494 Caroline McGill. 



VII. Tlie Behavior of MyofibrilljB During Contraction 529 



1. The continuity of the myofibrillje through the contraction 



nodes 529 



2. The increase in the thickness of the uiyotibrillsE during con- 



traction 530 



VIII. The Behavior of the Nuclei During Contraction 531 



1. The form of the contracted nucleus 531 



2. The behavior of the chromatin during nuclear contraction . . . 533 



3. The effect of contraction on the volume of the nuclei 534 



4. The effect of fatigue on the nuclei 534 



5. The effect of certain drugs on nuclear contraction 535 



IX. Chemical Changes in the Smooth iNIuscle Fiber Inuring Contraction 535 



X. Development of (Contractility ._ 536 



XI. Summary _ 538 



XII. Literature List 542 



XIII. Explanation nf Figun's _ 545 



I. Introduction, 



In a preliminary paper, McGill, (3) 1907, were described briefly 

 the finer structural changes which take place during the contraction 

 of the smooth muscle in the intestine of N^ccturus and some of the 

 mammals. In the present paper a more detailed account of contrac- 

 tion, not only for the intestinal muscle, but for smooth muscle in gen- 

 eral, is given. The structure of resting smooth muscle is described, in 

 the main, from the standpoint of its syncytial arrangement. The 

 general histology of resting smooth muscle is discussed at length by 

 Heidenhain, 1900, and will be considered only briefly in this paper. 



The structure of contracted smooth muscle was studied in both 

 living and fijced material. The general form of the contraction area 

 is here described, together with a more detailed account of the behavior 

 of the myofibrillse and of the nuclei. The myofibrillse have long been 

 considered the contractile elements of smooth muscle. There has, 

 however, been little proof that they are such. It has therefore seemed 

 worth while to study them carefully in an effort to obtain some evi- 

 dence along this line. Furthermore, the form of the contracted 

 smooth muscle nucleus has recently been much disputed. In this 

 paper an attempt is made to throw some light on this subject also. 



This paper is confined to a consideration of vertebrate muscle. 

 Enough work has been done on invertebrate muscle to show that it is 



