428 



are exceedingly fine, but in some fibers coarser rayofibrillae, which 

 appear to be formed by a fusion of the finer, are seen. They are not 

 confined to the periphery of the bundle as are the coarse myofibrillae 

 („Grenzfibrillen") described by Heidenhain ('00). 



In cross section (Fig. 1 a) the smooth muscle fibers appear as 

 rounded or oval areas. In the large intestine of the dog, the nucleus 

 is usually at or near the center of the fiber. In Necturus this is not 

 always the case. Very frequently in this form, the nucleus lies to 

 one side of the bundle, almost independent of the fiber (Figs. 1 and 2). 

 Neither Fig. 1 nor Fig. 2 is magnified sufficiently to show the nuclear 

 relations clearly, however. 



The nuclei in uncontracted smooth muscle are very much elongated, 

 and show a distinct chromatin reticulum. (Figs. 1 and 5). In none of 

 the forms studied was the chromatin found arranged in a spiral as 



Fig. 1. Cross-section of a portion of the intestinal wall of Necturus, entire thick- 

 ness of the serous and muscle coats, the muscle in the resting condition, a longitudinal 

 muscle; b circular muscle; c blood vessel; d mesothelium; e connective tissue; /cross- 

 section of a smooth muscle fiber. Fixation , Zenker's fluid ; stain, iron - ha»matoxylin. 

 Leitz obj. 5, oc. 2, drawing reduced ^/g, magnification, 188 diameters. 



described by Mltnch ('03). Sometimes at the ends of the nuclei a 

 small amount of granular protoplasm can be demonstrated (Fig. 3). 



Between the anastomosing bundles of myofibrillse is a loose reti- 

 culum of connective tissue. The connective tissue fibrfllie are chiefly 

 intercellular, though in places they pass through the protoplasm of 

 the connective tissue cells, and even occasionally make their way for 

 a short distance in among the myofibrilla3 of the smooth muscle fibers. 

 As will be shown in my paper on the histogenesis of smooth muscle 

 in the pig, this relation is easily understood, since myofibrillse and 

 connective tissue fibrillae arise in a common syncytium. 



Before discussing the changes which take place in smooth muscle 

 during contraction, a few of the terms used will be defined. By con- 



