Rinieturc of Smooth Muscle. 521 



and thickening. The term contraction wave will be restricted to the 

 deeply staining so-called homogeneous bands of firmly contracted 

 muscle which pass irregularly across the contraction areas, Figs. 1, 2, 

 11, 21, 30, 87. Contraction node refers to the deeply staining 

 thickened area in the individual muscle fiber, and internodal segment 

 to the fibrillated, uncontracted or weakly contracted portion of the 

 fiber between the contraction nodes. 



a. The form of the contraction wave. 



As the muscle of the digestive tract contracts, contraction waves 

 appear. They are irregular in outline and branch and anastomose. 

 Fig. 1, 80, 87. In Fig. 1 they are shown in a cross section through 

 the contracted large intestine of dog. They are seen in the circular 

 coats of the muscularis as irregular bands extending in places across 

 the entire thickness of the muscle coat. The section of course shows 

 them cmly in two dimensions. In serial sections their extent in the 

 third dimension was determined. In this direction up and down 

 the intestine they extend as irregular bands of about the same width 

 as shown in the cross-section of the iutestino. In the longitudinal 

 coat the waves- are similar in form to those found in the circular coat. 

 In fact, usually when a contraction Avave in the circular coat reaches 

 the outer margin, there is opposite it in the longitudinal coat a 

 similar wave. Fig. 1. When the contraction waves are cut in cross 

 section of the muscle they appear as irregular polygonal areas studded 

 thickly with the cut ends of contracted muscle fibers, Fig. 1, 1, m. The 

 contraction waves may extend quite obliquely across the area of 

 contraction. Fig. 1 c. In places the contraction waves are of even 

 contour, in other places of very diffuse and irregular outline. Xot 

 all contraction waves pass across the entire thickness of the mus- 

 cularis. They may involve only a few fibers or when just beginning 

 to form only a portion of a single filx-r. 



In the large intestine of dog and in the small intestine of l^ecturus, 

 in tissue firmly contracted the contraction waves are quite broad, 

 involving from one-fourth to one-half of the length of the muscle 

 fiber, Figs. 21, 28, 80. In muscle not so finnly contracted they 

 may be extremely narrow, Figs. 85, 89, 40. There are from one to 

 three of these waves in each contracted fiber. 



