- The human digestive system consists of the alimentary canal and the associated glands.
- The alimentary canal begins with an anterior opening – the mouth, and it opens out posteriorly through the anus.
- The mouth leads to the buccal cavity or oral cavity, which has a number of teeth and a muscular tongue and each tooth is embedded in a socket of jaw bone with a type of attachment called as thecodont.
- A set of temporary milk or deciduous teeth is replaced by a set of permanent or adult teeth, and the type of dentition is called diphyodont.
- An adult human has 32 permanent teeth, which are of four different types
- incisors (I)
- canine (C)
- premolars (PM)
- and molars (M).
- Arrangement of teeth in each half of the upper and lower jaw in the order I, C, PM, M is represented by a dental formula which in human is 2123/2123.
- The tongue is a freely movable muscular organ attached to the floor of the oral cavity by the frenulum, and the upper surface of the tongue has small projections called papillae, some of which bear taste buds.
- The oesophagus and the trachea open into the pharynx.
- A cartilaginous flap called epiglottis prevents the entry of food into the glottis, which is the opening of the wind pipe.
- The oesophagus is a thin, long tube which extends posteriorly passing through the neck, thorax and diaphragm and leads to a ‘J’ shaped bag like structure called stomach.
- The stomach has three major parts
- a cardiac portion into which the oesophagus opens
- a fundic region
- a pyloric portion which opens into the first part of small intestine.
- Small intestine is distinguishable into three regions
- a ‘U’ shaped duodenum
- a long coiled middle portion jejunum
- a highly coiled ileum.
- The opening of the stomach into the duodenum is guarded by the pyloric sphincter.
- Ileum consists of
- Caecum is a small blind sac from which a vestigial organ called vermiform appendix
- The colon is divided into three parts
- an ascending
- a transverse
- a descending part.
- The descending part opens into the rectum which opens out through the anus.
- The wall of alimentary canal from oesophagus to rectum possesses four layers
- Serosa is the outermost layer and is made up of a thin mesothelium and some connective tissues.
- Muscularis is formed by smooth muscles usually arranged into an inner circular and an outer longitudinal layer.
- The submucosal layer is formed of loose connective tissues.
- Mucosa is the innermost layer which forms rugae in the stomach and small finger-like foldings called villi in the small intestine.
- The cells lining the villi produce numerous microscopic projections called microvilli giving a brush border appearance.
- Villi are supplied with a network of capillaries and a large lymph vessel called the lacteal.
- Mucosal epithelium has goblet cells which secrete mucus that help in lubrication.
- Mucosa forms gastric glands in the stomach crypts of Lieberkuhn in different parts of the alimentary canal.
Fig. Human digestive system
Fig. Arrangement of teeth
Fig. Transverse section of gut