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Structure of the Bones—Bones are hard, yellow-white, insensitive objects, which form the skeleton and give attachment to soft structures (muscles, tendons and ligaments); they are of various sizes, forms and densities. In the limbs the bones are ordinarily more or less long, circular bodies, with expanded ends, effectually supporting the body, supplying leverage and attachment for soft structures, and forming the basis of all joints. Where cavities, such as the cranium, chest, and pelvic, enclosing the organs requiring protection and support, the bones tend to assume a flat, expanded form.

Living bone is bluish pink, insensitive, and elastic; on exposure to air it becomes diseased and blackened, very sensitive and painful; (the teeth excepted) which are harder and of a higher specific gravity than any other bone formation.

Bones are composed of two kinds of substance—animal, which makes the bone tough and flexible; earthy, which makes it hard and fragile. In young animals the animal matter forms about one-half of the bone substance; in the adult, it diminishes to about a third, while in old animals it is still less; hence the bones of very old animals are brittle, more liable to fracture and harder to mend.

Bones in a six-year-old horse contained, Phosphate of Lime, 54.37 per cent; Carbonate of Lime, 12.00 per cent; Phosphate of Magnesia, 1.83 per cent; Soluble Salts, 0.70 per cent, or mineral matter, 68.90 per cent. While they contain Cartilage, 27.99 per cent; Fat, etc., 3.11 per cent, or animal matter 31.10 per cent.

In bone tissue there are two modifications of texture, the compact and the cancellated. The former—hard, dense, and ivory-like, is always situated externally; the latter porous and spongy lies within.

Although the compact tissue appears uniformly dense, and destitute of porosity, yet, if we transversely sectate the shaft of a long bone, and examine it under the microscope, by transmitted light, it is found to contain numerous round openings. These are called Haversian canals. They transmit bloodvessels, and run in a longitudinal or slightly oblique direction, opening on either the outer or inner surface of the bone.

The external (outer) surface of every bone is covered by a tough, fibrous, inelastic membrane called periosteum, which can be seen by examining the bone of an animal which has recently died. The only exception to this is at the joints where one bone articulates with another, and where a tendon or muscle plays over a bone; here we find its place taken by articular cartilage. By its strength it sometimes retains bones in contact after an oblique fracture; in the young, it is thicker and more vascular than in the adult. Blood-vessels which penetrate the periosteum pass directly to the bone; the outer surface of the bone is always studded with numerous foramina through which these enter.

The periosteum owing to its inelasticity, is, when inflamed, the seat of intense pain; and should any part of it be stripped off, there is every probability of the denuded bone dying and separating.

Contents of Bone.—Red marrow is found in the extremities or near the ends of bones, white marrow is found in the shaft.

Classes of Bones.—Bones are classed as long, flat, and irregular. Long or cylindrical bones are found in the limbs or extremities, and serve as levers and pillars for traveling and to support the body. Descriptively, a long bone is divisible into a center or shaft and two ends or extremities. Flat bones are found where visceral organs need protection. As the shoulder or scapula and ribs, to protect the heart and lungs; pelvic or hip bones, to protect the rectum and urinary and genital organs; also the cranial bones, to protect the vital organ called the brain. Irregular bones are found in the spinal column and in the joints, such as the knee or carpus, hock or tarsus, where great strength is required. They usually possess many angles and indentations, with surfaces for articulation and tendonous attachment, and consist of a thin, dense, external (outer) case of compact bone enclosing cancellated tissue. In proportion to their size they present a much larger extent of articular surface and greater mechanical strength than any other class.

1. Skull, or skeleton of the head, the most anterior (forward) part of the horse’s skeleton, articulates with the first cervical vertebra (or atlas), from which it is suspended by its posterior (or back part) extremity, its anterior (forward) extremity being free. Its position varies with the attitude of the animal; but in our descriptions we shall always suppose it to be placed in a horizontal position.

In the young animal the skull is composed of a number of bones, all of which, with the exception of the lower jaw, the teeth, the bones of the tongue, and ossicles of the ear, become united by ossification (growing together) in the adult.

In speaking of the different points of importance in connection with the head bones of the horse, and other animals, suppose a cross or longitudinal section of the head is made. It will be noticed that it is full of cavities or sinuses. The uses of these are to lighten the head and also to warm the air as it passes into the nostrils, on its way down to the lungs. It will also be found full of foramen or small holes through which the nerves from the brain and various blood vessels pass to the organs situated in the head, such as the tongue, lips and the various glands in and around the head.

Then there are the cavities in which the eyes are situated, one on each side of the head, called the orbital fossa. In examining this fossa you will find a small opening or foramen, through which the optic nerve passes in coming from the brain to the eye. This is the nerve of sight. Then the most important part of all to consider is the cranial cavity in which that very important organ is situated called the brain, which controls all the various functions and movements of the body.

Another important point is the situation of the ear drum. It is situated in the hardest bone found in the whole skeleton, called the petrosal. The nerve that gives the function called hearing comes from the brain down to the petrosal bone and enters by a small foramen or hole into the drum of the ear to give hearing. This nerve is called the auditory nerve.

2. Cervical Vertebrae or Neck Bones.—These bones are seven in number. The first and second bones proceeding from the head receive special names. The first one is called the atlas, from which the head is suspended and to which it is attached; it somewhat resembles the body of a bird with wings extended. The second bone receives the name of dentata. This is the bone which allows the head to turn in any direction, hence it is sometimes called the axis or pivot of the neck. Between these two bones, on the upper surface, is the only place where the spinal cord is not covered with bone, a spot about three-fourths of an inch in diameter. The next four bones receive no name, and are about the same in size and length. The last, or the seventh bone, is only about one half the length of the preceding ones and receives no special name.

3. Dorsal Vertebrae or Back-Bone.—Dorsal bones are eighteen in number. The chief point of interest about them are the height of the spines on the upper surface of the bones. These large spines form the withers of the horse, as will be noticed in the skeleton. On either side of these bones the ribs are attached, 18 pairs corresponding with the number of bones in this region.

4. Lumbar Vertebrae (or the bones which form the skeleton of the loins). These bones are six in number, and they are situated immediately above the kidneys.

5. Sacral Vertebrae or Rump Bones.—There are five of these bones in the young horse, but in the adult they unite as a single bone, somewhat triangular-shape. These are situated between the upper hip bones and help to form the rump. Beneath these bones the bladder is situated.

6. Coccygeal Vertebrae or Tail Bones.—These are 18 to 20 in number. There is no complete canal, like in the previous vertebraes, for enclosure of the spinal cord.

7. Pelvic or Hip Bones.—The pelvic bones are flat, but somewhat irregularly-shaped, and they form the sides, floor, and part of the roof of the pelvic cavity. Above they are connected with the sacrum, and below united to each other, in the adult, by ossification (union of bone). In the young animal, as above stated, and especially in the foetus, each side consists of three parts, which retain their names of ilium, ischium, and pubis, even after union by ossification. They all three meet in the acetabulum, or articular cavity for the femur or hip bone.

8. Sternum or Breast Bone.—This bone is small and short in the horse and is situated on the lower surface of the chest cavity. The principle points to be noted in this bone are that of its softness, and that the first eight pairs of ribs are attached to it on either side. This bone, in the horse resembles the keel and cut-water of a boat.

9. Ribs.—In the horse the ribs usually number eighteen on each side. They extend in a series of arches of varying curvature from the dorsal vertebrae above, towards the sternum and sides of the abdomen below. Their shape, in a great measure, determines the conformation or shape of the thorax or chest cavity; they protect its contents, and materially aid in its contraction and expansion. They are continued downward and forward by a small piece of cartilage or gristle, and are just slightly attached to the breast bone or sternum; these are called the false ribs. Note that, starting with the first rib, they get longer until the ninth rib is reached; they then get shorter, the last rib being only a few inches long.

10. Scapula or Shoulder Blade.—The scapula is a flat bone situated on the antero-lateral (front side) surface of the thorax or chest cavity, it is triangular in shape, the base being turned upwards. The inferior or lower extremity articulates with the humerus or shoulder bone.

11. Humerus or Shoulder Bone.—The humerus is a long bone extending from the scapula to the radius and ulna in an oblique direction downwards and backwards. Like all long bones it possesses a shaft and two extremities and two articular surfaces.

12. Radius, or Bone of the Fore-arm.—The radius is a long bone, and occupies a vertical position between the humerus and the carpus.

13. Ulna, or Bone of the Fore-arm.—The ulna is an irregular bone, in form triangular, with the base uppermost, and is placed on the supero-posterior or (upper and back) part of the radius.

14. Carpus, or Knee.—The carpus, knee or wrist, as it is incorrectly termed in quadrupeds, is composed of seven and often of eight small, irregular bones arranged in two rows of three each, one above the other, the seventh being at the back of the three in the upper row, and the eight, when present, in a similar position with respect to the lower row. Their names are as follows: scaphoid, lunar bone, cuneiform bone, trapezium, trapezoid, os magnus, unciform, pisiform, which is not always present.

15. Os Metacarpi Magnus, Metacarpal or Cannon Bone.—The large metacarpal, or cannon bone, is a long, straight bone, placed in a vertical direction. Its superior or upper extremity articulates with the carpus and its inferior or lower extremity articulates with the os suffraginis and the two sesamoids.

16. Ossa Metacarpi Parva, or Splint Bones.—The two small metacarpal, or splint bones, tuberous in form (marked or covered with projections) at the carpus and tapering distally (lower portion), are attached to the large bone, one on each side of its posterior (back) surface, by ligaments in the young animal, and by ossification (union of bone) in most grown up, and in all old animals.

17. Proximal Phalanx, Os Suffraginis, or Large Pastern Bone.—The os suffraginis, or large pastern bone, passes obliquely downwards and forwards, and articulates with the cannon bone above, and the median phalanx or os coronae below. It belongs to the class of long bones.

18. Great Sesamoid Bones.—These bones, two in number, are placed side by side at the postero-inferior (back and lower) part of the metacarpus and postero-superior (back and upper) part of the os suffraginis; they are irregular in shape, their back parts are covered with cartilage, for the passage of the flexor tendons of the digit or the last four bones of the limb. Their superior or upper surface is roughened, and their sides, which are grooved for the reception of the suspensory ligament. (This is called the fetlock joint.)

19. Median Phalanx, Os Coronae, or Small Pastern Bone.—The second phalanx, os coronae, or small pastern bone, has no marrow canal, and belongs to the class of irregular bones. It is inclined, like the os suffraginis, obliquely downwards and forwards, and is partly covered by the hoof. The upper portion of this bone articulates with the os suffraginis and the two sesamoids, the lower part articulates with the os pedis.

20. Distal Phalanx, Os Pedis, or Coffin Bone.—The third, or ungual phalanx, os pedis, or coffin bone, is an irregular bone situated within the hoof, and, when in a healthy state, corresponding somewhat to it in shape, being semilunar in form, with the convexity to the front. This bone is very hard and porous, having many openings for the transmission of arteries and veins. In this bone we notice the wall, the sole, the tendonous surface, the articular surface, and the alae or wings. This bone articulates with os coronae superiorly (upper) and posteriorly it articulates with the os navicular bone.

21. Os Naviculare.—The naviculare, third or small sesamoid, or shuttle bone, is an irregular bone, situated with its long axis transversely, behind and below the os coronae, and behind the os pedis, with both of which it articulates, the articulation of the three forming the so-called coffin joint. The lower surface of this bone is important as it is covered with cartilage, and together form a kind of a pully over which plays the great flexor perforans muscle. The remaining portion of the anterior limb or front leg, will be considered under anatomy of the foot. The navicular bone can be plainly seen on Plate VII, anatomy of the foot.

22. Femur or Thigh Bone.—The os femoris, femur or thigh bone, the largest, thickest, and strongest bone in the body, belongs to the class of long bones, and is placed in a direction obliquely downwards and forwards, articulating with the cup-shaped cavity in the pelvic or hip bones superiorly (or upper), and with the tibia and patella inferiorly (or below). This bone is roughened for the attachment of the powerful muscles of the hip.

23. Patella, or Stifle Bone.—This, the kneepan or stifle bone, is placed in front of the pully-shaped groove of the femur. It is very compact, its front surface being irregular, round and very much roughened, for ligamentous attachment, and its posterior (or back) surface very smooth to articulate with the groove in the femur, presenting two depressions divided by a ridge, the inner being the larger, and in the fresh state enlarged still more by projecting cartilaginous lip, or elevation. The patella increases the power of the hind leg, and it is this bone that causes stifle joint lameness when it slips out of the groove in which it glides normally.

24. Tibia, or Legbone.—The tibia or leg bone, is a long bone, larger at its upper than its lower end, situated between the femur and the astragalus, slanting downwards and backwards. This bone is three-sided, possessing outer, inner and back surfaces, all of which are wider above than below. This bone gives attachment to the flexor muscles of the hip.

25. Fibula.—This is a long slender bone, little developed in the horse, and is an appendage to the tibia, being attached to the outer side of that bone, and extending from its head to its lower third, to which it is affixed by a ligament; the space between the two bones is called the tibial arch. The fibula gives attachment to the peroneus muscle, the muscle that is supposed to be severely contracted when an animal is affected with string-halt.

26. Tarsus, or Hock Bones.—The tarsus, or hock, corresponding to the ankle-joint of a man, is composed of six irregular compact bones, situated between the lower end of the tibia and the superior or upper extremity of the metatarsus; they are arranged in two series; one consisting of the cuboid and three cuneiform bones, the magnum, medium, and parvum, corresponds to the lower row or carpal bones; the other upper series consists of the astragalus and calcaneum; the first, forming with the bone above the mobile portion of the joint, may be said to correspond to the upper row of carpal bones, while the latter, being the lever bone, corresponds to the trapezium. These bones, like those of the carpus, are thickly covered with cartilage on their articular surfaces, which acts as a protection against concussion. It is these bones that become diseased and united when an animal is affected with bone spavin. The calcaneum bone which forms the prominent part, termed the point of the hock, and corresponds to the heel-bone of man. This bone gives attachment to the calcaneo-cuboid ligament, and it is this ligament that is sprained or ruptured in curb of the hock.

28. Metatarsus, or Os Metatarsi Magnum.—This bone presents the same general appearance as the large metacarpal or cannon bone, from which it differs principally in being about one-sixth longer and flattened from side to side. It is rounded and more prominent in front. This bone articulates above with the tarsus bones, and its lower portion articulates with the os suffraginis.

29. Ossa Metatarsi Parva, or Splint Bones of the Hind Leg.—These also present the same general form as the small metacarpal bones of the front leg; the outer is the longest and largest, and has the largest head, with two surfaces which articulates with the cuboid bone; the inner one has also three articular surfaces, two for small, and one for the middle cuniform bones. The two surfaces on each head articulate with corresponding ones on the large metatarsal bone. The remaining bones of the hind extremity, viz.: the three phalanges, with their three accessory bones, so closely resemble the corresponding bones of the fore extremity, that it seems at first sight difficult to distinguish one from the other; the chief differences being, that the first phalanx of the hind extremity is longer; its upper end larger, and its lower end smaller, than in the fore extremity.