The skeleton is the internal framework of the body and is made up of bones and cartilage. Bones provide support (when they are locked in position by the muscles and ligaments) and protection (the skull, rib cage, and pelvis protect their contents). Bones also produce motion, acting as levers when pulled by the muscles. Cartilage is a hard, yet elastic (resilient) tissue, and can be found in the ears, the nose, and at the ends of the ribs, where they directly create surface form.
Bones move against one another, or articulate with each other, at joints. Their articulating surfaces are covered with articular cartilage. Bones are held in their proper relationship to each other at the joints by strong, fibrous, nonelastic bands called ligaments, which keep them in contact yet permit normal motion. A ligament connects bone to bone (a tendon connects muscle to bone).
Relating to bone, a spine is a projecting point or a raised ridge (the entire vertebral column is also called the spine); a process is a projection of varying shape, a protuberance is a bony projection or bump, and a tuberosity is an enlargement. A crest is a high ridge. A long bone, such as the humerus, femur, or tibia, consists of a shaft with an expansion at either end. The rounded, regularly shaped expansion is called a head. Large prominences at the ends of bones, which articulate with other bones and are covered by a thin layer of articular cartilage, are called condyles. Small, nonarticulating prominences found on the condyles are called epicondyles.
Bone may directly create surface form (elbow, forehead, medial surface of the tibia, outer point of the hip), strongly influence it (rib cage, sacrum, back of hands and feet), or be concealed deep in muscle (neck vertebrae; most of the humerus, femur, and pelvis). Areas of bone just below the skin are subcutaneous and can be felt and often seen at the surface. Prominent subcutaneous points of bone are called bony landmarks, and may serve as important proportional measuring points of the body. Locating the bony landmarks in the living animal is the key to understanding the exact position of the entire skeleton, most of which is embedded in the soft tissues of the body. In muscular individuals, subcutaneous points and ridges my show as depressions surrounded by bulging muscles. In thin individuals, these same bony areas may appear as raised points and ridges. The proportions of an animal are created by the relative lengths of the bones. Proportions are usually best measured from these bony landmarks.
The skeleton can be visualized as a combination of volumes and axes. The skull and rib cage are three-dimensional volumes that greatly affect surface form. The bones of the pelvis make a rigid volume, which is for the most part deeply buried in musculature. The limb bones—long and thin—are axes, or lines of direction passing through the form of a fleshed-out limb.
A series of adjacent, somewhat parallel bones, such as the metacarpals of the hand or the individual ribs, can create a plane. Large, single bones, such as the braincase of the skull, also create planes. Planes are often very evident on the surface, and can be flat or curved. The shoulder blade is a plane with a raised ridge on its surface. The muscles that lie on the shoulder blade maintain a planar quality, reflecting the flatness of the underlying bone. It is important to note the orientation of a plane (which way it is facing).
When studying the live animal, the first, and perhaps most important step is to accurately visualize the skeleton inside the body, for onto this internal scaffolding are attached the numerous convex forms of the musculature.
The skeleton is divided into an axial (longitudinal) skeleton and a limb (appendicular) skeleton. The axial skeleton consists of the skull, the rib cage, and the spine (vertebral column). The limb skeleton consists of the four limbs. In primates, such as humans, apes, and monkeys, they are called the arms and the legs. In four-legged animals, the limbs in front are called the forelimbs, and include the scapula or shoulder blade, and the clavicle when present. The limbs in back are called the hind limbs, and include the pelvis.
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