Describe the tissue structure of the wall of the digestive tract.

(N/A) The wall of the alimentary canal from the oesophagus to the rectum possesses four layers:
$(a)$ Serosa: Serosa is the outermost layer and is made up of a thin mesothelium (epithelium of visceral organs) with some connective tissues.
$(b)$ Muscularis: It is formed by smooth muscles,usually arranged into an inner circular and an outer longitudinal layer.
$(c)$ Sub-mucosa: The sub-mucosal layer is formed of loose connective tissues containing nerves,blood,and lymph vessels. In the duodenum,glands are also present in the sub-mucosa.
$(d)$ Mucosa: The innermost layer lining the lumen of the alimentary canal is the mucosa. This layer forms irregular folds (rugae) in the stomach and small finger-like projections called villi in the small intestine.
The cells lining the villi produce numerous microscopic projections called microvilli,giving a brush-border appearance. These modifications increase the surface area enormously.
Villi are supplied with a network of capillaries and a large lymph vessel called a lacteal.
Mucosal epithelium has goblet cells which secrete mucus that helps in lubrication.
Mucosa also forms glands in the stomach (gastric glands) and crypts in between the bases of villi in the intestine (Crypts of Lieberkuhn).
All the four layers show modifications in different parts of the alimentary canal.