DIGESTIVE SYSTEM

On the basis of the embryonic origin, the alimentary canal of vertebrates can be divided into three parts –

(A) Fore gut / Stomodaeum : Ectodermal. It includes buccal cavity / oral cavity.

(B) Mid gut / Mesodaeum : Endodermal. It includes pharynx, oesophagus, stomach, small intestine, and large intestine.

(C) Hind gut / Proctodaeum : Ectodermal. It includes anal canal and anus.

Digestive system of man consists of alimentary canal and some accessory digestive organs.

The Alimentary Canal:

It is a coiled muscular tube about 6 -9 metres long extending from mouth to anus.

Mouth:

The mouth is an opening bounded by upper and lower lip.

Lips are attached on the inner side with the gums by thin transparent fold called Labial frenulum.

The space between lips and teeth is called vestibule.

The roof of buccal cavity is palate consisting of hard palate (maxilla, premaxilla and palatine bones) anteriorly and soft palate posteriorly.

Mucus epithelium has thick transverse folds called palatine rugae.

Terminal part of soft palate hangs in the throat called uvula. On sides of uvula tonsils are present which are made of lymphatic tissue.

The floor of buccal cavity is occupied by a muscular tongue attached at base by a fold called lingual frenulum.

Teeth present on upper and lower jaws are

(i) Incisors: for cutting, have one root.

(ii) Canines: for tearing, have one root.

(iii) Premolars: for crushing, grinding and chewing, in upper premolar 2 roots and lower premolar 1 root.

(iv) Molars: for chewing, in upper molar 3 roots and lower molar 2 roots.

Arrangement of different types of teeth in the jaws on
one side and the sockets on the other side

 

Dental formulae

In man, 20 teeth grow twice during life time i.e., diphyodont; (premolars and last molars absent In primary dentition) and 12 teeth appear only once i.e., Monophyodont.

Child =

17 yr. old =

Adult =

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 called the sulcus terminalis divides the upper surface of the tongue into anterior oral part and posterior pharyngeal part.

The apex of the sulcus terminalis projects backwards and is marked by a small median pit, named the foramen caecum.

The foramen caecum is an embryological remnant and marks the site of the upper end of the thyroglossal duct. Oral part of the tongue has papillae on its surface.

These are :

(i) Filiform papillae : smallest, most abundant and have no taste buds.

(ii) Fungiform papillae : appear as red dots on tongue and contain taste buds.

(iii) Foliate papillae: absent in man.

(iv) Circumvallate papillae: largest in size and knob like, also contain taste buds.

The different areas of tongue are demarcated as follows:

Tip – sweet

Tip and sides – salt

Sides – sour

Base – bitter

Sweat glands of dogs are present on tongue (panting of dog).

Salivary Glands

Four pairs of salivary glands open in the mouth cavity:

(i) Parotid: Largest -present below and in front of ears -Stenson's duct.

(ii) Submaxillary: Medium sized -present at the angles of jaw lower -Wharton's duct

(iii) Sublingual : Smallest -located below the tongue - Rivinus duct.

(iv) Infra orbital : Absent in man, otherwise present below eyes e.g. , in rabbit.

Daily Secretion of saliva is 1.5 litres, (pH of saliva is 6.7) and has salivary amylase (ptyalin), maltase and lysozyme.

Salivary glands are stimulated to secrete saliva by parasympathetic innervation while sympathetic nerves causes reduced secretion leading to drying of mouth.

Clare required for activation of salivary amylase. Mumps is viral infection of salivary glands (mainly Parotid).

Pharynx opens through gullet into the oesophagus and through glottis into the larynx.

An elastic cartilage plate, epiglottis, covers the glottis at the time of swallowing. Food mixed with saliva in buccal' cavity-Bolus.

Oesophagus

It is a long and thin tube, 25 cm long that pierces the diaphragm and enters the abdominal cavity.

Oesophagus is characterised by :

(i) Absence of visceral peritoneum. Its outermost fibrous (non-coelomic) covering is called tunica adventitia.

(ii) Absence of digestive glands. It has mucus-secreting goblet cells.

(iii) Presence of mucous membrane formed of non-keratinised stratified squamous epithelium some cells of which are ciliated.

(iv) Presence of voluntary (upper 1/3rd) and involuntary muscle fibres (lower 2/3rd).

Stomach

Stomach is oval and pouch like organ, divisible into cardiac, fundic, main body and pyloric parts.

Cardiac sphincter is present at the opening of oesophagus into stomach and prevents the regurgitation of food into oesophagus.

The pyloric part opens into small intestine and opening is guarded by pyloric sphincter.

The wall of stomach has three layers of muscles, outermost longitudinal layer, middle circular layer and innermost of oblique layer.

Mucosa has folds called rugae and cardiac, fundic and pyloric glands.

Only fundic glands secrete gastric juice.

These contain neck cells (secrete mucus and present in all three types of glands), oxyntic or parietal cells (secrete HCl and Castle's intrinsic factor for absorption of B12).

HCl of gastric juice converts Fe3+ into Fe2+ which makes the absorption of iron possible.

Non-secretion of HCl (achlorhydria) or gastrectomy can lead to iron-deficiency anaemia.

The peptic cells or chief cells or zymogenic cells release large quantity of pepsinogen and other enzymes.

Small Intestine

First part of small intestine is duodenum. It is 25 cm long, U or C-shaped in humans and receives the opening of hepatopancreatic duct (Bile duct + Pancreatic duct).

A small swelling is present at the opening of hepatopancreatic duct and is called 'Ampulla of Vater' or Hepatopancreatic ampulla and the opening is regulated by sphincter of oddi.

Other parts of small intestine are jejunum and ileum.

The wall of intestine has thin layers of longitudinal and circular muscles.

Mucosa has folds called plicae circulare (folds of Kerkrings or Valvulae conniventes) and villi towards lumen of the intestine.

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 (secrete digestive enzymes) and argentaffin cells (probably secrete hormones).

In duodenum, Brunner's glands are also present (located in submucosa) which secrete mucus.

 Diffused patches of lymphoid tissues are present throughout the small intestine and are aggregated in ileum to form Peyer's patches.

Large Intestine

It is about 1.5 m long and consists of three parts -Caecum, Colon and Rectum.

A blind pouch of caecum is vermiform appendix.

These parts help in digestion of cellulose in herbivores.

Wall of colon has sac like haustra.

Histologically, wall of colon has three bands of longitudinal muscles called taeniae coli.

Another characteristic of colon surface is the presence of small fat filled projections called epiploic appendages.

The colon is divisible into ascending, transverse, descending and sigmoid colon.

Ascending colon is the smallest part and lacks mesentry.

Last part of rectum is anal canal having a strong sphincter. It opens outside by anus.

DIGESTIVE GLANDS

Liver

Largest digestive gland. It lies in the upper right side of the abdominal cavity just below diaphragm.

The liver is divided into two main lobes-right and left.

Between the right and left lobes, falciform ligament is present.

The right lobe is differentiated further into right lobe proper, a quadrate lobe and caudate lobe.

Liver is surrounded by Glisson's capsule, its trabeculae divide liver lobes into hexagonal lobules.

Polyhedral hepatocytes are arranged in cords around a central venule.

Portal triads contain hepatic artery, portal venule, bile ductule and lymphatics.

Blood sinusoids are present.

Kupffer cells are present in sinusoids and are phagocytic.

Gall bladder is situated on the inferior surface of right lobe. It is 8 cm long and 2 cm wide.

Bile is secreted by hepatocytes into the bile canaliculi, a series of narrow spaces between adjacent liver cells.

The canaliculil drain via bile ductules into bile ducts, which run in portal tracts; the bile duct themselves discharge into the right and left hepatic ducts which unite to form the common hepatic duct at the hilum of the liver.

A part of transverse section of mammalian liver

Gall bladder has a capacity of 30 to 50 ml. It consists of smooth muscles lined by columnar epithelium.

It fills and empties via cystic duct which joins the common hepatic duct to form the bile duct; this in turn empties into the duodenum through the ampulla of Vater (hepatopancreatic ampulla).

At the point of its entry into the duodenum, the bile duct and adjacent pancreatic duct join each other.

The sphincter of Boyden surrounds the opening of bile duct.

Sphincter of oddi surrounds the ampulla of Vater.

Pancreas

It is a racemosely branched gland, situated between stomach and duodenum.

Pancreas consists of acini (which secrete digestive enzymes) and islets of Langerhans (which secrete insulin and glucagon hormones).

Pancreas has two ducts within it.

The first is duct of Santorini which is accessory or nonfunctional, opening directly into duodenum and the other is duct of Wirsung which is functional and combines with bile duct to form common hepatopancreatic duct.