On Innervation, Especially, Sensory Innervation of Rectum and Anus in Canine Fetus

Abstract

In a canine fetus of the later fetal stage, the anal mucous part, containing the zonae intermedia and columnaris, is much smaller in area than that in human fetus. Tne intermediate zone is lined with a bladder-like tsratified epithelium poorer in development than in human fetus. No papillae are formed out of the propria into this epithelium, as little in fetal as in adult dogs. The propria in this as well as in the columnar zone is composed of a connective tissue containing abundant spindle-formed connective tissue cells, and no longitudinal muscle fibres extending down from the muscularis mucosae of the rectum is yet formed between the propria and the submucosa of a similar tissue, as was in the case of the human anus.In the zona columnaris, the columnae are lined with a 3-4-rowed cylindrical or a flat epithelium, and the sinus with a tall cylindrical epithelium. Unlike that in human fetus, papillae are found in formation out of the propria into these epithelia, even rather markedly in the part nearer the rectum. Anal canals are formed beginning in the epithelium of the zona columnaris and running down along the subepithelial layer before ending in blind sacs, but in number, these are much smaller than in human fetus. The epithelium here is of stratified cylindrical type.The proctal glands are already in formation. These are found, beside within the mucosa and the tunica muscularis of the anus, also not rarely extending into the rectal mucous membrane. But in fetal dog, they are as yet not nearly full-grown, their end parts being mere epithelial cords and the ducts being indicated by a 2-3-rowed cylindrical epithelium, no sign of their specific structure being yet apparent.The muscularis of the rectum is composed of an inner circular and an outer longitudinal layers extending down as far as the intermediate zone of the anus, the former layer thence passing over into the m. sphinter ani internus. On the other hand, the outer longitudinal layer partly reaches down along the outside of the m. sphincter ani externus, and gaining in strength, finally arrives at the inside of the coxal bone to end there. I wish to call this the m. dilatator ani.The ganglia in the AUERBACH's plexus are formed, beside between the two muscle layers above, often enough inside the muscle layers as well and in a small number in the m. dilatator ani too. Smaller ganglia are formed also in the MEISSNER's plexus. Such ganglia are found in the anus as well, but in development these are much poorer than those in the rectum. The ganglion cells are still very ill developed and their multipolarity is as yet sufficiently manifest, so that they are seen as more apolar round cells.The nerve fibres coming into the distal part of the rectum run via the AUERBACH's plexus into the MEISSNER's plexus and come into closest relation with these plexuses. Beside the very fine vegetative fibres, a small number of thick sensory fibres are also found among them. The terminations of the vegetative fibres are already well developed into full-grown STOHR's terminal reticula.The sensory fibres, in company with the vegetative fibres, run also via the AUERBACH's plexus into the MFISSNER's plexus, and thence, parting from the vegetative fibres in most cases, into the subepithelial connective tissue layer to form their terminations there. Since in the dog fetus, the formation of the muscularis mucosae in the rectum is as yet not clearly defined, it is often undefinable in which layer a sensory fibre ends, but it seems that most of them end at the plane where the boundary between the submucosa and the muscularis mucosae is to be formed, except a comparatively few that penetrate further into the propria and attain the vicinity of the intestinal crypts. Their terminal mode is much simpler than those in adult dogs, unbranched terminations occupying the largest majority, and only a few bifurcated terminations representing the branched terminations found here.