Structure of Smooth Muscle. 507 



For differentiating the connective tissue in which the smooth muscle 

 cells are embedded, Mallory's anilin-blue connective tissue stain, Van 

 Gieson's picrofuchsin stain, Weigert's elastic tissue stain and various 

 iron-hsematoxylin mixtures gave good results. Mallory's stain, at the 

 same time, was found to be a good differential for muscle. With this 

 stain the myofibrillse of relaxed muscle stain intensely red. In con- 

 tracted muscle the entire contraction node stains yellow. The nuclei 

 are yellow, all collagenous fibrils blue. Van Gieson's mixture stains 

 contraction nodes intensely yellow, collagenous fibrils bright red. 

 Van Gieson's stain at the same time is an excellent nuclear stain. 

 Elastic fibers were brought out best by Weigert's resorcin stain, 

 though iron-hsematoxylin often showed them distinctly. 



IV. Steucture of Resting Smooth Muscle. 



1. General structure of smooth muscle. 



JSTo attempt will be made here to go into the details of the structure 

 of resting smooth muscle. In this paper the subject will be considered 

 chiefly from the standpoint of the histogenesis of the tissue, which was 

 not considered by Heidenhain. Hence before discussing the histol- 

 ogy of the adult tissue it has seemed advisable to give a short resume 

 of the process of development. For a review of the literature on this 

 subject, together with a detailed description of the histogenesis of 

 smooth muscle, the reader is referred to a paper by the author, 

 McGill (1), 1907. 



In the digestive and respiratory tracts of the pig, smooth muscle 

 arises in common with the interstitial connective tissue, from the 

 mesenchymal syncytium surrounding the endodermal tube. The 

 differentiation of smooth muscle begins in the mid-oesophagus of the 

 5 mm. pig embryo. A condensation of the mesenchyme, with the 

 elongation of the mesenchymal cells initiates the process. As the 

 nuclei elongate, the myofibrillse appear in the surrounding protoplasm, 

 They arise as coarse, varicose, deeply staining fibrils, which grow 

 rapidly and soon run for long distances through the syncytium with- 

 out regard to cell boundaries. In later development these coarse 

 myofibrillse break up, in large part, to form finer myofibrillse, but 

 some may persist as the coarse myofibrillse found in some adult 

 smooth muscle. 



