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Alimentary Canal Diagram: Mouth, Oesophagus, Stomach, Small Intestine and Alimentary canal wall

The human digestive system is composed of two components namely the alimentary canal and other associated glands.

Human alimentary canal

The pathway through which the food enters and leaves the body is called the alimentary canal. It has two openings - the mouth and the anus. The human alimentary canal consists of the following parts.
1. Mouth and the Buccal cavity
2. Oesophagus
3. Stomach
4. Small intestine
5. Large intestine

Mouth and the buccal cavity

  • The mouth, the anterior opening of the alimentary canal, is the doorway to the oral cavity.
  • The oral cavity consists of a muscular tongue and tooth. Every tooth is found embedded in the socket of the jaw bone. Such a type of attachment of teeth to the jaw bone is called the thecodont. Mammals have the diphyodont type of dentition. They form two sets of teeth - temporary deciduous (milk) teeth and permanent adult teeth.
  • The set of teeth present in an adult human being includes incisors, canines, premolars, and molars.
  • The arrangement is as follows: 2123/2123

Note: this arrangement only represents half the jaw as there is symmetry in the left and right sides.

  • The surface of the teeth is composed of enamel and is responsible for the mastication of the food. The muscular tongue moves freely and is found attached to the buccal cavity with the help of the frenulum. Small projections called papillae are present on the upper surface of the tongue. A few papillae contain taste buds.

Oesophagus

  • The buccal cavity opens into the pharynx. The pharynx is the common passage for respiratory and digestive pathways i.e. food and air.
  • The windpipe and oesophagus lead into the pharynx. A cartilaginous structure called epiglottis prevents the food from entering the glottis.
  • The oesophagus is a thin and long tube extending from the next (posteriorly) to the stomach.

Stomach

  • The oesophagus leads to the stomach and is regulated by a muscular gastroesophageal sphincter.
  • The stomach is a J-shaped (pouch-like) organ situated in the upper left region of the peritoneum cavity. The stomach consists of three portions namely, cardiac, fundus, and pyloric.
  • The cardiac portion of the stomach is the part into which the oesophagus opens. The fundus is the middle portion of the stomach. The pyloric portion leads to the first part of the small intestine ( duodenum).

Small Intestine

  • The small intestine consists of 3 regions - duodenum, ileum and jejunum.
  • The duodenum is C-shaped and its opening into the small intestine is guarded by the pyloric sphincter. The jejunum is the middle portion of the small intestine. The ileum, the last portion of the small intestine, is highly coiled and leads to the large intestine.

Large Intestine

  • The large intestine consists of three portions - caecum, colon, and rectum.
  • Caecum, a tiny sac, contains some helpful micro-organisms. Vermiform appendix, a fingerlike projection, originates from the caecum. The vermiform appendix is a vestigial organ.
  • The three parts of the colon - ascending colon, the transverse colon, and descending colon.
  • The descending colon leads into the rectum.
  • The rectum opens into the exterior via the anus.

Alimentary canal wall

The human digestive system is composed of two components namely the alimentary canal and other associated glands.

  • Serosa: The outermost layer of the alimentary canal is the serosa. It is composed of a thin mesothelium along with a few connective tissues.
  • Muscularis: The layer next to serosa is the muscularis. This layer is composed of smooth muscles, hence the name muscularis. The smooth muscles are arranged into two layers - the outer layer of longitudinal muscles and the inner layer of circular muscles. An additional layer of oblique muscles may be present in a few regions of the alimentary canal.
  • Sub-mucosa: The third layer of the alimentary canal is the submucosa. It is composed of loose connective tissues and comprises blood, nerves, and lymph vessels.
  • Mucosa: Mucosa is the innermost layer present in the wall of the human alimentary canal. The mucosal layer forms special folds in the stomach and small intestine.

    In the stomach, the mucosal layer folds itself irregularly to form rugae. In the small intestine, the mucosa folds itself to form fingerlike foldings named villi. The cells present on the villi produce many microvilli. Microvilli are microscopic projections, which give a brush-border-like appearance in the intestine. Microvilli improve the surface area of the intestine to a significant extent. Villi are supplied and supported with lacteal (a large lymph vessel) and a huge network of capillaries. The Mucosa consists of mucus-secreting cells called goblet cells. The mucus secreted by these cells aids in lubrication. The mucosal layer also forms gastric glands (present in the stomach) and crypts of lieberkuhn (present in between villi bases).
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