The skeleton is a jointed structure of bones controlled by muscles. It supports the whole body and contains the central nervous system. It also stores minerals and marrow for the body's use.
The bones are living organisms whose form, structure, and composition vary during the animal's life.
A dog has approximately 320 bones. The axial skeleton consists of 134 bones, including the skull, the spinal column, breastbone, ribs, and hyoid bone. The appendicular skeleton consists of 186 bones, including those of the limbs, and in male dogs the penile bone.
There are four types of bones: long bones such as the femur and tibia in the leg; flat bones such as the scapula (shoulder blade) and cranium (skull); irregular bones such as the vertebrae; short bones such as the phalanges (bones of the paws); and the sesamoid bones, which are really ossified portions of tendons.
A dog's spinal column runs horizontally between the shoulders and the pelvis. It consists of 7 cervical, 13 thoracic, and 7 lumbar vertebrae, as well as 3 sacral vertebrae fused to form the sacrum. There are also caudal vertebrae which vary in number according to the breed and the individual dog. At the level of the thorax, the spinal column joins the ribs and the sternum (breastbone).
The skeleton of the head is made up of two sets of bones: the cranial bones, which protect the brain (cerebrum, cerebellum, and brain stem), and the facial bones around the mouth and nasal cavities. The proportions of these bones vary from one breed to another. The limbs are connected to the axial skeleton by girdles. The shoulder girdle consists solely of two scapulae, one on each side of the body, since dogs, like other running animals, have no clavicles (collar bones). The pelvic girdle links the hind legs to the spine. Together with the sacrum it forms the pelvis.
The bones of the limbs correspond to those in humans. In the foreleg (arm) the upper part is supported by the humerus; the lower part is supported by the radius and the ulna; the forepaw (hand) is supported by the carpus, the metacarpus, and five toes (fingers) of three phalanges each, except for the dew-claw (thumb), which has only two. The hind leg consists of the thighs, supported by he femur; the leg the leg supported by the tibia and fibula; the foot, containing the tarsus and metatarsus; and four toes, each having three phalanges. The dog is a digitgrade animal - that is, it walks on its toes. Its thid phalanges rest on the ground, protected by the pads on the sole of the foot. When a dog runs, its feet are straight and elongated. When a dog is at rest, the bones of its limbs are at angles to each other.
Joints enable the bones to move in relation to each other. There are three kinds: fixe, mobile, and semi-mobile.
The fixed joints, such as those of the cranium, are called sutures.
The most mobile joints are those of the limbs. The surfaces of these bones are covered with cartilages and lubricated by synovial fluid so that they can glide smoothly over each other. The bones are held in place by a fibrous envelope called the articular capsule, which encloses the joint and is reinforced with ligaments. Muscles and tendons surround each joint to direct and control their actions. The semi-mobile joints, as their name indicate, permit only limited movement. There are semi-mobile joints after each vertebra in the spinal column.
There are three types of muscle: the striated (striped), or skeletal muscle, which make movements of the skeleton, the skin and certain organs possible; the cardiac muscle, which is also striated but is controlled by the nervous system; and the smooth muscles of the digestive tract and other internal organs.
The muscles of the skin are well developed in dogs. Their contraction allows the animal to shake itself dry after a bath. Among the head muscles, however, only the muscles of the ears and eyelids have an important tole; the other muscles do not enable the animal to alter its facial expressions as we do. The skeletal muscles vary a great deal in their form. They may be long or short, and their attachment to the bones may be direct or by way of a tendon. Dogs, like other mammals, also have a muscle called diaphragm. This separates the thoracic and abdomino-pelvic cavities.