444 



MANUAL OF PHYSIOLOGY. 



the transverse striation of the contractile substance appears reg- 

 ular, and is easily recognized. 



Each fibre consists of a delicate case of thin, elastic, homo- 

 geneous membrane, forming a sheath 

 called sarcolemma, within which the 

 essential contractile substance is in- 

 closed. The soft contractile substance 

 completely fills and distends the elastic 

 sarcolemma, so that when the latter 

 is broken its contents bulge out or 

 escape. After death, particularly if pre- 

 served in weak acid (HC1), the striation 

 becomes more marked, and the dead and 

 now rigid contractile substance can be 

 easily broken up into transverse plates 

 or disks. 



Besides the transverse striation a longi- 

 tudinal marking can be seen in the mus- 

 cle fibre which indicates the subdivision 

 of the contractile substance into thin 

 threads called primitive fibrillse. Each 

 primitive fibril shows a transverse mark- 

 ing, corresponding with the transverse 

 striation, which divides the fibrils into 

 short blocks called sarcous, or muscle ele- 

 ments. These markings, as well as the 

 transverse striations of the muscle fibre 

 in general, depend on different parts of 

 the contractile substance having different 

 powers of refraction, which give the ap- 

 pearance of dark and light bands. 



In the muscle fibre are found long 



TWO fibres ot striated mus- granular masses like protoplasm; these 

 are the nuclei of the contractile sub- 

 stance. They must not be confounded 

 with the nuclei of the sarcolemma, which 

 are much more numerous along the edge 



cle, in which the contractile 

 substance (m) has been rup- 

 tured and separated from the 

 sarcoleruma (a) and (*); (p) 

 space under sarcolemma. 

 (Ranvier.) 



