SMOOTH MUSCLE AND HEART IMUSCLE 127 



gethcr are larger and larger (Fig. 30). The arrangement 

 of the bundles with reference to one another and to the 

 muscle as a whole will be described in more detail in a 

 subsequent paragraph. Here we may say that the shape 

 of the muscle is determined by the arrangement of the 

 bundles; that is, they may be placed end to end, making 

 a long and slender muscle ; or side by side, giving a short, 

 thick muscle. Examples of these various shapes are 

 numerous and familiar to most of us in our own bodies. 



Make-up of Smooth Muscle 

 AND Heart Muscle. — In some of 

 the lower animals the smooth "^ 

 muscle fibers are bound together in 

 fairly definite muscles. In others, 

 they form rather large masses, for 

 example, the foot of a clam is a 

 mass composed of smooth muscle yig. so. — A Piece of 



fibers. But in the higher animals Muscle Showing Five 



the fibers of smooth muscle are X:.ed^ora'"„'lb:rof 



usually bound together in the form fine bundles. The latter, 



of sheets, and so do not make up ^" *"'^' '^'' "^^t «T ""^ 



. ,. . , i^ c* V. uip microscopic muscle fibers. 



mdividual muscles which can be 



dissected out as can particular skeletal muscles. Thus 

 the muscular part of the lining of the stomach consists 

 of two layers which can be separated from each other 

 but which can not be distinguished as made up of 

 individual muscles. 



The fibers of heart muscle are bound together to make 

 up a muscular bag in which it is difficult to distinguish 

 individual muscles. 



The Structure of Skeletal Muscle Fiber. — A 

 single fiber of skeletal muscle presents under the micro- 

 scope a very interesting appearance, since it shows a regu- 

 lar sequence of cross-markings or striations (Fig. 31). 

 But apparently these cross-markings do not have much 

 significance. Careful examination of the fiber shows that 

 its protoplasm is of two general sorts. There is a series 



