It continues its transabdominal course as the transverse colon until it arrives at the splenic flexure, where it courses inferiorly as the descending colon. In the left inguinal region, it takes an “S”-shaped course as the sigmoid colon before terminating at the rectosigmoidal junction. Along this course, undigested and unabsorbed residues (food, drugs etc) are condensed and converted to faeces by increasing luminal water absorption.
Histologically, the large intestines can be distinguished from the small intestines by the absence of villi, plicae circularis, and Paneth cells (in adults). Simple columnar epithelium lines its mucosa. The crypts of Lieberkühn are deeper in the colon and goblet cells become more abundant.

The enteroendocrine cells are dispersed throughout the mucosa and the surface epithelium is equipped with brush borders. It is also noteworthy that the aggregations of lymph nodules are more solitary that those observed in the small intestines. The double layered muscularis mucosae that is typical of the GIT is also present.

MBbs, LBC (2021, October 28). Histology of the lower digestive tract . Kenhub. Retrieved April 27, 2022, from https://www.kenhub.com/en/library/anatomy/histology-of-the-lower-digestive-tract

The gall bladder is stimulated to contract and expel the bile into the duodenum, by the hormone cholecytoskinin pancreozymin (CCK) produced by the endocrine cells of the duodenal mucosa. The production of this enzyme is stimulated by the presence of fat in the proximal duodenum. The contractions expel bile into the common bile duct, and the bile is then carried to the duodenum.

The inner surface of the gall bladder is covered by the mucosa. The sufrace is made up of a simple columnar epithelium. The epithelial cells have microvilli, and look like absorptive cells in the intestine. Underneath the epithelium is the lamina propria. The wall of the bladder does not have a muscularis mucosae and submucosa.

The muscularis externa (muscle layer) contains bundles of smooth muscle cells, collagen and elastic fibres. Underneath this, on the outside of the gall bladder is a thick layer of connective tissue, which contains large blood vessels, nerves and a lymphatic network. Where this layer is attached to the liver, it is called the adventia. In the unattached region, there is an outer layer of mesothelium and loose connective tissue (the serosa).

Paxton, S. (nd). Digestive: The Histology Guide . Histology.Leeds.Ac.Uk. Retrieved 27 April 2022, from https://www.histology.leeds.ac.uk/digestive/gallbladder.php

The appendix is composed of the four layers characteristic of the gastrointestinal tract.
Mucosa (or mucous membrane)
Epithelium – neither villi nor plicae circularis are present.
Absorptive Cells (or Enterocytes) – simple columnar cells with microvilli (or brush border).
M-Cells (or Micro-Fold Cells) – cover nodules and have a lower profile than absorptive cells. They have small folds on their surface versus microvilli on absorptive cells).
Goblet Cells – secrete mucus for lubrication.
Crypts – fewer than in other regions of the colon.
Lamina Propria – abundant between the infrequent crypts.
Lymph nodules (#1, #2 and #3) are common.
Plasma cells, lymphocytes, eosinophils and macrophages are present.
Muscularis Mucosae – layer of smooth muscle.
Submucosa – lymph nodules (#1 and #2) may extend into the submucosa.
Muscularis Externa – two orthogonal layers of smooth muscle (inner circular and outer longitudinal).
Serosa – covers the outer surface of the appendix.

Appendix (large intestine) | Gastrointestinal Tract . (sd). Histologyguide.Com. Retrieved April 27, 2022, from https://histologyguide.com/slideview/MH-122-appendix/14-slide-1.html