_Long Bones._—These bones are usually used as a system of levers to confer the power of locomotion. A long bone consists of a shaft and two extremities. The shaft is a hollow cylinder within which is the medullary canal. The extremities are somewhat expanded for the purpose of articulation, and to afford a broad surface for the attachment of muscles. The long bones are as a rule curved in two directions to give greater strength to the bone. Some examples of this class of bone are the clavicle, radius, ulna, humerus, femur, tibia, fibula, metacarpal, metatarsal, and the phalanges. _Short Bones._—These bones are placed in that part of the skeleton where there is need for strength and compactness, and where the motion of the part is slight and limited. Some examples of this class of bone are the bones of the carpus and tarsus (in the hand and the foot). _Flat Bones._—Flat bones are found where the principle requirement is either extensive protection, or the need of a broad surface for the attachment of muscles. Some of the bones of this class are the occipital, parietal, frontal, nasal, lachrymal, vomer, scapula, sternum, and the ribs. _Irregular Bones._—These bones are such as from their peculiar shape and form can not be grouped under any of the preceding heads. Some of the bones of this class are the vertebrae, sacrum, coccyx, temporal, sphenoid, ethmoid, etc. If the surface of a bone is examined, certain articular and non-articular eminences and depressions will be seen. _Articular Eminences._—Examples of this class are found in the heads of the humerus and the femur. _Articular Depressions._—Examples of this class are found in the glenoid cavity of the scapula and the acetabulum. _Non-articular Eminences._—These are designated according to their form. A tuberosity is a broad, rough, and uneven elevation. A tubercle is a small, rough prominence. A spine is a sharp, slender, pointed eminence. A ridge, line, or crest is a narrow, rough elevation, running some way along the surface. _Non-articular Depressions._—These are of variable form, and are described as notches, sulci, fossae, grooves, furrows, fissures, etc. These non-articular eminences and depressions may serve to increase the extent of surface for the attachment of ligaments and muscles or may receive blood vessels, nerves, tendons, ligaments, or portions of organs. Canals or foramina are channels or openings in bone through which pass the nerves and blood vessels. =Teeth.=—In the human body we find two sets of teeth. One appearing in childhood, and are known as milk teeth, twenty in number, the permanent teeth replacing these about the sixth year. There are thirty-two permanent teeth, divided into four incisors, two canines, four bicuspids and six molars. Teeth are made up of three different substances, which are known as enamel, dentine and cement.