SKELETAL SYSTEMS AND MOVEMENT 



505 



Skeletal muscle 



Skeletal muscle is what we know as flesh. 

 It makes up about 40 per cent of the weight 

 of a man. Most of it is located in the limbs, 

 where it effects locomotion, and in the body 

 wall; it is present also in the diaphragm, 

 tongue, upper third of the esophagus, phar- 

 ynx, larynx, and around the eyeballs. When 

 viewed with a microscope, each myofibril 

 in skeletal muscle appears to consist of alter- 

 nate light and dark segments; these seg- 

 ments are arranged in a muscle so that all 

 of the light ones lie side by side, and all of 

 the dark ones also lie side by side, giving 

 the whole muscle a striated appearance. The 

 blood supply is rich and appears in amounts 

 of from HOO to 4000 capillaries per cubic 

 millimeter, which, however, is much less 

 than in heart muscle. Since these striated 

 muscles are the type that are attached to 

 the skeleton, they are called skeletal mus- 

 cles; and since their movements are more or 

 less under voluntary control, they are known 

 as voluntary muscles. 



The fibers of skeletal muscles are spindle- 

 shaped and may be from one to 40 mm. 

 long and from 0.01 to 0.15 mm. in diameter. 



Each is a multinuclear cell, the nuclei usu- 

 ally lying just beneath the surface mem- 

 brane. Myofibrils run lengthwise through 

 the cell. The fibers are bound together into 

 muscles by connective tissue, and the mus- 

 cles thus formed are attached to bones by 

 means of tough strands of connective tissue 

 called tendons. The end of the muscle that 

 is attached to the more stationary part of 

 the skeleton is called the origin, and the 

 other end, which is attached to the part 

 moved when the muscle contracts, the in- 

 sertion. Some of the skeletal muscles or 

 man are shown in Figs. 374 and 375, and the 

 origin, insertion, and function of a few of 

 these are given in the accompanying table. 

 Skeletal muscles contract more quickly than 

 smooth muscles, and since each fiber may 

 act separately, various* gradations in the 

 force of the contractions are possible. Mus- 

 cles used in locomotion are usually present 

 in pairs that are "antagonistic." For exam- 

 ple, when the biceps contracts it bends the 

 arm at the elbow; since muscles exert no 

 force when they relax, an opposing muscle, 

 the triceps, on the opposite side of the arm 

 is required to extend the forearm again as 

 shown in Fig. 232. 



THE NAME, ORIGIN, INSERTION, AND FUNCTIONS OF SOME OF THE 

 SKELETAL MUSCLES OF MAN (Illustrated in Fig. 374) 



NAME OF MUSCLE 



ORIGIN 



INSERTION 



FUNCTION 



* Abduct means to draw away from (L. ab, from; ducere, draw). 

 ** Flex means to bend (L. flcxus, bent). 



t Supinate means to bring the palm upward (L. supinus, bent back). 

 tt Adduct means to draw toward (L. ad, toward). 



