The Histogenesis of Smooth Muscle in the Alimentary Canal etc. 231 



longitudinal which is comparatively thin, a middle circular which is 

 throughout extremely thick, and an inner, the muscularis mucosae, 

 which varies in amount and arrangement in different regions. In the 

 oesophagus it consists ot irregular, longitudinal bundles scattered 

 throughout the subepithelial connective tissue, in some places approach- 

 ing in amount the muscle of the circular layer, in other places being 

 represented merely by scattered fibers. In the stomach and intestine, 

 the muscularis mucosae consists of two very thin layers, an outer 

 longitudinal and an inner circular. 



The muscle of the alimentary canal, with the exception of a few 

 cross-striated fibers in the outer longitudinal coat of the oesophagus, 

 is made up of smooth muscle from the oesophagus, at the region of 

 the bifurcation of the trachea, to the rectum. Throughout the extent 

 there is great variation in the size of the muscle fibers and in the 

 amount of interstitial connective tissue. The largest muscle cells are 

 found in the muscularis mucosae of the oesophagus. 



When viewed in longitudinal section it is easily demonstrated that 

 in adult smooth muscle, as in development, there is a persistent pro- 

 toplasmic syncytium, the whole mass of smooth muscle cells being 

 united together by larger or smaller protoplasmic strands so that indi- 

 vidual, independent muscle cells do not exist (Figs. 21 to 24). The myofi- 

 brillae extend throughout the syncytium, without regard to cell territories. 



The idea that smooth muscle is a syncytium is by no means new. 

 Most of the writers who have so described it, however, have found 

 the cells to be united only by very delicate protoplasmic strands, the 

 so-called intercellular bridges. Among these investigators may be 

 mentioned Leydig (1849), Wagener (1869), Flemming (1878), Kul- 

 tschitzky (1888), Barfurth (1891), Heidenhain (1893), Werner (1894), 

 Bohemann (1895), De Bruyne (1895), Trieple (1897), Hertwig (1898) 

 and Eohde (1905). In recent years, the tendency has been to regard 

 smooth muscle cells, except by possible anastomoses at the ends, as 

 entirely separate and distinct elements, bound together merely by con- 

 nective tissue. Henneberg (1900), Heidenhain, M. (1900) and Benda 

 (1902), hold to this view. Eecently, however, Rolide (1905), in a com- 

 parative study of the smooth muscle in both vertebrates and inverte- 



