The digestive tube is lined internally by a continuous mucous membrane, which begins at the mouth and ends at the anus. Beginning with the Oesophagus, the alimentary canal has a wall made up of four layers or tunics, which have some common general, features and specific regional characteristics. The four coats are from within outwards.
- Mucous membrane
- Submucosa
- Muscularis
- Serosa
Mucous Membrane
- This consists of three layers
- Epithelium,
- Lamina propria and
- Muscularis mucosa.
- Epithelium
- The type of epithelium varies in relation to the function of that part of the tube, it lines. In some parts, it is primarily protective, in other places it is secretory and in some other places it is absorptive in function. It is of the stratified squamous type in oesophagus, rumen, reticulum and omasum of the ruminant stomach and simple columnar from stomach to large intestine and the anus being again lined by stratified squamous epithelium.
- Lamina Propria
- This is a layer of loose connective tissue over which lies the epithelium. It is made up of fine interlacing fibres and contains fibroblasts and macrophages. It is frequently infiltrated with lymphocytes and plasma cells. From the stomach to large intestine, simple tubular glands are found in the lamina propria, isolated lymph nodules referred to as Solitary glands may also be present. In the small intestine minute projections, with a core of lamina propria covered by epithelium called villi are formed.
- Muscularis mucosae
- It is a thin layer of plain muscle fibre and when present separates the lamina propria from the submucosa. It may be arranged in two layers, longitudinal and circular, but the two layers are not always clearly recognizable. It consists of only isolated bundles of plain muscle fibres in the oesophagus except near its termination, but is distinct from stomach onwards. It is absent in rumen and reticulum.
Submucosa
- It consists of loose connective tissue and connects the mucous membrane to muscularis.
- Glands may be present in this layer as in oesophagus and duodenum and in ileum dense aggregations of lymphoid tissue called Peyer’s patches are present.
Tunica Muscularis
- The muscular coat consists of the layers of plain muscle fibres – outer longitudinal and inner circular layers.
- In the oesophagus it is made up of voluntary muscle, except near its termination in some species.
Tunica Serosa
- This consists of mesothelium (simple squamous epithelium with an underlying layer of loose connective tissue) which is adherent to the deeper muscular coat.
- Since these are reflections of the serous membranes lining the body cavities (pleura of thoracic cavity and peritoneum of abdominal and pelvic cavities), no serous membrane will be covering organs not contained in these cavity (Oesophagus in the cervical region) and a fibrous coat of loose connective tissue will replace the serosa.
