20 THE ELEMENTARY TISSUES 



few of the muscles connected with the middle ear, tongue, 

 pharynx, larynx, diaphragm, and generative organs. This 

 group of muscular tissue 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 



FIG. 17. Striated muscular tissue of the heart, 



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



(Y**,) 



and the sarcoplasm. The fibrils which are long and thread- 

 like, running the entire length of the fiber, consist of alternating 

 light and dark segments which fall together in the different 

 fibrils and give the muscle its characteristic striated appear- 

 ance. 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. 



