Fig. 192.
From a transverse section of the large intestine of Rana temporaria, moderately distended.—G. H.
| A | Denser portion of submucous coat. |
| B | Looser portion of submucous coat. |
| C | Artery, cut obliquely. |
| D | Circular muscle-layer. |
| E | Longitudinal muscle-layer. |
a. General description. The large intestine is a flask-shaped viscus, lying in the median line. The small intestine opens into it by an abrupt curve at its anterior end. The large intestine is the widest part of the alimentary canal, is thin-walled, and diminishes in width towards its hinder end, where it opens into the cloaca above the opening of the bladder.
b. [Minute structure.
(1) The serous coat resembles that of the small intestine, etc.
(2) The muscular coats resemble those of the small intestine, but are thinner; the longitudinal layer is, however, proportionally thicker (Wiedersheim).
(3) The submucous coat (Fig. 192) resembles that of the small intestine in its general structure; the portion lying immediately beneath the mucous membrane (B) is denser in structure.
This layer has no Muscularis mucosa.
(4) The mucous coat (A) is a simple layer of large, columnar, epithelial cells, with large oval nuclei. The cells have a hyaline free border, but this possesses no striation. The mucous membrane is usually described as possessing numerous simple follicles (glands of Lieberkühn); in those specimens which I have examined they have been entirely absent (Figs. 192, 193). At six to eight points in a transverse section of a rectum moderately distended the submucous coat is thinner, and so throws the mucous coat into slight, longitudinal grooves; but these do not in the least resemble the glands of Lieberkühn.
Fig. 193.
Transverse section of large intestine of Rana esculenta; the mucous membrane thrown into longitudinal folds in consequence of the organ being contracted. Arteries injected with carmine.—G. H.
| A | Large circular vessels within the muscular coats. |
| B | Fine anastomosis to the mucous coat. |
(5) The epithelium possesses goblet- or chalice-cells (Hoffmann), the number of which probably depends upon the period of the year and the state of digestion. In the rectum, from which the section for the figures 192 and 193 were cut, not one goblet-cell was found in a complete series of sections.
If the rectum be contracted, the mucous membrane is thrown into longitudinal folds.
(6) The blood-vessels (Fig. 193) have a simple arrangement. The arteries are large, and form oblique loops around the intestine, lying under the peritoneum; from these branches are given off to form a series of rings in the submucous membrane (Fig. 193 A); from these fine twigs are given off to form a fine anastomosis under the mucous membrane (B).
(7) The lymphatics of the large intestine are arranged in two chief systems: one under the serous coat resembles the corresponding system of the small intestine. The second set forms a network of rounded loops in the submucous coat, which give rise to a secondary set of smaller vessels towards the mucous membrane; this secondary system forms a sort of trellis-work standing on the rounded loops, and so maps out small blocks of the thick submucous coat. Towards the cloaca the arrangement is simpler; the secondary lymphatics are lost, while the primary lymphatics tend to form elongated, longitudinal loops (Langer).]