The dog’s internal structure and mammal reproduction

The dog’s digestive tract consists of the mouth cavity, pharynx, gullet, stomach, and small, large and rectal intestines. In the mouth teeth grind food, salivary glands wet it with saliva, and the tongue tastes and mixes. In the stomach food is partly digested by gastric juice, and in the small intestine it is fully digested by intestinal and pancreatic juices and bile from the liver. The undigested remainder passes into the large intestine and leaves through the rectum. Air goes through the nasal cavity, larynx, windpipe and bronchi to the lungs. The rib cage is separated from the abdomen by the diaphragm. On inhaling, the ribs rise, the diaphragm flattens, the chest widens and air enters the lungs; on exhaling, the ribs lower, the diaphragm rises like a dome and air leaves. The heart is four-chambered: two atria and two ventricles. There are systemic and pulmonary circuits, and arterial and venous blood do not mix. The excretory organs are a pair of bean-shaped kidneys in the abdomen on both sides of the lumbar vertebrae; urine flows through the ureters to the bladder and out. Because breathing and circulation are perfect, metabolism is rapid and body temperature constant: about 38–39 °C in a dog. The brain has five parts, but the cerebral hemispheres are complex, with many cortex folds – the more folds, the more cells; conditioned reflexes depend on cortex activity. The cerebellum controls complex movements. Land animals tell smells well (a dog recognises prey and relatives by smell). The hearing organ has outer, middle and inner parts; the pinna amplifies sound and finds its direction. The eye distinguishes shapes well. The skin is also a sense organ: it senses pain, hot and cold, hard and soft; long hairs near the nose and eyes – whiskers – also serve touch.