Note. Inasmuch as most of the experimental work of this chapter depends upon the use of the microscope and also necessarily assumes a knowledge of facts which are discussed later, it would be well to postpone experiments in histology until they can be more satisfactorily handled in connection with kindred topics as they are met with in the succeeding chapters.] Chapter II. The Bones. 27. The Skeleton. Most animals have some kind of framework to support and protect the soft and fleshy parts of their bodies. This framework consists chiefly of a large number of bones, and is called the skeleton. It is like the keel and ribs of a vessel or the frame of a house, the foundation upon which the bodies are securely built. There are in the adult human body 200 distinct bones, of many sizes and shapes. This number does not, however, include several small bones found in the tendons of muscles and in the ear. The teeth are not usually reckoned as separate bones, being a part of the structure of the skin. The number of distinct bones varies at different periods of life. It is greater in childhood than in adults, for many bones which are then separate, to allow growth, afterwards become gradually united. In early adult life, for instance, the skull contains 22 naturally separate bones, but in infancy the number is much greater, and in old age far less. The bones of the body thus arranged give firmness, strength, and protection to the soft tissues and vital organs, and also form levers for the muscles to act upon. 28. Chemical Composition of Bone. The bones, thus forming the framework of the body, are hard, tough, and elastic. They are twice as strong as oak; one cubic inch of compact bone will support a weight of 5000 pounds. Bone is composed of earthy or mineral matter (chiefly in the form of lime salts), and of animal matter (principally gelatine), in the proportion of two-thirds of the former to one-third of the latter. Illustration: Fig. 10.—The Skeleton. The proportion of earthy to animal matter varies with age. In infancy the bones are composed almost wholly of animal matter. Hence, an infant’s bones are rarely broken, but its legs may soon become misshapen if walking is allowed too early. In childhood, the bones still contain a larger percentage of animal matter than in more advanced life, and are therefore more liable to bend than to break; while in old age, they contain a greater percentage of mineral matter, and are brittle and easily broken. Experiment 3. _To show the mineral matter in bone_. Weigh a large soup bone; put it on a hot, clear fire until it is at a red heat. At first it becomes black from the carbon of its organic matter, but at last it turns white. Let it cool and weigh again. The animal matter has been burnt out, leaving the mineral or earthy part, a white, brittle substance of exactly the same shape, but weighing only about two-thirds as much as the bone originally weighed.