Structure of Smooth Muscle. 533 



segment. Then the ends of the nucleus contract, while the center 

 remains unchanged. This frequently occurs in the intestinal muscle 

 of Wectuiiis, where the uuclci are extremely long, Fig. 28. 



2. The behavior of the chromatin during nuclear contraction. 



In the resting smooth muscle nucleus the chromatin is arranged 

 as a very delicate central reticulum and as a thin layer just beneath 

 the nuclear membrane. Figs. 22, 24, 27, 42, 77, 85. During con- 

 traction the chromatin is massed at the two ends of the nucleus. 

 There is left a space relatively free from chromatin at the middle 

 of the nucleus. Figs. 28, 29, 51, 89. As the chromatin masses at 

 the two ends of the nucleus there is a streaming and rearrangement 

 of the meshes. At the same time it becomes much more deeply 

 staining. 



The massing of the chromatin at the ends of the contracted nuclei 

 is most marked in the intestinal muscle of Necturus and dog. Figs. 

 28, 29, 51, 52. Necturus material is very favorable for study be- 

 cause the nuclei are so extremely large. 



As the nucleus begins to enlarge at the point of contraction the 

 chromatin reticulum breaks up into very fine threads. Fig. 86. These 

 pass, as if by distinct streaming, toward the poles of the nucleus 

 where they arrange themselves in loops or festoons. Figs. 28, 87, 

 88. As the process continues the fine strands of chromatin fuse to 

 form exceedingly coarse threads. These may remain in a loose 

 festoon at the end of the nucleus or else break up there into a coarse 

 reticulum. Figs. 29, 59, 89. At the same time some of the chro- 

 matin collects in a heavy layer just beneath the nuclear membrane. 

 The strands of chromatin in the fully contracted nucleus are much 

 coarser than are those in the resting nucleus, Figs. 27, 28, 29. Figs. 

 98-95, 96-99, show similar chromatic changes in the nuclei of 

 Necturus where the smooth muscle had been stimulated to contrac- 

 tion by pilocarpine. 



The chromatic changes in the nuclei of smooth muscle of dog 

 intestine are quite similar to those observed in ^Tectums, Figs. 42-63. 

 However, in resting nuclei the chromatin strands tend to run as 

 fine longitudinal fibrils, Fig. 42. These are more pronounced in 

 partially contracted nuclei. Figs. 43-46. As the nucleus contracts 



