The skeleton of the dog is very similar to those of humans and other mammals. A skeleton is the framework that both supports the soft structures of the body and enables locomotion. A dog's complete skeleton includes an axial skeleton, comprising the skull, vertebral column, ribs and sternum, and an appendicular skeleton that includes the thorax, pelvis, forelimbs and hind limbs. The skeleton of any dog includes about 319 bones. How those bones fit together -- their lengths relative to each other, and the resulting angles formed between them -- determines how efficiently a dog can move and do work. Structure and function are at the heart of conformation standards set by dog breeders' associations.
Aside from size, one of the largest differences in dogs from breed to breed is head shape. Although each breed has a distinctive head, canine skulls can be categorized into three main shapes: brachycephalic, mesocephalic, and dolichocephalic. Brachycephalic breeds, such as boxers and bulldogs, have a wide skull with a muzzle that is less than half the length of the head. Mesocephalic skulls are most similar to wild type or wolf skulls, and are seen in dogs such as German shepherds and Siberian huskies. Dolichocephalic breeds, such as the collie and the borzoi, have skulls that are narrow and long. Breed standards describe the desired shape of an ideal skull for that breed. In many cases, the standard or the breed history will explain how the prescribed shape fits the work the dog was bred to do.
Neck and Spine
The vertebrae of the neck and spine support the neck and body and protect the spinal cord from damage. The vertebrae also are the bridge that links the dog's hindquarters and forequarters and provides attachments for the back muscles that transmit locomotive force generated by the back legs and joints to the front end. The neck and head also serve as balancers that shift the moving dog's center of gravity as needed. Five types of vertebrae make up the vertebral column: seven cervical, 13 thoracic, seven lumbar, three sacral and 20 caudal vertebrae, which make up the tail. Dogs with naturally short tails have fewer caudal vertebrae. The dog's 13 pairs of ribs connect to the thoracic vertebrae; all but the last of these connect at the other end to the sternum, or breastbone. The depth and breadth of the rib cage or thorax determine how much space the dog has for efficient heart and lung function.
The long bones of the dog's front legs must be straight and strong enough to support his weight as it is transferred to the forequarters by the powerful force of the rear legs when the dog is running. The front legs connect to the spinal column via tendons at the scapula, or shoulder blade; the hind legs connect at the hip joints in the pelvic girdle. The forelimbs are equivalent to human arms, with the humerus closest to the shoulder and the radius and ulna in the lower limb. Likewise, the hind limbs are made up of the femur in the upper limb and the tibia and fibula in the lower. The relative lengths of these bones and the angles they form with each other determine how effectively the dog can move and carry out his work.
The feet play a major role in a dog's well-being. Comprising 29 bones in the front and 30 in the rear, the feet function as the dog's primary shock absorbers. In the front legs, the pastern connects the leg bones to the foot through the carpus, which is equivalent to the human wrist, down to the metacarpus and phalanges, analogous to the hand and fingers in humans. The length and angle of the pastern determines how much shock will be cushioned when the dog's weight is transferred to the front end during locomotion. The hind feet transition from the hock joint down the tarsus, which is equivalent to, but longer than, the human heel. The metatarsus and phalanges are analogous to the human foot and toes.
- The Dog in Action: McDowell Lyon
- dogs image by lena Letuchaia from Fotolia.com