human beings, the digestive system consists of alimentary canal and the associated glands (these glands help in digestion of food).
Alimentary canal:– It start with mouth and opens outside through the anus. The mouth leads to the buccal cavity or oral cavity. The oral cavity has a number of teeth and muscular tongue.
Teeth:-
- Majority of mammals including human beings, have two sets of teeth during their life g one temporary and one permanent set.
- A set of temporary milk of deciduous teeth is replaced bu a set of permanent or adult teeth. Such type of dentition is called Each tooth is embedded in a socket of jaw bone. This type of teeth attachment is called thecodont.
- An adult human has 32 permanent teeth which are of four different types, namely incisors (I) canines (C), premolars (PM) and molars (M). Such type of dentition is called Heterodont
- The arrangement of teeth in each half of the upper and lower jaw in the order I,C, PM and M is represented by a dental formula.
The dental formula of an adult human being is 2123/2123 X 2 = 8/8X2 = 16/16 = 32
- Out of these 32 teeth, premolars and last molar teeth appear only once e are monophyodont and the remaining teeth are diphyodont.
- The hard chewing surface covering the teeth is made up of enamel which helps in the mastication of food. Enamel is the hardest substance of the human body, secreted by ameloblasts (ectodermal in origin).
Each tooth has three parts, namely crown, neck and root. Crown remains capped with hardenamel, formed principally of calcium phosphate; it forms the chewing surface of the tooth
Both crown and root are composed of dentine, secreted by odontoblasts (mesodermal in origin), inside which a central pulp cavity is present in which thre is nerve and blood supply
- Human teeth are:-
(a) Thecodont:- Embedded in sockets and have well-developed roots (mammals and crocodiles), according to position.
(b) Bunodont:- Small, blunt and rounded cusps e.g human, as per arrangement of enamel and dentine.
Digestive system Tongue:-
- The tongue is a voluntary, muscular and glandular structure which occupies the floor of the mouth. It is attached to the floor of the mouth by a fold called the frenulum of the tongue.
- An inverted V-shaped furrow termed the sulcus terminalis divides the upper surface of the tongue into anterior oral part and posterior pharyngeal part. Papillae present in the tongue detect different tastes.
The different areas of tongue are demarcated as follows:-
Tips – sweet
Tips and sides – salt
Sides – sour
Base – bitter
- There are no taste buds for the tasting chillies. The burning sensationis due to stimulation of pain receptors.
Pharynx:- The oral cavity leads into pharynx. It is common passage for food and air (breathing).
Note:-
Tonsils:- Lymphoid tissue of pharynx is called tonsils. It includes:-
(i) Nasopharyngeal/pharyngeal tonsil/adenoids.
(ii) Palatine/faucial tonsils
(iii) Lingual tonsils
(iv) Tubal tonsils
The pharynx leads into oseophagus. Glottis is the opening of the windpipe. A cartilaginous flap called epiglottis prevents the entry of food into the glottis during swallowing. The oseophagus 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 opening of oseophagus into the stomach is regulated by amuscular (gastro-oesophageal) sphincter.
Oesophagus is characterized by:-
- Absence of digestive glands.
- Presence of mucus-secreting goblet cells.
- Presence of voluntary (in anterior 1/3rd) and involuntary muscle fibres (in posterior 2/3rd ).
- It is located in the upper left portion of the abdominal cavity; has four major parts- a cardiac portion into which the oesophagus opens, a fundic region, body and a pyloric portion which opens into the first part of small intestine
- Cardiac sphincter is present at the opening of oesophagus into stomach and prevents regurgitation of food into oesophagus. The pyloric part opens into duodenum (small intestine) and its opening is guarded by pyloric sphincter.
The mucosa of stomach has gastric glands. Gastric glands have three major types of cells, namely:-
(i) Mucus neck cells which secrete mucus.
(ii) Peptic or chief cells which secrete the proenzyme pepsinogen.
(iii) Pariental or oxyntic cells which secrete HCL and intrinsic factor (factor essential for absorption of vitamin B12 in intestine).
It is distinguished into three regions:-
(i) Duodenum (C-shaped)
(ii) Jejunum (Long and coiled middle portion)
(iii) Ileum, (Highly coiled, longest portion).
- Ileum, the last part of small intestine leads into the large intestine.
- The wall of intestine has thin layers of longitudinal and circular muscles. Epithelial cells lining the villi have microvilli which further increase the absorptive area.
- Intestinal glands or Crypts of leiberkuhn have epithelial cells (secrete mucus), paneth cells (secretes digestive enzymes) and argentaffin cells ( probably secretes hormones). IN duodenum, Brunner’s glands are also present (located in submucosa) which secrete mucus.
- DIfused patches of lymphoid tissues are present throughout the small intestine ans are aggregated in ileum to form Peyer’s patches.
Large intestine consists of caecum, colon and rectum.
- Caecum: It is a small blind sac which hosts some symbiotic micro-organisms. Vermiform appendix (a finger like tubuler projection) arises from the the caecum and is considered vestigial in human beings. The caecum opens into the colon.
Colon: it is divided into three parts, an ascending, a transverse and descending part. The descending part of colon opens into rectum. Ascending colon is the smallest part and lacks mesentry
- Rectum: It is the last part of alimentary canal. It is the storage site for faecal matter. It opens outside through anus.
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