INTRODUCTION 701 



muscles of the common fowl, the white fibers constitute most of the muscle. 

 Also, in the M. quadratus femoris of the cat or the M. semitendinosus of 



the rabbit, the red fiber predominates. In general, the more continuously 

 active muscles contain the greater number of red fibers, while the less con- 

 tinuously active contain pale fibers. Pale fibers react more quickly and thus 

 contract more readily than the red fibers. However, they are exhausted more 

 rapidly. 



Connective tissue, mostly of the white fibrous variety, associates the muscle 

 fibers (cells) into groups called muscles. Muscles, such as the Mm. biceps 

 brachii, biceps femoris, sartorius, rectus abdominis, etc., are a mass of asso- 

 ciated muscle fibers, tied together by connective-tissue fibers. 



The surrounding connective tissue of a particular muscle is known as the 

 external perimysium (fig. 325D). The external perimysium extends central- 

 ward into the muscle and separates it into smaller bundles of fibers, or 

 fasciculi. Thus each fasciculus is a group of muscle fibers, surrounded by 

 the internal perimysium. The perimysium around each fasciculus extends into 

 the fasciculus between the muscle cells, where its fibers become associated 

 with the sarcolemma of each muscle fiber (cell). 



The connection between the muscle fibers and their tendinous attachment 

 has attracted considerable interest. One view holds that the myofibrils pass 

 directly into the tendinous fibers. An alternative and more popular view main- 

 tains, however, that it is the sarcolemma which attaches directly to the ten- 

 dinous fibers. Hence, the pull of the muscle is transmitted through the sar- 

 colemmas of the various muscle cells to the tendon. 



b. Cardiac Muscle 



Cardiac muscle is characterized by the presence of alternating dark and 

 light bands as in skeletal muscle. The striations are not as well developed, 

 however, as in skeletal muscle, nor is the sarcolemma around the muscle fibers 

 as thick. Another distinguishing feature of cardiac muscle is the fact that 

 the fibers anastomose and thus form a syncytium, although M. R. Lewis ('19) 

 questions this interpretation. Still another characteristic structure of cardiac 

 muscle is the presence of the intercalated discs (fig. 325C). These discs are 

 heavy transverse bands which extend across the fiber at variable distances 

 from one another. A final feature which distinguishes cardiac muscle is the 

 central location of the nuclei within the anastomosing fibers. 



c. Smooth Muscle 



Smooth muscle fibers are elongated, spindle-shaped elements which may 

 vary in length from about 0.02 mm. to 0.5 mm. The larger fibers are found 

 in the pregnant uterus. The diameter across the middle of the fiber approxi- 

 mates A \o 1 IX. This middle area contains the single nucleus. The fiber 



