PHYSIOLOGY OF ORGANS 



157 



cleus, but after a timo division of the nucleus is not followed by division 

 of the cell. As a result the cells become very large and they have many 

 nuclei. The fibrils which are not present in the very young cells finally 

 are developed as the result of secretion. In the adult muscle (Fig. 117) 

 one may recognize that the separate cells or fibers are surrounded each 

 by a dense tough sheath, the sarcolemma. Within the sarcolemma are a 

 large number of fine fibrilliB scattered through the cytoplasm. The 

 cytoplasm of muscle cells is usually called sarcoplasm. The fibrillae, 

 also called sarcoshjles, are composed of bands of material which present 

 the appearance of alternate dark and light cross stripes. The structure 

 of the sarcostyles may best be seen in cells which have been gently torn to 



A B c 



Fig. 117. — General appearance of striated muscle. A, part of a muscle fiber of a frog; 

 B, part of a fiber teased out to show fibrillae; dh, dark bands; Ih, light bands; /, fibrilla; 

 n, nucleus; s, sarcolemma; C, a sarcostyle (fibrilla), diagrammatic; dh, dark band; Ih. light 

 band with a thin band of dark material dividing it into two portions. (.-1 and B from 

 Parker and Haswell's Textbook of Zoology.) 



pieces with fine needles. Figure 117, C, illustrates the appearance of 

 a sarcostyle. That the dark and light bands indicate different sub- 

 stances has been determined by the use of the polariscope. The signifi- 

 cance of these bands is indicated below in the statement of a theory of 

 muscular contraction. 



Theory of Muscular Contraction. — There is good reason to believe 

 that the sarcostyles are composed of non-living material which is elabo- 

 rated and kept in working condition by the sarcoplasm which fills the 

 spaces between the sarcostyles. The latter are machines for contraction. 

 Under various conditions of contraction the appearance of the dark and 

 hght bands of the sarcostyles changes. The dark bands apparently elon- 



