Striictnre of Smooth Muscle. 529 



VII. The Behavior of the Myofibrillae During Contraction. 



Although the myofibrillae have long been considered the contrac- 

 tile elements of smooth muscle, previous investigators have not 

 demonstrated that they are such. The myofibrillae can be easily 

 seen, even in fresh material, in uncontracted muscle and in the inter- 

 nodal segments of contracted muscle. Fig. 2, 5-36. The contraction 

 nodes, even by recent workers on smooth muscle, have been described 

 as entirely homogeneous, Henneberg, 1901 ; Heiderich, 1902 ; 

 Soli, 1906. The author in a preliminary paper, McGill (3), 1907, 

 showed that in material properly differentiated the myofibrillae may 

 be traced through the so-called homogeneous nodes, and that they 

 apparently thicken as they pass through. 



In material fixed and stained in the ordinary manner these nodes 

 do appear perfectly homogeneous, Figs. 2, 3, 5, 11, 20, 22, 31. 

 However, in muscle fixed in Zenker's fluid, then over-stained in 

 iron-haematoxylin and the haematoxylin carefully extracted, it is pos- 

 sible in many instances in the contraction nodes to show a distinct 

 fibrillation, en in Figs. 6, 18, 33, 34, 35, 36, 39, 40. When exam- 

 ined under high magnification, the individual fibrillae may some- 

 times be traced continuously through one contraction node and inter- 

 nodal segment into the next contraction node, Fig. 40. 



The myofibrillae when they can be traced through the contraction 

 nodes run just as straight a course as in the uncontracted muscle 

 or in the internodal segments of contracted muscle, Figs. 6, 33, 34, 

 39, 40. If they were not contractile elements one would expect 

 them to be folded and wavy as are the elastic fibers when the muscle 

 fiber shortens. 



When uncontracted muscle fibers are caught between contracted 

 fibers the myofibrillae, as well as the whole uncontracted fiber, may 

 be thrown into zigzag waves due to passive shortening. Likewise 

 when small pieces of muscle are removed and allowed to contract 

 in the absence of tension, the fiber and consequently the myofibrillae 

 even in contracted muscle may take a wavy course. But in muscle 

 contracted with normal amount of tension, then fixed and sectioned, 

 the myofibrillae in both the contracted and uncontracted fibers run 

 comparatively straight. The zigzag form of contraction so frequently 



