STRIATED MUSCLE. 



8 9 



FIG. 106. STRIATED MUSCLE 

 FIBER OP FROG, TEASED 

 APART IN WATER, BEING 

 TORN AT X, AND SHOWING 

 THE SARCOLEMMA AT s 

 and s 1 . 



in -fresh fibers within which the fibrils have been ruptured and have drawn 

 away from the membrane (Fig. 106). It resists acetic acid and has been 

 considered elastic. These fibers arise from myo- 

 blasts which at first have single nuclei within 

 their central portions. As the cells elongate their 

 nuclei divide rapidly, at first by mitosis and later, 

 it is said, by amitosis. It is generally denied that 

 the adult fibers are due to a fusion of myoblasts. 

 The first fibrils are homogeneous structures at the 

 periphery of the cells. It has been observed that 

 the activity of certain muscles in living embryos 

 begins at the time that their fibrils appear. As 

 the fibrils multiply and fill the cell the nuclei 

 migrate toward the sarcolemma. The striations 

 which have been described under cardiac muscle, 

 are most perfectly developed in the voluntary mus- 

 cles. All that can ordinarily be seen of them, 

 however, is shown in Fig. 107, namely, the alter- 

 nating dark and light bands, the latter bisected 

 by the ground membrane. Sometimes, though 

 rarely, as a result of treatment with alcohol the 

 muscle fiber breaks into transverse discs, called 

 sarcous elements, each having the thickness of a 

 muscle segment. These elements are single layers 

 of cuboidal blocks, one for every longitudinal fibril, 

 and these blocks may separate from one another. 

 Neither the elements nor their small pieces are 

 now considered significant. 



The extremities of the muscle fibers are round- 

 ed or conical, the end toward the tendon being 

 more obtuse than the other. Near the tendon 

 the fiber contains many nuclei both peripheral 

 and deeply placed. They divide by amitosis and 

 provide for lengthwise growth of the fiber. Con- 

 nection with the tendon is established by the peri- 

 mysium which is continuous with the tissue of the 

 tendon. The sarcolemma ends with the muscle 

 substance. Such striated muscle fibers as are in- 

 serted in the skin or mucous membranes may be 

 pointed or branched (Fig. 108) . Their perimysium 

 is prolonged in the form of elastic fibers which 

 blend with the surrounding connective tissue. 



FIG. 107. PART OF A LONGI- 

 TUDINAL VIEW OF A HU- 

 MAN STRIATED MUSCLE 

 FIBER. 



a., Anisotropic; i., isotropic 

 band; k., nucleus; q., 

 ground membrane (Z) . 

 X 560. 



FIG. 108. BRANCHED STRI- 

 ATED MUSCLE FIBER PROM 

 THE TONGUB OP A FROG. 



