Histological Studies of the Rectum of Monkey, with Special Reference to its Innervation

Abstract

In the mucous membrane of the upper part of the rectum of monkey, there are reticular folds in formation occasioned by the undulating structure of the tunica muscularis. Anal columns are found extending into the mucous membrane of the lower rectum, lined with a rectal epithelium.The development of the lymphatic follicles in the rectum is particularly good in the lower part, often showing formation of lymphatic follicles opening out in the so-called pori folliculares. It is of interest that tubular pits without connection with any lymphatic follicles are also found in existence frequently in the rectal mucosa.In the upper part of the rectum, the intestinal crypts are rather densely arranged, their narrow interspaces being filled with smooth muscle fibres coming from the muscularis mucosae, only a very limited infiltration of free cells being observed. In the lower part of the rectum, however, the crypts are formed stout but short and are more loosely arranged, the propra is mucosae being rich in lymphocytes.Anal canals (SETO) beginning in the sinus anales are found very often running into the mucous membrane of the lower rectum. These canals are lined by two-, or more rarely, three-rowed columnar epithelium. Their branches run between the intestinal crypts to open out on the surface of the rectal mucosa.The intestinal crypts in the rectal mucosa contain numerous argentophil cells in their basal 1/3. Goblet cells are numerous in the lower part of the rectum, but very rare in the upper part. The inner circular layer of the muscularis mucosae fades out when the anorectal line is approached, but the outer longitudinal layer extends down as far as into the zona columnaris ani, even thickening into the RÜDIGER's m. dilatator ani. Also the striated muscle fibres originating in the vicinity of the anal orifice participate in the formation of this dilator muscle.The incoming nerve fibres supplied to the rectum, especially, its lower part, come in along the upper surface of the floor of the pelvis up to the rectal wall, then after forming nervous plexus in the adventitia and the connective tissue septa in the outer longitudinal layer of the muscularis, spread out in the layers of the rectal wall. These nerve bundles comprise many myelinated sensory fibres of spinal nature beside fine vegetative fibres. Most of the sensory fibres presumably originate in the pelvic nerves.The sensory terminations in the rectal wall are first formed in the adventitia and the outer longitudinal layer of the muscularis, that is, in its connective tissue septa. Such terminations are of the branched type consisting of 4-5 branch fibres each, which usually run very perceptibly winding courses, then taper off slowly, to end in sharp points.In the outer longitudinal muscle layer itself too are found sensory terminations, which are somewhat more complex in formation. Their branch fibres often show change in size during their long wavy courses and end either sharply or bluntly. Besides, branched terminations are frequently found along the small veins in the connective tissue of these parts. In such a termination, a sensory stem fibre accompanying fine vegetative fibres divides into ever thinner branch fibres by repeated bifurcations, the branches ending sharply around the small veins after running long courses.The sensory fibres hardly form any of their terminations in the inner circular layer of the muscularis, the submucosa or the muscularis mucosae, but form numerous terminations upon reaching the propria mucosae. These terminations are of the unbranched and the simple branched types. Their stem fibres usually accompany fine vegetative fibres, show little change in size and divide out into wavy terminal fibres after losing their myelin sheaths, to end sharply around the intestinal crypts. In particular, in the lower part of the rectum