34 



THE MUSCLES 



FIG. 12. MUSCULAR TISSUE 



a, &, Striped muscular fibres ; c, The same more 

 highly magnified 



Most skeletal muscles taper at one or both, ends until the 

 muscular tissue finally disappears, and the sheath gradually 

 changes to white, fibrous, inelastic connective tissue which 

 forms tendons, by means of which most muscles are attached 

 to bone. 



3. When we examine 

 these fibres under the mi- 

 croscope we discover that 

 they in turn are made up 

 of still finer fibres, or ft- 

 brillce, as' shown in Fig. 

 12. The fibres are beau- 

 tifully marked by par- 

 allel wavy lines, about 

 ten thousand to an inch, 

 which give the fibre its 

 name of the striped mus- 

 cular fibre. All of the voluntary muscles present this appear- 

 ance. 



Striped muscle cells are from one to one and one-half inches 

 in length, and from ^^ inch to ffa inch in breadth. These 

 cells lie parallel to each other in bundles. All action of the 

 muscles is accomplished by changes in the shape of these cells, 

 which act in concert under the control of the nervous system. 



When these cells shorten and thicken, the whole muscle 

 undergoes a corresponding change, it contracts ; when the cells 

 narrow and lengthen, a similar change occurs in the shape of 

 the whole muscle, it relaxes. Each cell is encased in a delicate 

 membrane, called sarcolemma, which during life adheres to the 

 protoplasm within. Each cell possesses many nuclei which 

 lie just under the sarcolemma. Figure 12, a, shows a number of 

 these adhering fibrillae ; &, shows these bundles torn asunder 

 (the sarcolemma is visible at c) ; c, shows how the same cells 

 cleave laterally when treated with chemicals such as ammonium 

 carbonate. 



3. What is the composition of the fibres ? How are the fibres marked ? How long are 

 the striped muscle cells ? How broad ? Describe the correspondence between action of the 

 muscles and the shape of the cells. 



