Internal Anatomy of the Alimentary Canal

The human digestive system comprises the alimentary canal and associated glands. The alimentary canal starts from an anterior opening, the mouth which leads into the buccal cavity or oral cavity. The length of the human alimentary canal is 6-9 meters.

Buccal Cavity

The space between lips and teeth is called Vestibule. The roof of the buccal cavity is the palate, consisting of the hard palate, maxilla, pre-maxilla, palatine bones, and soft palate. Hard palate epithelium has thick transverse folds called Palatine Rugae. The terminal part of the soft palate hangs in the throat as uvula. On the sides of the uvula, tonsils are present which are made up of lymphatic tissue. The floor of the buccal cavity is occupied by a muscular tongue attached at the base by a fold called the lingual frenulum. On the tongue, various papillae like filiform papillae, fungiform papillae, foliate and circumvallate papillae are present.

Pharynx

It is a common passage for swallowing food and breathing. The gullet is the aperture that leads into the esophagus. Glottis is the aperture that leads into the larynx and trachea. Epiglottis is the structure that prevents the entry of food into the windpipe

Oesophagus

This portion is involved in deglutition or swallowing of food. The esophagus is a long and thin tube that connects the mouth, passing through the neck, thorax, and diaphragm and into the stomach. The regulation of opening and closing is taken care of by the muscular sphincter. In a male body, the esophagus is as long as 10 inches and does not undertake secretion of digestive enzymes and instead, it secretes mucus to ease the transportation of food.

Stomach

It is a J-shaped bag-like structure and is found in the top left part of the abdominal cavity. It consists of three parts- the cardiac portion which allows the esophagus to open, a fundic, and the pyloric region which opens into the small intestine.

The gastric mucosa is the inner part of the lining of the stomach and it comprises several types of cells. These cells handle secretions of different enzymes and mucus. The mucosa receives the secretion of the gastric glands as gastric juices. It comprises the following types of cells:

• Peptic cells: They secrete protease enzymes in proactive or precursor form; pepsinogen and pro rennin.
• Parietal and Oxyntic cells: They are present along the sides and secrete HCl and a protein factor called Castles Intrinsic Gastric Factor. This helps in the absorption of vitamin B12 so if a gastrectomy is done, the person will suffer from Pernicious Anaemia. Iron deficiency anemia can also occur because the conversion of ferric into ferrous form occurs in presence of HCl.

Small Intestine

The small intestine can be observed in three regions namely, the C-shaped duodenum, coiled middle jejunum, and the highly coiled ileum. The opening between the stomach and the duodenum is regulated using a pyloric sphincter and similarly, the ileum and large intestine are connected.

In the entire small intestine, the mucous membrane is thrown into folds called Plicae Circulares or Folds of Kerckring.

These folds are prominent in the jejunum. In the entire small intestine, diffused patches of lymphoid tissues are also found and are called GALT (Gut Associated Lymphoid Tissue). In Ileum, these patches get aggregated to form Payer’s Patches, also known as abdominal tonsils.

Large Intestine

The large intestine consists of the caecum, colon, and rectum. The caecum is observed as a small blind sac that manages some symbiotic microorganisms. The vermiform appendix is a narrow finger-like tubular structure that is a vestigial organ, and it arises from the caecum. The caecum and colon are connected via an opening and it contains three parts, namely ascending part, descending part, and traverse part. The descending part is connected to the rectum and opens out through the anus.

The first part of the large intestine is the caecum, and it opens into the colon. The serosa surrounding the colon has fat storage areas called Epiploic Appendages. In the colon, there are three longitudinal bands of smooth muscle fibers called Taeniae coli. They contract to form a pocket-like structure called Haustra.

Histology of Human Gut

The outermost layer of the gut is a fibrous coat, called the Serosa/mesothelium or visceral peritoneum. Inner to Serosa is muscularis externa, which is formed of an outer layer of longitudinal smooth muscles and an inner layer of circular smooth muscle.

Between circular and longitudinal muscle layers, a motor plexus called Myenteric or Auerbach’s Plexus is present. It controls peristaltic movement in the GIT. Inner to muscularis externa is sub-mucosa which is a loose connective tissue layer with blood and lymph vessels and nerve supply.

The submucosal plexus is Meissner’s plexus. It is a sensory plexus and controls exocrine and endocrine secretion of GIT. The mucosa is made of an innermost layer of epithelium, middle Lamina Propria, and external muscularis mucosa in contact with the submucosa.