526 Caroline McGill. 



was found in the muscle of the digestive tract of chicken, Fig. 2, of 

 dog, Fig. 21, and in that of the urino-genital tract of man. Figs. 

 3 and 31 show the marked thickening which may take place in the 

 fiber during contraction. In Fig. 3 the contraction nodes are stained 

 black, in Fig. 31 orange. The mortising of the contraction nodes of 

 neighboring fibers together, as is shown in the contraction wave in 

 Fig. 2, is further proof of the large amount of thickening of the fiber 

 which takes place at this point. The occurrence of thickening of 

 the contraction nodes in fixed material is probably due to the fact 

 that the fixative used (Zenker's fluid) did not produce the shrinkage 

 of the contraction nodes which was obtained by Henneberg and 

 Heiderich in their work. 



When the contraction nodes are seen in cross section many of them 

 are oval or elongated, indicating that the fiber at this point is dis- 

 tinctly flattened. The flattening is probably due to unequal 

 pressure of neighboring nodes. It is seen most frequently where 

 there is little connective tissue separating the nodes. In the circular 

 muscle of both intestine and blood-vessels especially nearest the lumen 

 the contracted fibers show this flattening. In cross section of these 

 fibers the long diameter extends from nearest the lumen outward. 

 Since, when the muscle of these tubes is contracted the lumen is nar- 

 rowed, this direction of flattening is just what one would expect if it 

 be due to pressure. The uncontracted fibers, intemodal segments 

 and even the contraction nodes, M'hen they are separated by much 

 loose connective tissue, are typically round in cross section. 



In the muscle from the contracted areas of the small intestine of 

 N'ecturus the deeply staining contraction nodes are of just about the 

 same thickness as the internodal segments. In this the muscle 

 approaches that described by Henneberg. But even here when the 

 nodes and internodal segments of the contracted areas, Fig. 23, are 

 compared with the fibers in completely relaxed muscle, Fig. 24, it will 

 be seen clearly that lx)th are of much greater caliber than are the 

 relaxed fibers. Furthermore, from the shortening of the nuclei and 

 the condensation of the connective tissue, it will be seen that the 

 fibrillated internodal segments have also undergone some contraction. 

 So even in tectums muscle it seems that at all times the contracted 

 fibers are of greater caliber than are the relaxed fibers. 



