Bone consists of one-third animal or organic matter and two-thirds earthy or inorganic matter. These proportions, however, vary with age. In youth it is nearly half and half, while in the adult the earthy is greatly in excess. It also varies with disease. With some defect of nutrition, the bone is deprived of its normal proportion of earthy matter, while the animal matter is of unhealthy quality, and we have as a result, a disease called rickets, so common in the children of the poor. The earthy or inorganic matter consists of phosphate, carbonate, fluoride of calcium, sodium chloride, and phosphate of magnesium. The animal matter consists of fat collagen, which when boiled with water is resolved into gelatin. To illustrate the two substances, take a bone and place it in dilute hydrochloric acid. The acid will eat out all the mineral matter and we have left only the animal matter. After this operation one can take the bone and can bend it into any position whatever, which experiment shows that the animal matter gives elasticity to the bone. The second experiment would be to put the bone on a bed of hot coals and burn it. Only the animal matter will burn and we will have the mineral matter remaining. After this operation one will find that the bone is very brittle and will easily break, which experiment shows that the mineral matter gives stability and support to the bone. [Illustration: FIG. 5—Cross section of bone. (Sharpey)] If a cross section is made of any long bone, such as the humerus, and this section placed under the low power of the microscope, the Haversian canal system can be discerned. The Haversian canal system consists of the numerous small openings or canals through which the blood vessels ramify in distributing the nourishment to the bone. Around each individual canal are seen smaller spaces arranged in a circle. These are known as the lacunae (small lakes). Going from the lacunae are smaller canals which take on the name canaliculae, and joining all the lacunae together, making the appearance of concentric circles, we have the lamellae. The outside covering of the bone is called the periosteum and the inside covering is called the endosteum. Most of the long bones and many of the smaller bones are supplied by a nutrient artery, which enters the bone near its center, enters the bone marrow, and divides into two branches, one going up and the other down in the marrow. The blood is then distributed through the Haversian canal system. Veins emerge from the long bones in three places: 1. One or two large veins accompany the nutrient artery. 2. Numerous veins emerge from the articular extremities. 3. Many small veins arise in and emerge from the compact substance. Bones are divided, according to shape, into four classes: long, short, flat and irregular.