CLASSES OF BONES.--Bones are classed as long, flat, and irregular. Long or cylindrical bones are found in the limbs or extremities, and serve as levers and pillars for traveling and to support the body. Descriptively, a long bone is divisible into a center or _shaft_ and two ends or extremities. Flat bones are found where visceral organs need protection. As the shoulder or scapula and ribs, to protect the heart and lungs; pelvic or hip bones, to protect the rectum and urinary and genital organs; also the cranial bones, to protect the vital organ called the brain. Irregular bones are found in the spinal column and in the joints, such as the knee or carpus, hock or tarsus, where great strength is required. They usually possess many angles and indentations, with surfaces for articulation and tendonous attachment, and consist of a thin, dense, external (outer) case of compact bone enclosing cancellated tissue. In proportion to their size they present a much larger extent of articular surface and greater mechanical strength than any other class. 1. SKULL, or skeleton of the head, the most anterior (forward) part of the horse’s skeleton, articulates with the first cervical vertebra (or atlas), from which it is suspended by its posterior (or back part) extremity, its anterior (forward) extremity being free. Its position varies with the attitude of the animal; but in our descriptions we shall always suppose it to be placed in a horizontal position. In the young animal the skull is composed of a number of bones, all of which, with the exception of the lower jaw, the teeth, the bones of the tongue, and ossicles of the ear, become united by ossification (growing together) in the adult. In speaking of the different points of importance in connection with the head bones of the horse, and other animals, suppose a cross or longitudinal section of the head is made. It will be noticed that it is full of cavities or sinuses. The uses of these are to lighten the head and also to warm the air as it passes into the nostrils, on its way down to the lungs. It will also be found full of foramen or small holes through which the nerves from the brain and various blood vessels pass to the organs situated in the head, such as the tongue, lips and the various glands in and around the head. Then there are the cavities in which the eyes are situated, one on each side of the head, called the orbital fossa. In examining this fossa you will find a small opening or foramen, through which the optic nerve passes in coming from the brain to the eye. This is the nerve of sight. Then the most important part of all to consider is the cranial cavity in which that very important organ is situated called the brain, which controls all the various functions and movements of the body.