520 Caroline McGill. 



As the small intestine of ]^eeturus contracts the lumen decreases 

 in diameter, due to the contraction of the circular muscle. The 

 simultaneous contraction of the longitudinal muscle has, however, 

 little effect upon the length of the intestine. A number of measure- 

 ments were taken upon segments of intestine, both before and after 

 contraction. In every instance the length remained nearly constant, 

 tliough sections showed in the contracted segments distinct contraction 

 waves in the longitudinal fibers. Though the longitudinal fibers were 

 distinctly shortened at the contraction waves, they were unduly 

 stretched between, which possibly accounts for no decrease in length. 

 The work was done on animals which had been kept in captivity for 

 some time and had been fed little, so contraction may not have been 

 entirely normal. 



2. In arteries. 



In arteries during contraction there is a marked thickening of the 

 media with accompanying decrease in the size of the lumen. When 

 the fresh carotid of the ox is severed the proximal end contracts and 

 the distal end relaxes. As tlie muscle contracts the Avail of the 

 vessel thickens and the lumen decreases in diameter, as it relaxes the 

 reverse changes take place. Since there is little longitudinal muscle 

 there is little change in the length of the vessel. 



VI. Forms of Coktraction i:s^ Smooth Muscle. 



In smooth muscle two main types of contraction have been de- 

 scribed: 1. Peristaltic contraction, where one or more contraction 

 nodes appear in the fil)er, with uncontracted areas between; 2. Total 

 contraction where the entire fiber shortens and thickens. In this 

 study, muscle wdth contraction of type 1 was easily demonstrated. 

 Afuscle of type 2 was seldom found, though in blood-vessels this type 

 is in places nearly approached. 



1. Peristaltic contraction. 



Before discussing the changes which take place in smooth muscle 

 during the peristaltic type of contraction, a few of the terms used will 

 be defined. By contraction area is meant the entire mass of muscle, 

 all or a part of the fibers of which have undergone active shortening 



