2O THE ELEMENTARY TISSUES 



phragm, and generative organs. This group of muscular tis- 

 sue is composed of bundles of fibers, each fiber of which is 

 derived from a single cell which has many nuclei. Each 

 fiber is enclosed in a thin, homogeneous, elastic membrane, 

 the sarcolemma. The fibers are composed of a semi-fluid 

 and viscous material which is called the muscle plasma. The 

 muscle plasma consists of two elements, the fibrils and the 

 sarcoplasm. The fibrils which are long and thread-like, run- 

 ning the entire length of the .fiber, consist of alternating light 



FIG. 17. Striated muscular tissue of the heart, 



Showing the trelliswork formed by the short branching cells, with central 

 ^ nuclei. (Yeo.) 



and dark segments which fall together in the different fibrils 

 and give the muscle its characteristic striated appearance. 

 The sarcoplasm, which varies greatly in the striated muscle 

 of different animals, fills in the space between the fibrils. 

 From a study of comparative physiology it is assumed that 

 the fibrils are the contractile element of the muscle fiber, 

 while the sarcoplasm serves a general nutritive function. 



Striated muscular tissue is very richly supplied with .blood- 

 vessels. The larger arteries and their accompanying veins 



