44 FRANKLIN PARADISE JOHNSON 



columnar cells. This epithelium is similar to that found in other 

 parts of the digestive tube in its early stages. Differentiation 

 was first noted in an embryo of 55 mm. But few changes take 

 place from this time until birth. The following description, 

 although taken largely from an embryo of 240 mm., will hold good 

 for any of the later stages. 



The pars analis recti is the region in which takes place the tran- 

 sition between the mucosa of the digestive tube and the skin. 

 This transition does not take place gradually but in three sepa- 

 rate steps, thus forming three distinct superimposed zones, the 

 zona columnaris, the zona intermedia, and the zona cutanea. 

 The last of these zones, however, does not belong to the pars 

 analis, properly speaking, but to the outer skin. 



The zona columnaris is the region of the rectal columns, but 

 these, however, are not always confined to this zone alone. They 

 ma}^ extend up into the first part of the rectum for a short distance, 

 and they may also be continuous with the anal skin folds which 

 extend inward and upward from below. The zona columnaris 

 is lined by a stratified columnar epithelium of from two to three 

 layers of cells. The transition from the simple columnar to the 

 stratified columnar epithelium takes place rather gradually. It 

 forms the linea ano-rectale. In the upper part of the zone are to 

 be found usually a few intestinal glands and goblet cells. 



In the lower part of the zona columnaris, arising from the 

 rectal sinuses, are found tubular gland-like structures. These 

 may be of a simple tubular form lying completely within the sub- 

 mucosa, or they may be branched tubules which pierce the under- 

 lying muscle layers. The latter type are known as the intra- 

 muscular glands. There are seldom more than six or eight of 

 these in any one rectum. The main ducts of the glands extend 

 outward, and usually downwards, and penetrate the internal 

 sphincter muscle. Here an ampulla-like swelling is usually met 

 with. Extending beyond this ampulla are several tubular branch- 

 es which continue through the internal sphincter and end blindly 

 in the intramuscular connective tissue. Occasionally a tubule 

 is seen piercing the longitudinal muscle layer. Around the ter- 

 minations of the tubules which sometimes take on the appearance 



