An animal can be visualized as being constructed of a series of simplified, three-dimensional, somewhat geometric volumes (head, forearm, thigh). Each of these volumes has one dimension that is longer than the others. A line projected through the center of the mass of this volume on its longest dimension is called its axis (plural, axes). For the most part, especially in the limbs, these axes follow the skeleton, so that a line drawn through the long dimension of a bone is on, or close to, the axis of the volume of that region (for example, the position of the radius is close to the axis of the forearm).
One of the more confusing regions of the body is the volume of the upper arm. The humerus (upper arm bone) is mostly deeply buried in muscle, and lies toward the front of this muscle mass, with the massive triceps muscle located at its rear. Therefore, the axis of the humerus does not coincide with the central axis of the upper arm volume. It is important, though, that the actual position of the humerus be clearly understood, because its upper end forms the point of the shoulder, and its lower end is an important site for the origination of the forearm muscles.
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