Structure of Smooth Muscle. 519 



As the large intestine of dog contracts, aside from an increase in 

 thickness of both muscular coats and a decrease in the lumen of the 

 tube, there is an accompanying decrease in the length of the contract- 

 ing segment. The decrease in lumen is due to contraction of cir- 

 cular muscle, the decrease in length to the contraction of the longi- 

 tudinal muscle. 



In the small intestine of ISTecturus, dog and cat the peristalses are 

 very irregular. The waves are short and follow each other rapidly. 

 Seldom does a wave extend entirely around the circumference of the 

 intestine. Thus in a single cross-section of the small intestine all 

 gradations from completely contracted to relaxed muscle may be 

 found. Here, as in the large intestine of dog, a very constant ratio 

 exists between the thickness of the longitudinal and that of the cir- 

 cular muscle coat. In a preliminary paper (McGill (3), 1907), Fig. 

 2 is a cross section from a contracted area, Tig. 1 from an uncon- 

 tracted area of the same section of small intestine of ISTecturus. 

 Because of the incomplete contraction several measurements could be 

 made on a single cross section. The following measurements are 

 taken from cross sections of the small intestine of ISTecturus: 



Section 1. 



a, uncontracted area 



circular coat, 0.262 mm. 

 longitudinal coat, 0.125 mm. 



b, partially contracted area 



circular coat, 0.2 mm. 

 longitudinal coat, 0.1 mm. 



c, uncontracted area 



circular coat, 0.162 mm. 

 longitudinal coat, 0.081 mm. 



The ratio of thickness of the longitudinal to the circular coat in all 

 three measurements is ai)proximately as 1 to 2. A number of measure- 

 ments were taken from diiferent sections, all with like results. 



