126 MOVEMENT IN ANIMALS 



like the skeletal muscle in being quick-actinp^ and power- 

 ful; it differs from all other kinds of muscle in its property 

 of contracting and relaxing automatically. In a para- 

 graph above was emphasized the importance of the con- 

 trol of muscular movement. This control requires that 

 the muscles in general shall be inactive except when ac- 

 tivity is especially called for. The heart presents quite 

 a different case, since its proper functioning as an organ 

 for pumping blood requires that its muscle shall contract 

 and relax in regular sequence. Corresponding to this 

 difference in requirement we find the difference in be- 

 havior already mentioned, namely, that the heart muscle 

 works automatically, while the other types of muscle are 

 under control and work only when called upon to do so. 



Muscle Fibers. — All muscles are composed of living 

 cells, the shape of the cells being in general cylindrical, 

 that is, they are comparatively long and slender. For 

 this reason they are commonly referred to as muscle 

 fibers. The size varies. Single cells, or fibers, of skeletal 

 muscle may range in length from 1 mm. to 35 mm. (1/25 

 inch to 1 2/5 inch); and in diameter, from .034 mm. to 

 .055 mm. (1/7.50 inch to 1/450 inch). 



In the higher animals smooth muscle fibers are much 

 smaller, averaging not more than .042 mm. (1/600 inch) 

 in length, but in the lower animals, in which they con- 

 stitute the chief type of muscle substance, they may 

 attain to larger sizes. 



Make-up of Skeletal Muscle. — The fibers are 

 bound together in bundles or sheets of various kinds to 

 form the muscle as a whole. A good idea of the structure 

 of the skeletal muscles can be obtained from an examina- 

 tion of raw lean meat, where it can be made out that 

 individual fibers are held together in bundles by means 

 of a binding tissue known as connective tissue. These 

 small bundles are bound together into still larger bundles, 

 and these again into still larger, the connective tissue be- 

 coming coarser as the bundles that are to be bound to- 



