RECTUM 275 



tary nodules are present. The muscularis mucosae, submucosa, and cir- 

 cular layer of smooth muscle also resemble those of the colon, but the 

 three taeniae spread out and unite so as to form a continuous layer of 

 longitudinal muscle. In the upper part of the rectum this layer is spe- 

 cially thickened dorsally and ventrally. As the rectum loses its mesen- 

 tery, the tunica serosa is replaced by adventitious connective tissue. 



The pars analis recti is the region of transition from mucous mem- 

 brane to skin. This transition is not gradual but takes place in three 

 steps, thus forming three distinct superimposed zones. From above 

 downward these are the zona columnaris, zona intermedia, and zona cutanea 

 (Fig. 270). The last, however, does not belong to the pars analis, properly 

 speaking, but to the outside skin. 



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

 not always limited to this zone. They may extend upward into the first 

 part of the rectum for a short distance, and they may also be continuous 

 downward with the so-called anal skin folds. In the upper part of the 

 zona columnaris the simple columnar epithelium of the superior portion 

 of the rectum becomes two- or three-layered. Its outer cells are columnar, 

 with finely granular protoplasm. The transition takes place gradually 

 at the linea ano-rectalis . In the upper part of the zone there are usually 

 a few intestinal glands containing numerous goblet cells, and a few goblet 

 cells are found also in the surface epithelium. In the lower part of the 

 zona columnaris, arising from the rectal sinuses, there are a few branched 

 tubular gland-like structures, the intra-muscular glands (Fig. 270). 

 There are seldom more than six or eight in any one rectum. The main 

 ducts of these glands extend outward, and usually downward, and pene- 

 trate the internal circular muscle (internal sphincter). Here a flask- 

 shaped swelling is usually met with. Extending beyond this ampulla there 

 are several tubular branches which continue through the internal sphincter 

 and end blindly in the intra-muscular connective tissue. Occasionally a 

 tubule is seen piercing the longitudinal muscle layer. Around the termi- 

 nations of the tubules, which are sometimes swollen, there is a small amount 

 of lymphoid tissue. The epithelium lining the main ducts of these glands 

 consists of several layers of polygonal cells, but the ampullae and branches 

 are lined with one or two layers of cuboidal cells. Secretory cells are 

 present in the embryo and at birth, but are apparently wanting in the 

 adult. 



The transition between the zona columnaris and zona intermedia is 

 marked by a rather abrupt change in the epithelium, which becomes many 

 layered and squamous. This transition takes place at the level of the 

 anal valves, but between the valves it extends upward on the rectal col- 

 umns. Thus it follows a zig-zag line, the linea sinuosa analis (ano-cutane- 

 ous line of Hermann). Within the zona intermedia the epithelium, com- 



