128 



MOVEMENT IN ANIMALS 



of dense protoplasmic strands running the length of the 

 fiber but not occupying its entire bulk; the spaces be- 

 tween and among these fibers are filled with very thin 

 watery protoplasm which is probably not much firmer 

 (i.e. viscous) than the body fluid or lymph which bathes 

 the fiber on the outside. Enclosing the entire fiber and 

 holding its parts in place is a delicate membrane or 

 sheath. It is probable that the in- 

 dividual contractile strands within 

 the muscle fiber are comparable 

 with the protoplasmic strands 

 which were described at the begin- 

 ning of the chapter as occurring in 

 many of the fixed protozoa. Just 

 as these strands have the power of 

 contracting forcibly, so have these 

 in the muscle fiber, and it is their 

 combined action that makes up 

 the contraction of the fiber as a 

 whole. 



Smooth Muscle Fiber. — The 

 fiber of smooth muscles is much 

 simpler in structure than is that 

 of the skeletal muscle. In fact, on 

 account of its simplicity of con- 

 struction it may be compared di- 

 rectly with the protoplasmic con- 

 tracting strand in the Protozoa, 

 although as an apparatus for 

 doing work it is probably a great deal more effi- 

 cient than is the contracting strand in the one-celled 

 organism. 



Heart Muscle Fiber is intermediate in structure be- 

 tween smooth and skeletal muscle. The individual cells 

 contain protoplasmic strands similar to those in skeletal 

 muscle; but the fibers, instead of being enclosed each in 

 its own sheath, merge together into a continuous proto- 



Fio. 31. — A Small 

 Piece of a Single Mu.scle 

 Fil)er. At (a) the fil)cr 

 ha.s been crushed and 

 twisted to show the out^r 

 sheath separated from the 

 protoplasm Ix'neath. At 

 (ft) the dense and rlosely 

 parked longitudinal 

 strands are suggested, 

 together with the char- 

 acteristic cross-markings. 



