Dogs are mammals and have a physiology much like other quadruped mammals. The skeletal system gives rigidity to the body with bones acting as levers, stores of minerals and sites for blood formation. Long bones are mainly used in locomotion and include the humerus, radius and ulna of the foreleg and the femur, tibia and fibula of the hind leg. The skull of the dog shows greater variation than in other species ranging from the "compressed" (brachycephalic) format of the bull breeds through to the narrow skull of the sighthounds. Tail length also shows great variation.
The dog's circulatory system has a four chambered heart similar to that of man. It is also subject to some of the same inherited diseases including patent ductus arteriosus and conotruncal septal defects. In essence arteries carry oxygenated blood from the heart and veins carry unoxygenated blood back to the heart.
The digestive system starts with a mouth containing twenty-eight deciduous teeth which are gradually replaced in early puppy-hood by forty two permanent teeth (12 incisors, 4 canines, 16 premolars and 10 molars). There is a simple stomach and a small and large intestine - the large one is the shortest and simplest of all domesticated animals. The dog is a carnivore but less genuinely than the cat. It is more like an omnivore. In the wolf the reproductive system is monoestrus - the animal comes into season once a year. Domestic dogs usually cycle twice a year. Puberty begins when puppies are about six to nine months. The estrus cycle consists of a proestrus period (about 9 days), an estrus (5 to 20 days) and a metestrus (80 to 90 days). Pregnancy lasts about 63 days. Litter size in the wolf is usually four cubs. In the dog it varies over a wide range. In broad terms litter size is positively influenced by body weight and/or wither height. Small breeds have from 1 to 4 pups and larger breeds have anything up to 22. A breed the size of the German Shepherd has a range up to 17 pups, with 7.7 on average.
Features of the dog (IMAGE)