SMOOTH MUSCLE 



83 



alkaline solutions it is possible to dissolve the substance cementing 

 adjacent cells and shake them apart, so that isolated smooth muscle 

 cells may be observed. 



Smooth muscle cells are variously organized. In the connective 

 tissue of the villi or folds of the intestine a few isolated smooth 

 muscle cells occur; associated with hairs are small bundles which 

 form the arrector pili muscles; in more complex organizations, the 

 cells are arranged in bundles or sheets. When organized into groups 

 or muscle coats the ta])ering ])ortions of the cells overlap and a 

 network of elastic and reticular fibers extends between adjacent 

 cells from the surrounding fibroelastic connective tissue. When 

 such sheets or bands of smooth muscle are cut in longitudinal 

 section (Fig. 44), the characteristic spindle shape of the cells is 



_ »L -Inner circular coat 



Fig. 44. — Diagram of cross-section and longitudinal section of smooth muscle. 



evident, but in cross-sections roughly circular sections of different 

 sizes appear adjacent to each other and nuclei appear only in those 

 sections through the central region of the cells. This is what one 

 would expect from o\'erlapping of the tapering ends. 



In the wall of the alimentary tract from the lower esophagus to 

 the anus or cloacal o])ening there are usually two distinct sheets of 

 smooth muscle ; an inner coat of muscle cells encircle the tube, and 

 an outer coat has cells arranged lengthwise. (Fig. 45.) Constric- 

 tions of the inner coat decrease the lumen, and constrictions of 

 the longitudinal coat cause a shortening of the tube at the points 

 affected. During life, waves of contraction pass along these coats 

 simultaneously and cause the peristaltic movements essential in 

 the functioning of the digestive system. 





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