Structure of Smooth Muscle. 539 



thickest in the neighborhood of the nucleus and tapers toward the 

 poles. When such isolated fibers appear in section, absence of end 

 anastomoses may be due to the fact that the section is cut slightly 

 obliquely. In macerated material the anastomoses are usually 

 destroyed, and there results the spindle-shaped smooth muscle cell 

 described in the text-books. 



6. In adult smooth muscle, just as in development, two types of 

 myofibrillss occur ; very fine fibrillse corresponding to the elementary 

 fibrillae of Apathy and to the "Binnenfibrillen" of Heidenhain ; 

 coarse fibrillse similar to the primitive fibrilla3 of Apathy which in 

 some respects resemble the ''Grenzfibrillen" of Heidenhain, in others 

 more nearly the coarse myofibrillge of Benda. Some muscle fibers 

 have only fine myofibrillse, others only coarse myofibrillse, while still 

 others have both types. When coarse myofibrillse are present they 

 may be arranged as a peripheral layer similar to the "Grenzfibril- 

 len" of Heidenhain or they may be scattered throughout the sarco- 

 plasm, as are the coarse myofibrillffi of Benda. Each myofibrilla 

 throughout its length is fairly uniform in caliber. The myofibrilla 

 run continuously from cell to cell through the anastomoses. 



7. The nucleus of resting smooth muscle is a much elongated 

 spindle-shaped structure ranging in length in the material studied 

 from twenty micra in the digestive tract of the chicken to eighty 

 micra in Necturus. An eccentric position of the nucleus was fre- 

 quently observed. The nuclear membrane is of even contour. The 

 chromatin is arranged in fine granular reticulum supported by a 

 fine linin network. From one to five plasmasomes are present. 



8. The connective tissue of resting smooth muscle appears like 

 ordinary areolar tissue, in the meshes of which are embedded the 

 muscle fibers. The branched connective-tissue cells anastomose with 

 each other and occasionally with the muscle cells. The collagenous 

 fibers are arranged as a loose reticulum, as a heavy reticulum or 

 as distinct membranes. The elastic fibers form a loose network. 

 The fibers which run parallel with the muscle fibers are com]iara- 

 tively straight. 



B. The structure of contracted muscle. 



1. During the contraction of the digestive tract both layers of 



