Digestion : enzymatic conversion of complex food substances to simple absorbable forms in the alimentary canal.
DIGESTIVE SYSTEM :
Includes:
- Alimentary canal
- Digestive glands or associated glands.
Alimentary canal :
- The alimentary canal begins with mouth and ends with anus.
- Mouth leads to buccal cavity or oral cavity.
- Oral cavity has teeth and muscular tongue.
- Each tooth embedded in a socket of jaw bone: such attachment called thecodont.
- Diphyodont : human has two sets of teeth in their life time:
- Milk teeth or deciduous teeth
- Permanent teeth.
- Heterodont : teeth are of unequal shape and size.
- Incisor (I)
- Canine (C)
- Premolar (PM)
- Molar (M).
- Dental formula : arrangement of teeth in each half of the upper jaw and lower jaw.
- Dental formula of human adult is
- The hard chewing surface of the teeth made up of enamel.
- The tongue is a freely movable muscular organ attached to the floor of the oral cavity by the frenulum.
- The upper surface of tongue has small projections called papillae, some of which bears taste buds.
- The oral cavity leads into a short pharynx which serves as a common passage for food and air.
- Oesophagus and the trachea open into the pharynx.
- Opening of wind pipe or trachea called glottis, and that of oesophagus is called gullet.
- The cartilaginous epiglottis prevents the entry of food into the glottis during swallowing.
- Oesophagus connects pharynx with stomach.
- Opening of oesophagus is regulated by gastro-oesophageal sphincter.
- The stomach has three parts:
- Cardiac: into which oesophagus opens.
- Fundus: air filled portion of stomach.
- Pyloric: portion opens into the small intestine.
- Small intestine distinguished into three parts:
- Duodenum: ‘U’ shaped first part.
- Jejunum: longer, coiled middle portion.
- Ileum: highly coiled posterior part.
- The opening of stomach into the duodenum is guarded by pyloric sphincter.
- Large intestine consists of three parts:
- Caecum is a small blind sac which hosts some symbiotic micro-organisms.
- Caecum has a finger-like blind tubular projection called vermiform appendix.
- The Caecum opens into colon, which has three distinct part-
- Ascending colon
- Transverse colon
- Descending colon
- The descending colon opens into rectum which opens to out through anus.
Histolology of alimentary canal :
- Alimentary canal from oesophagus to rectum has four layers.
- Serosa.
- Muscularis.
- Sub mucosa.
- Mucosa.
- Serosa is the outermost layer and is made up of a thin mesothelium with some connective tissues.
- Muscularis is formed by smooth muscles arranged outer longitudinal and inner circular layers.
- Sub-mucosa is formed by loose connective tissues containing nerves, blood and lymph vessels.
- Mucosa is the innermost layer made of endothelium.
- Mucosa forms irregular folds (rugae) in the stomach and small finger like folding 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 for absorption.
- Villi are supplied with a network of capillaries and a central lymphatic vessel called lacteal.
- Epithelial cells of mucosa contain secretory cells which secretes digestive enzymes.
- Mucosa also forms glands in the stomach (gastric gland)
- Mucosa forms crypts in between the bases of villi in the intestine called Crypts of Lieberkuhn.
