THE MUSCULAR SYSTEM 39 



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 'Svhite" 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, fascic- 

 uli, 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 carbo- 

 hydrates, proteins, and fats. These are conveyed by the 

 blood to the muscles. Heat is generated by muscular con- 

 traction. Carbon dioxide and other w^aste 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 com- 

 pound muscle has several origins or insertions. Sometimes 

 parts of a tendon may ossify, producing a sesamoid hone, 

 such as the patella. The fleshy portion of a muscle is 

 called the helly. 



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 



