Human anus

Abstract: The inner circular layer of the lamina muscularis mucosae of the rectum of cat goes out of existence as the anus is approached, but the outer longitudinal layer gains in development and reaches down to the distal part of the zona columnaris and, dissimilar from that in dog. The outer longitudinal layer of the tunica muscularis of the rectum disappears when the anus is reached, while its inner circular layer forms a weak m. sphincter ani int, around the zona intermedia ani.The zona intermedia ani is far narrower in extent than that of man and no papillae, as found in the human counterpart, are formed in this part of cat anus. Neither longitudinal muscle fibres are found beneath the propria. The zona columnaris ani consists of columnae rectales and sinus rectales, quite as in man and dog, but unlike in the case of man, a prominent papillary formation is observed in the feline anus.In the cat anus, proctal glands found in powerful development in the dog anus and the anal canals conspicuous in the human anus are both absent. But in the feline anus, very peculiar anal bags are found in formation. These are formed by the mucous membrane in the inferior portion of the zona intermedia ani running far into the outer sphincter muscle and stretching out bag-wise. Sebaceous and sweat glands are found in powerful development in the subepithelial tissue of these bags.Sensory fibres run into the adventitia of the distal part of the cat rectum accompanying many vegetative nerve fibres which come into close connection with the AUERBACH's and also the MEISSNER's plexus there. The ganglion cells in the AUERBACH's plexus show indeterminate multipolarity, numerous apolar cells being found among them. The small ganglia in the MEISSNER's plexus are formed in the vicinity of the muscularis mucosae in the greatest majority, and the nerve processes sent out by the cells therein are particularly poor in development. The fine vegetative fibres form STOHR's terminal reticula in the lowest portion of the rectum as well.The sensory fibres mostly form their terminations upon reaching the propria but some of them end in simple branched terminations in the submucosa and the lamina muscularis mucosae. Sometimes these fibres end bluntly or sharply in looped terminations after running rather complicated looping courses without much ramifying. The sensory fibres ending in the propria, after loosing their myelin sheaths, form unbranched or simple branched terminations around the intestinal crypts. Their terminal fibres sometimes run very long courses and terminate subepithelially in sharp points, never penetrating into the epithelium.AUERBACH's and MEISSNER's plexus are formed in the anus of cat too, but the ganglia in them are poorer in development than those in the inferior rectum above and the structure of the ganglion cells therein is also much simpler.The sensory fibres running into the mucous membrane of the cat anus are more numerous than those in the inferior rectum and mostly end subepithelially. These fibres are found more abundant in the zona columnaris where the papillary formation is prominent than in the zona intermedia where no papillae are in formation, and their terminations are more complex in structure in the former zone. In the proximal part of the zona columnaris such development of sensory fibres is particularly notable. These complex terminations, however, originate mostly in medium-sized fibres and consist in fairly complex branched terminations. In the distal part of the zona columnaris, on the other hand, most of the terminations originate in thick stem fibres, but are simpler in structure here than in the preceding and are mostly of the unbranched type. Intraepithelial fibres are found in the zona columnaris ani in cat too, but these are much less powerful than in man and dog, being limited to unbranched type in most cases.