SKELETAL MUSCLE 



125 



abundant; but the muscles of the extremities are poor in elastic tissue, 

 containing only fine, chiefly longitudinal fibers, found especially in the 

 perimysium externum. 



Cross sections of striated muscle fibers are readily recognized. They 

 have rounded-polygonal outlines formed by the sarcolemma and fibrous 

 membrane, within which are the myofibrils, often shrunken from the 

 membrane. The fibrils stain intensely with eosin. They appear as coarse 

 granules, but their rod-like form becomes evident as they are followed up 

 and down by changing the focus. The shifting picture thus presented is 

 quite characteristic. Some fibers stain more darkly than others, owing to 

 the varying abundance of sarcoplasmic granules. 



External perimysium. 



Muscle bundles. 



Internal perimysium. 



Cross section of artery. 



Muscle spindle. 



Cross section of nerve 



FIG. 116. FROM A CROSS SECTION OF THE OMOHVOID MUSCLE OF MAN. X 60. 



In many animals, as in the rabbit, two sorts of striated muscles may be recognized 

 red muscle (e.g., the M. semitendinosus and M. soleus); and pale or white muscle 

 (e.g., the M. adductor magnus). Correspondingly there are two sorts of fibers. 

 First, there are dark fibers with abundant sarcoplasm, well defined longitudinal 

 striation, and poorly developed transverse markings, having in general a small 

 diameter; these occur in red muscles. Secondly, there are pale fibers, with less sar- 

 coplasm and better defined transverse striations, having a greater diameter. These 

 are the more highly differentiated fibers. Although in some animals these two sorts 

 of fibers are found in separate muscles, in others, as in man, they are mingled in 

 single muscles. In general the most constantly active muscles (cardiac, ocular, 

 masticatory and respiratory) contain the most fibers with abundant sarcoplasm. 

 The muscles having many fibers with scanty sarcoplasm contract more quickly but 

 are exhausted sooner. 



