which the longitudinal fibrils are imbedded. Alternate light and dark regions in these fibrils produce the cross striated appearance. Each fiber contains many nuclei, but it is believed by some to be a single cell. The nuclei occur near the surface in the "white" muscles, or deeper in the fiber between the fibrils in the "red" muscles. "Red" muscles contract more slowly than the "white," and are more resistant to fatigue.
Striated muscle fibers are aggregated into bundles, fasciculi, and these in turn into masses of varying size and shape, known as muscles. In general these fibers run parallel to one another, and are bound together into bundles by connective tissue. The contraction of the muscle is initiated by a nervous stimulus. The energy for muscular work is derived from chemical changes in carbohydrates, proteins, and fats. These are conveyed by the blood to the muscles. Heat is generated by muscular contraction. Carbon dioxide and other waste products of muscular work are carried away by the blood.
As a rule a muscle is attached at each end, the less movable attachment being the origin; the more movable is the insertion. The attachment may be by means of a tendon, a non-elastic cord sometimes of considerable length, or by a sheet of connective tissue, an aponeurosis. A compound muscle has several origins or insertions. Sometimes parts of a tendon may ossify, producing a sesamoid bone, such as the patella. The fleshy portion of a muscle is called the belly.
Most muscles move bones and cartilages, though some may induce movement in soft parts only, such as the facial muscles which pass from a bone to the easily movable skin. Some which lie beneath a concave or convex surface merely tend to straighten out this surface when they contract. The processes of many of the bones act as levers moved by
