As a whole the bony framework serves to keep the soft parts in place, to support and protect them, and to aid in locomotion. The bones of the head and trunk support and protect organs; those of the arms are for tact and prehension; those of the lower extremities are for support and locomotion.
Normally bones have little sensibility, but when inflamed they are extremely sensitive and painful.
Joints.—The bones are connected with and move upon one another by means of joints. These joints are of three kinds: 1. Immovable, where the adjacent margins of the bones are closely applied, with little fibrous tissue between, as in the sutures of the head; 2. those with limited motion, which are very strong, the parts being connected with tough fibro-cartilage; and 3. freely movable. In this last group the articulating surfaces are covered with cartilage, which again is lined with a delicate synovial membrane which secretes a small amount of lubricating fluid, the synovial fluid, to reduce friction. Their surfaces are also sometimes deepened by the presence of inter-articular fibro-cartilages. Bursæ or sacs of synovial membrane occur outside the joints under tendons and ligaments to reduce friction.
The nature and extent of the motion of a joint is defined and the bones are held together by strong bands of fibrous tissue or ligaments, these ligaments being more fully developed in joints where there is great freedom of motion or where there is great weight to be supported. In a ball-and-socket joint, such as the hip, there is a ligament in the form of a strong capsule which surrounds the joint on all sides and limits its motion, while hinge joints, like the elbow, and pivot joints, such as that formed by the atlas on the axis, have lateral ligaments that allow of freer motion. In the shoulder-joint, which is the most freely movable joint in the body, the capsular ligament is very lax.
In general the kinds of motion possible in joints may be said to be flexion, extension, abduction, adduction, circumduction, and rotation.
When much violence is applied to a joint and no dislocation results, as in a sprain, there is often much stretching and even laceration of the ligaments.
Muscle.—The flesh, which forms a large proportion of the weight of the body, consists of muscular tissue. Of this two kinds are found: 1. The striated or striped muscle of animal life, which is under the control of the will and so is known as [voluntary muscle], and 2. the unstriped or smooth muscle of organic life over which we have no control, that is, the involuntary muscle. Each fiber of striped muscle has an elastic, membranous sheath, the sarcolemma, and consists of rod-shaped cells with a nucleus along the edge, set end to end and having [crosswise striations]. In unstriated muscle the fibers, which have no sarcolemma, consist of oval or spindle-shaped cells, with a nucleus much smaller than that of striped muscle and situated in the middle. In both kinds of muscle the fibers are bound together with connective tissue and blood-vessels into fasciculi or bundles, and many bundles go to make up a muscle. The muscle in turn has a connective tissue envelope or sheath, the fascia. These fasciæ are found throughout the body, the superficial ones being just beneath the skin, while the deep ones not only form sheaths for the various muscles but form partitions between them and serve to strengthen their attachments. The striped muscles are those of motion, while the unstriped occur in the hollow organs, surrounding the cavity and in some cases lessening its capacity by their contraction.
An intermediate form of muscle known as cardiac muscle occurs in the heart. Here the fibers have striations but the nucleus is generally in the middle of the cell and the fibers branch and run together.
