hypothetical process of evolution and how the middle finger nail finally becomes converted into hoof. The horse still is a three-toed animal, for the splint bones are merely relics of the second and fourth toes in the last stages of their disuse.  
Another point of interest is the evolution of the tail of a Horse which explains certain characteristics in early sculptures and drawings. Figs. 7 to to show the tail of common Donkey, Zebra, Prjevalski's Horse of Siberia, and domestic Horse. Prjevalski's Horse is believed to be a living type of the original wild horse of Europe, and we find the exact type of tail which is depicted on all drawings of horses by prehistoric man in France. It is also quite typical of many of the coarser and underbred horses which may be found in Northern Europe to-day, but not that of the Southern parts where the influence of the Barb or Arabian Horse is greater. The tail of the Arab is set on the body in such a way that the spinal column is continued a few inches in a horizontal direction before slowly and gracefully curving downwards, whilst the long plumes of hair commence at the root. 
Our race of English Thoroughbreds are descendants from an early Arabian ancestor, and consequently all carry the plumed tail in contrast to that of the English Shire Horse, or Cart Horse, where the tail is curved immediately upon leaving the body, and has a series of short stiffer hairs upon upper part, and plumed upon lower portion only, a similar type well shown on vases and ornaments of Early Greek Art.  PLATE IV. ENGLISH THOROUGHBRED TYPES OF HORSES AS SHOWN BY SKULLS (Heavy Draught Horse left, Arabian Thoroughbred right). Upon the Parthenon Frieze will be found the Hog Mane of the Prjevalski Horse, and the Ass tribe. For all ordinary purposes of proportion, the following simple measurements will be sufficient. A well-bred Hunter (Plate III) or Polo Pony should stand in a square. A line drawn from top of withers to ground should equal one drawn from point of shoulder to point of rump. The elbow should be exactly half way. The length of head should equal length of neck from root of ear to shoulder, or length of shoulder from withers to point of shoulder. The height of a horse is about two and half times the length of head. In other types of horses, however, these measurements slightly vary. The Thoroughbred Racehorse may stand higher at the withers, due to increased length of leg, although this may be again counteracted by greater slope of shoulder and length of quarters, and the head may be longer. The difference is actually slighter than it appears, and the same applies to the Cart Horse (Plate m). The ideal type should look long and low, and is due to increased depth of ribs; elbow set slightly lower, so shortening and strengthening the legs ; thickening of neck, especially at the base, and so developing chest muscles ; and thickening of the head and jaw. Both this breed and the Race Horse are specialized types for excessive but short bursts of speed, or strength combined with weight, but the Hunter is the natural combination of both, and conforms to the same standard measurements which apply to other animals requiring similar qualities, notably, the Greyhound and some of the Deer tribe.  STUDY OF THE HEAD OF A HORSE In human portraiture the expression of the sitter lies very largely in the eyes and mouth, but in animal portraiture this is not the case. Very little expression is shown in the eyes of a horse beyond the extremes of fear or anger, and none whatever is shown in the mouth. Ears play a far more important part in expression, the prick forward or otherwise giving some indication of their feelings. The individual character of every horse is, however, shown to a very marked degree at one point—namely, the contour of the frontal bone. Plate iv clearly shows the concave contour of the Arabian as compared to the more convex or roman-nosed form of the Cart Horse, whilst the thoroughbred Racehorse is a modified form of both. This can be more easily realized if a tracing be made over each skull. In the foreshortened view this is even more apparent, and in portraiture should always be carefully studied. A criticism from the owner will often be to the effect that the head is too heavy, and a careful revision of this line will nearly always rectify the fault. It is a curious point how seldom either owner, trainer, or groom can point out exactly where the portrait fails. They will remark at once whether the likeness is good or otherwise, but words fail them to indicate the precise variation. The explanation is perhaps that they, by con-tinual knowledge of horses, have only unconsciously absorbed these details, to them the result of years of experience and practice. But the artist with properly trained eye and by visualizing the accepted type can pick out unerringly small variations, and by the faintest exaggeration of same depict the individual character (Plate v). There is no necessity for the artist to be familiar with the separate bones of the horse's head. It is sufficient for him that they consist of upper and lower jaw, the most important point being the Zygomatic Ridge, no other animal having it developed in quite a similar manner. 
Note the shape and size of the ridge of bone above the eye socket (Fig. 1). Over this ridge rests the forehead strap, attaching to the cheek straps which lie behind the ears, and which follow the line of the Zygomatic Ridge down to the corner of the mouth. The nose band should lie exactly the width of two fingers below the point of Zygomatic Ridge. 
The neck vertebra (Plate vi) has little of interest to the artist owing to the heavy covering of powerful muscles. The Atlas bone is called after the mythological giant who supported the heavens upon his shoulders the head being supported by this bone. The Axis is the next bone of the vertebral column, and, as its name denotes, is the axis upon movements are made. The Ribs on either side number eighteen. The bones of the forearm are Scapula or shoulder-blade. The Humerus. The Radius and Ulna, the latter two being fixed and inseparable. For this reason a horse cannot rotate the arm as in the Human or the Cat and Dog tribe. The action of the forearm of a Horse is merely to and fro, therefore it is unable to change direction suddenly when travelling at full speed, but must either take a wide sweep or reduce speed and pivot upon the hind legs. 
The knee (Figs. 13 to 15), so called, of a horse is identical with the wrist of the human forearm. The collection of small bones are bound together, and enclosed by ligaments and covered by skin only ; it is, therefore, important to understand the general shape of the whole. The Metacarpus consists of one large bone and two rudimentary ones, known as Splints. These, as previously pointed out, are the rudimentary second and fourth toes. 
The Greater and Smaller Pasterns (Fig. 16) compare with the human finger joint with its bony covering of finger nail or Hoof. The Pelvis (Fig. iz) has two very important projections, the Ilium and the Ischium, both of which have a tremendous influence upon the form, the Pubis bone being hidden. The bones of lower limb are : The Femur. The Tibia and the Fibula. The Patella or Knee-cap (Fig. 17), as in the human knee, is merely attached to the Femur and Tibia by ligaments, but itself serves as a point of attachment for some important muscles, and in drawings of movement its position requires to be well understood, and in a similar manner, Figs. 18 to 19 show the ligaments which bind the hock or heel of the Horse. The heel portions marked black in Plate yr are two fatty cushions which serve as buffers to lessen the effects of jarring or concussion upon the foot. It will be noticed that no mention has been made of a Clavicle or Collar-bone. The purpose of this bone is to brace out the shoulders so as to give a free use to the arms. It is found in nearly all the birds, and in many of the climbing, burrowing, or flying animals, such as Squirrel, Mole, and Bat, but in the Horse and Dog it is quite absent, and in the Cat tribe is found in a very undeveloped state. The important muscles of the head are : The Levator Nasolabialis or MaxiIlaris, which is the elevator of upper lip and corner of mouth. Zygomaticus, the action of which is to pull down the lower portion of the Orbicularis, or muscle of the eye, and to raise the corner of the mouth. The Nasalis, which is attached to the cartilage of the nose and dilates the nostrils. The Caninus, with similar action, also elevating the upper lip. NOTE.—A Horse under the effects of excitement or violent action brings all the above muscles into movement. They, therefore, have an important influence upon light and shade and expression, and are made very visible by the cast shadow from nose band of bridle. Muscles of lesser importance, inasmuch as they alter but little in shape, are the Masseter, giving the closing movement to the lower jaw ; the Temporalis, with similar action, and the Buccinator, which flattens the cheeks, thus pressing the food between the teeth and also retracting the corner of the mouth.
The Longer Colli is also a deep-seated muscle, originating from the first six dorsal vertebra and attaching to underside of Atlas. Its action is to curve the whole neck downwards, and is partly covered by the Splenius. The Serratus Magnus, although not a superficial muscle, is of great importance, as its peculiar form is always well marked in well-bred and thin-skinned horses. It originates from the first seven ribs and the last five cervicle vertebra by a series of digitations, and is inserted in the inner side of the Scapula. Its action is to attach and raise the trunk between the shoulder blades, and also to raise the ribs in inspiration, consequently may be plainly visible when the horse is breathing rapidly, such as after a race. The Caracohyoidaeus is merely a flat membraneous tendon which covers the Carotid Artery and the main portion of the wind-pipe, or Trachea. The Sterno-Mastoideus arises at the top of the Sternum and inserts into the lower jawbone under the Parotid Gland. On the top of this lies the Jugular Vein. Two small muscles shown in one plate only are the Sterno-Thyroideus, which arise from the Sternum and becoming tendinous partially cover the wind-pipe and insert into the Thyroid cartilages. NOTE.—The Splenius passing over the Atlas and Axis bones becomes very fleshy and forms one single mass very noticeable in all sideways movements, casting a heavy shadow upon the neck.
The Shoulder Muscles (Plate vii) are the Triceps, powerful muscles in three well-marked parts or heads. It originates from nearly the whole of the lower border of the Scapula, and also the inner and lower side of the Humerus, and inserts by tendons to the Olecranon or elbow. Its action is to extend the forearm. FIG. 21. DEEP LAYER OF MUSCLES A small muscle whose influence upon the surface can be plainly seen is Teres Minor. It originates from the lower edge of Scapula, and running across the biceps inserts in the Humerus near the top of the Deltoid ridge. It has an important action in the flexing of the shoulder joint. 
Infraspinatus, as its name denotes lies upon the lower side of the spine of Scapula, and Supraspinatus upon the whole surface above the spine of Scapula. It also extends the shoulder joint and assists in preventing dislocation.  FIG. 22. MUSCLES OF THE HORSE The Trapezius. The Longissimus Dorsi is the most important muscle in the trunk, and is again coveted by the fascia-like Latissimus Dorsi. 
The Pectoralis is an important muscle, inasmuch as it varies considerably in the different breeds of horses. Much of the pulling power depends upon this muscle, its action being to draw the shoulder and arm downwards and backwards. In a similar manner to the Pectoralis of the human figure, this muscle splits up into bundles, very marked in the chest of a man of greatphysical development, and the same tendency may be found in the chest of a powerful, heavy draught horse. 
The three most important muscles in the movement of the front leg are the Biceps and the Bracialis, the action of both being to flex the forearm, whilst the Triceps, a prominent muscle with its clearly marked four divisions, extends the forearm. There are six Extensors in the forearm, but only three require notice. Extensor Carpi Radialis is a powerful muscle arising from the outer side of Humerus and becoming tendinous as it passes over the knee, inserts upon the upper end of the Metacarpal; its action is to extend the forearm. Extensor Digitorum Communis and Extensor Mgt torum Breris passing over the knee downwards to finally unite and insert upon the lower pastern. Their action is to extend the foot. There are seven Flexors. The Perforans or Deep Flexor arises on the inner Epiconely/e of the I-Istmerut, and becoming tendinous, on the inner side of Pisiform bone passes , and over the Sesamoids finally dividing and inserting into the Cofn Bone or, Navicula; NOTE.—This latter bone is small and embedded in the hoof. (See The Perforates or superficial Flexor arises from immediately above the previous muscle, and follows the same course until it passes over the Sesamoids, when it divides and inserts into the lower edge of either side of the lower pastern. Flexor Carpi Ulnaris (Plate rx) is a very obvious muscle, which inserts into the Pisiform. The action of all these, as the name denotes, is to flex the arm and foot. NOTE.—Both Perforans and Perforatus, although deep-seated muscles, have important bearing upon the drawing of a horse's leg, if the fact of their becoming tendinous is properly realised. After passing behind Pisiform, and held in their place by a ligamentous wrapping, they stretch down the leg and over the Sesamoids in a perfectly straight line. Any curving of this line at once depicts weakness and lameness. The muscles of the hind limb are the Gluteus, which practically covers the upper part of the Pelvis. The Gracilis (Fig. zr), which arises from the Pubic region and inserts into the inner side of the Tibia ; its action is to adduct the thigh. The Biceps Femoris, which flexes the leg. The Semitendinosus (Fig. 22), which originates on the Ischiatic tuberosity and passes by a tendon into the Tibia and the Semimembranosus, which arises from almost the same spot with similar insertion, but showing as two separate muscles in their fleshy passage.  SHOWING THE DIRECTION OF THE HAIR TRACTS Their action is to adduct the thigh and flex the stifle joint. The Sartorius is a muscle upon the inside of the thigh and is visible in jumping or rearing positions (Fig. 24). Examination of Plate v will suggest a more complicated muscular development than is shown by the diagrams in certain portions of the body, notably the chest.  STUDY OF A HORSE SHOWING PLAY OF LIGHT AND SHADE UPON THE SHEEN OF THE COAT This appearance, however, is caused by the play of light falling upon the hair which at some points entirely changes direction.
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In the proportions of the head, the lower point of the Zygomatic Ridge should about equally divide it in length, and the eye socket should again lay about midway between this and the top of the skull. These points are important as giving the position of the harness.
The neck is supported by a complicated and powerful series of deep-seated muscles, called the Comp/exus, but this is again covered by the Splenius muscle, originating in the mastoid process at the base of the skull and attaching to the transverse processes of the upper cervicle vertebra. Its action is to extend or incline the head.
back view.)
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