SMOOTH MUSCLE. 



79 



End of a muscle fiber. Nerve cell. 



FIG. 91. APPARENT INTERCELLULAR BRIDGES OF SMOOTH MUSCLE FIBERS. 

 A, Transverse section of the intestine of a rabbit. B, Longitudinal section of the intestine 

 of a guinea pig. X 420. 



alternating light and dark bands, very distinct in photographs. The 

 fibrils of cardiac and striated muscles are always banded in this way. 

 Some investigators consider that the border 

 fibrils are the contractile elements. Others hold 

 that by their elasticity they cause the muscle 

 cells to elongate after contraction, thus being an 

 obstacle to contraction. The elongation of the 

 relaxed muscles, either in the blood vessels or in 

 the intestinal wall, may be accomplished by the 

 pressure of the contents of these organs, or by the 

 elastic connective tissue which is outside of the 

 muscle cells. In the endoplasm of smooth muscle 

 cells, and thus surrounded by the border fibrils, 

 minute inner fibrils have been described and said 

 to be contractile. Among them is the unaltered 

 protoplasm. Where the fibrils diverge to pass 

 around the nucleus, that is, at the ends of the 

 nucleus, the granular protoplasm is most readily 

 distinguishable. In the intestine it has been 

 observed to contain pigment. Surrounding the 

 smooth muscle cells there is probably a delicate 

 cell membrane, but the nature of the structures 

 observed is still under discussion. The cell mem- 

 brane of a muscle cell is called a sarcolemma; its 

 protoplasm is named sarcoplasm; and the entire 

 cell is called a muscle fiber. Fibril is applied to the 

 filaments within the fibers. 



Smooth muscle cells are bound together so 



that they may act in unison. They may be joined end to end by the border 

 fibrils. Protoplasmic bridges have been described between them (Fig. 91). 



FIG. 92. FIBROUS TISSUE IN 

 RELATION WITH SMOOTH 

 MUSCLE FIBERS, PROM THE 

 BLADDER OP A PIKE. 

 (After Prenant.) 



c., Connective tissue network; 

 n., p., f., nucleus, granular 

 protoplasm, and fibrillar 

 protoplasm of a muscle cell. 



