HUMAN BIOLOGY 



FIG. 40. MUSCLE BUNDLES bound to- 

 gether by connective tissue sheaths. 



lean flesh is voluntary 

 muscle. If a piece of beef 

 is thoroughly boiled, it 

 may be easily separated 

 into bundles the size of 

 large cords. These bun- 

 dles may, by the use of 

 needles, be picked apart 

 and separated into thread- 

 like fibers (Fig. 40). 

 Microscopic Structure of Muscles. These threadlike 

 fibers may, under a magnifying glass, 

 be separated into fine strands called 

 fthrils. These last are the true muscle 

 cells; they are shown by the micro- 

 scope to be crossed by many dark lines 

 (Fig. 48). Hence voluntary muscles are 

 called striated_jir_strt^ed muscles. Pro- 

 longed boiling and patient picking with 

 a needle are needed to dissect muscle, 

 because the bundles are held together 

 by thin, glistening sheets of connective 

 tissue by which they are surrounded. 

 This connective tissue surrounds and 

 holds in place the separate fibers of each bundle (Fig. 40). 



The fibrils of invol- 

 untary muscles are 

 sp indie - shaped (see 

 Fig. 42). There are no 

 cross lines on the fibrils ; 

 hence involuntary mus- 

 cles are called smooth 



n 



FIG. 41. Two MUS- 

 CLE FIBERS OF 

 HEART. 



FIG. 42. INVOLUNTARY MUSCLE CELLS 

 (or fibers) . 



or unstriped muscles. 



