On the Histology and Innervation of Rectum and Anus of Snapping Turtle

Abstract

The author has studied the histology and the innervation of the region between the lower part of the rectum and the cloaca of a male snapping turtle using preparations stained with SETO's impregnation.
The distal part of the rectum measures 4.5mm in diameter and several longitudinal mucous folds protrude into the rather spacious lumen, which is lined by a one-rowed cylindrical epithelium and crypts are formed in the propria. These crypts are lined by a one-rowed cubic epithelium probably of mucous nature. The muscularis mucossae is very ill-developed, and the tunica musclaris here is composed of an inner circular and an outer longitudinal layers.
The proximalmost portion of the anal part is 3mm in diameter, has no crypts in the mucosa, the epithelium is of a stratified cylindrical epithelium, and lymphonodules are found scattered in the propria. The muscularis mucosae is markedly well developed, and the tunica muscularis is similar to that in the rectum. In the part a little more distal, the anal canal enlarges to 4-4.5mm in diameter and the first pair of anal sacs formed by depression of the epithelium makes appearance in the submucosa on both the lateral sides. Going a little more distalwards to the level of the orificium urethrae internum, the anal canal narrows down to 2-3mm in diameter, the tunica muscularis becomes particularly well developed on the ventro-lateral sides, and the second pair of anal sacs comes into view on both the lateral sides extending even beyond the tunica muscularis. The ductus ejaculatroii are found running on their ventral side.
Upon going further caudalwards, at the height of the orificium urethrae externum, the second pair of sacs as well as the ductus ejaculatorii come nearer to the anal canal. The muscularis mucosae is here further better developed. A little further downwards, the anal canal enlarges to 4.5mm in diameter, its lumen is broaden and the longitudinal folds grow larger. The second pair of sacs open out here into the anal canal and the proximal part of the corpus cavernosum penis comes into view on the ventro-lateral sides on this level. On the level a little further below, the anal canal attains the largest diameter of 5.5mm. The ductus ejaculatorii enter the tunica muscularis on both the lateral sides and soon open out on the ridge of a pair of large folds standing out from the mucosa on the ventral side. Another large mucous fold protrudes from the dorsomedian part of the mucosa.
The anal canal loses in size further distalwards, narrowing down to 2-3mm. It is provided with many small narrow folds in this part and the lumen is narrowed down. A spongy body that must correspond to the corpus cavernosum urethrae of mammals appears now on the ventral side. A pair of blind canals is formed in the central part of that body. More dorsal to the spongy body, these two canals unite into one, which runs more caudalwards and finally commnnicates with the anal canal, then opens out into the cloaca. These blind canals represent the third pair of anal sacs.
The AUERBACH's plexus in the tunica muscularis is very poor in development in the lower part of the rectum, and the incoming nerve fibres consist in fine vegetative fibres and a small number of thick sensory fibres. The MEISSNER's plexus formed also in the submucosa here is even worse developed than the AUERBACH's plexus. The vegetative fibres here too end always woven into STOHR's terminal reticula.
The sensory fibres coming into the rectum sometimes end in unbranched and simple branched snake-like terminations in the muscularis or spreading thence further into the submucosa, but otherwise also run further into the propria and form unbranched and simple branched terminations there. The latter have terminal fibres running wavy courses and end sharply in the vicinity of the rectal crypts. As in the same part of mammals, intraepithelial fibres are not found here.