TISSUES OF SPECIAL FUNCTION. 



91 



When seen in longitudinal section it is difficult to trace a given 

 muscle-fibre for any considerable distance, because the occasional 

 anastomosing branches of the cells cause a blending of the neigh- 

 boring fibres with each other. In cross-section the cells have a 

 round, oval, or polygonal shape, and vary considerably in size, 

 owing to the branching. Their cut surfaces are dotted with the 

 minute polygonal cross-sections of the elements of the contractile 

 substance, which give the cell its appearance of longitudinal stri- 

 ation. These elements are called the " sarcostyles." 



Cardiac muscle occurs only in the heart. It is not under the 

 control of the will, but differs from the other involuntary muscles 

 in the force and rapidity of its contractions, which resemble those 

 of the voluntary muscles. 



3. Striated Muscular Tissue (Figs. 76-79). The voluntary muscles 

 have for their characteristic tissue-element greatly elongated, multi- 



FIG. 76. 



FIG. 77. 



Striated muscular tissue. 



Fig. 76. Portion of a muscle-fibre from a mammal. (Schafer.) This figure represents the 

 appearances of the fibre when the surface is in sharp focus. 



Fig. 77. Termination of a muscle-fibre in tendon. (Ranvier.) c, contractile substance ; p, 

 retracted end of contractile substance, separated from the sarcolemma during the prep- 

 aration of the specimen ; m, sarcolemma, slightly wrinkled ; s, sarcolemma in contact 

 with fibrous tissue of tendon ; t, tendon. 



