46 FRANKLIN PARADISE JOHNSON 



I 



linea ano-cutanea, but it is not well marked. It has been defined 

 as the place where the first hair follicles appear. The skin in the 

 immediate neighborhood of the anus is known as the zona cuta- 

 nea. Sweat glands, the circum-anal glands of Gay, are found 

 only in this zone. 



The muscularis mucosae in the first part of the rectum is similar 

 to that of the colon. It is said to extend downward in the form 

 of muscle columns into the rectal columns. In the embryos I 

 have examined nothing in the way of distinct muscle columns 

 were to be found. The muscularis mucosae extends into the zona 

 columnaris as a thin sheet of fibers lying quite close to the epithe- 

 lium. The fibers forming this sheet soon disappear, however. 

 In two stages, 240 mm. and at birth, small bundles of voluntary 

 striated muscle fibers are present in the larger rectal columns. 

 These are composed of a number of fibers which arise from the 

 external sphincter muscle turned obliquely toward the submucosa 

 and then upward into the columns. A few fibers of the internal 

 sphincter join some of these columns in the 240 mm. embryo. 



The submucosa, while at first composed of mesenchyma, later 

 becomes fibrous. It contains numerous nerve fibers and blood 

 ves.sels, and Pacinian corpuscles as described by Pilliet ('92). 



The muscularis of the rectum is the downward continuation of 

 that of the colon. Both layers form, however, continuous sheaths 

 around the rectum. The circular was first seen in an embryo of 

 16 mm., while the longitudinal appeared at 17 mm. The circu- 

 lar layer presents, in the region of the pars analis recti, a thick- 

 ening, the sphincter ani internus. This does not become appar- 

 ent until relatively late embryonic life (99 mm. embryo). The 

 longitudinal muscle coat terminates below, after the manner 

 described by Beraud ('58) and Roux ('81), by a spreading out of 

 its fibers. These end in small connective tissue fibers, which, 

 continuing downward, spread out fan-shaped, and as septae, 

 di\'ide up the muscle bundles of the external sphincter. 



The external sphincter appears quite early. It was first defi- 

 nitely made out in an embryo of 22.8 mm. According to Otis 

 ('05) the external sphincter arises from the mesenchj^ma within 



