Sli'uclniv (if Siii.M.lli Muscle. 541 



soglnciit. 1'lie coutrncl iiiii nodes, liotli in living and in fixed material, 

 show as con.si)ien(»ns tliiekenings of the fiber. In arterial muscle 

 tlicv are l<»ng, so that a single one may include most of the length 

 of llie muscle fiber. in the muscle of the digestive tract they are 

 usually nai'vowcr and a number may be present in each fiber. The 

 nodes stain more distinctly with eosin and iron-hsematoxylin than 

 do the internodal segments. With Mallory's anilin-blue connective- 

 tissue stain the nodes arc colored orange, the internodal segments 

 i-ed. \\\t\\ ordinary difi"er(Mitiation the nodes appear homogeneous, 

 the internodal segments are distinctly fibrillated. Although the con- 

 traction i.odes of smooth muscle have always heretofore been de- 

 scribed as homogeneous, when the material is carefully stained in 

 iron-ha-matoxylin the myofibrilla' nuiy be traced through the nodes 

 and be shown to l)e continuous with the myofibrillse of the inter- 

 nodal segments. As the myofibrilhie enter the nodes they show a 

 distinct increase in caliber. Where the contraction nodes are nar- 

 row and close togetlier the myotibrilla? appear segmented. That the 

 contraction nodes rei)resent contracted portions of the muscle fiber 

 is shown by the following: In both fresh and fixed material the 

 smootli muscle fibers are thicker at the contraction node than else- 

 where. Tlie myofibrillae run straight through the nodes and as 

 mentioned above apparently thicker as they pass. In the nodes the 

 nuclei are shorter and thicker than in the internodal segments. In 

 the region of the contraction node the collagenous fibers are condensed 

 and tlie elastic fil)ers take a wavy course. 



."). During Contraction the nuclei of smooth muscle undergo an 

 acti\'e shortening and thickening. This change in shape can be seen 

 in living muscle. Ty]ueally, with the possible exception of some 

 arterial nuch i, and the nuclei of passively shortcut d nniscle fibers, 

 there is no folding or spiral winding of muscle nuclei during con- 

 traction. 



G. In general, as the nucleus contracts there is a rearrangement 

 of the chromatin. The fine strands collect into coarser fibers, which 

 arrange themselves in loops or festoons at the two ends of the 

 nucleus. During the process there is an ap])arent streaming of the 

 ehromatin toward the poles. 



