SMOOTH MUSCLE 



117 



OF SMOOTH MUS- 

 CLE FIBERS FROM 

 THE ARTERY OKA 

 DOG. 



probable, however, that the spiral nuclei occur in relaxed fibers, which 

 have been crumpled together by the contraction of adjacent fibers. 

 Along one side of the nucleus the centrosome may be found, occupying 

 a shallow indentation of the nuclear membrane. 



At the poles of the nuclei there is often an accumulation of granular 

 protoplasm (Fig. 104, p. 115) which is sometimes pigmented. The fibrils 

 diverge to pass by the nucleus, and the granular protoplasm occupies the 

 conical non-fibrillated space which is thus produced. 



The surface of the smooth muscle fibers is covered by ^^^^^^^^ 

 a delicate membrane, which is sometimes thrown into 

 transverse wrinkles t>y the contraction of the fiber. 

 Possibly the fibrils terminate in it. They do not appear t 



J FIG. 106. NUCLEI 



to become more compact as they extend into the tapering 

 ends of the fibers and presumably they do not all extend 

 the whole length of the cell. 



In transverse sections the fibers present rounded or polygonal outlines 

 (Fig. 107). They vary in size, since some are sectioned through the 

 tapering extremity and others through the thick central part which 

 contains the nucleus. In the figure the substance between the fibers ap- 

 pears solid, and it has sometimes been described as cement, or as a mem- 

 brane rather than as a reticulum. 



The relation of the myoglia, reticulum and muscle fibers to the process 

 of contraction has never been adequately explained. In the intestine, with 

 the normal accumulation of food, the diameter of the tube becomes four 



^ times as great as in the contracted state, 



and the muscle layer becomes reduced 

 to somewhat less than one-fourth of its 

 original thickness. The muscle cells 

 appear to slip by one another and to 

 form a layer only a few fibers thick. 

 After a certain amount of distention 

 the tube will expand no further, and 

 added pressure causes it to rupture. 

 Presumably the elastic and white fibers 

 aid in restoring the normal caliber. 



With extreme contraction, however, the white and elastic fibers no longer 

 aid the muscles, but become crumpled into coarse folds, as seen fre- 

 quently in contracted arteries. As to the muscle fibers themselves, 

 Meigs concludes that during contraction fluid passes from them into the 

 intercellular spaces, so that the fibers shrink in size and become darker; he 

 states that they decrease greatly in length but remain of about the 

 same diameter, while the spaces between the fibers become larger 

 (Amer. Journ. Physiol., 1908, vol. 22, pp. 477-499). According to Miss 







FIG. 107. CROSS SECTION OF SMOOTH MUSCLE 

 FIBERS FROM THE STOMACH. X 560. 



a, Connective tissue septum; b, section of a 

 fiber distal to the nucleus; c, section of a 

 fiber through the nucleus. 



