Main Parts of Human Digestive System (With Diagram)

The following points highlight the two main parts of human digestive system. The parts are: 1. Alimentary Canal 2. Digestive Glands.

Part # 1. Alimentary Canal:

It comprises the following parts:

I. Mouth:

Human mouth consists of two parts.

(a) Vestibule:

The vestibule is a slit-like space bounded externally by lips and cheeks and internally by the gums and teeth.

(b) Oral Cavity (Buccal Cavity):

It is inner portion of the mouth which has the following parts.

(i) Palate:

The roof of the oral cavity (buccal cavity) is called palate. Anterior part of the palate is known as hard palate which bears transverse ridges, the rugae. The posterior part of the palate is smooth and is termed the soft palate. The hinder free part of the soft palate freely hangs down as a small flap, the uvula.

(ii) Tongue. The tongue is attached to the floor of the mouth by a fold called the lingual frenulum. An inverted V-shaped furrow termed 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 backward and is marked by a small median pit, named the foramen caecum.

Papillae:

The upper surface of the tongue has four types of papillae (little projections).

(a) Vallate papillae or Circumvallate papillae are usually about 8 to 12 in number. Each vallate papilla contains up to 100 taste buds. These papillae are the largest of the four types of papillae.

(b) Filiform papillae are the smallest and most numerous of the four types. They are conical. They are found mainly near the centre and most of the upper surface of the tongue. These papillae contain tactile (touch) receptors but not taste buds.

(c) Fungiform papillae are much less numerous than the filiform papillae. They are rounded but smaller than vallate but larger than filiform papillae. They are most numerous near the tip of the tongue. Each fungiform papilla contains about five taste buds.

(d) Foliate papillae are not developed in human tongue. These are leaf-like and are situated at the sides of the base of the tongue. On each border there are four or five vertical folds. Their taste buds degenerate in early childhood.

Human tongue has four taste areas (sweet, salt, sour and bitter). Areas of sweet and salt can overlap.

Functions of the Tongue:

The tongue acts as an accessory digestive organ.

(i) It helps in chewing the food.

(ii) It aids in swallowing the food,

(iii) It acts as a brush to clean the teeth,

(iv) It plays a role in speech,

(v) It is an organ of taste.

Teeth:

(a) Characteristics:

Men have diphyodont (two sets of teeth— milk or de­ciduous and permanent), thecodont (teeth are embedded in the sockets of the jaw bones) and heterodont teeth (different types of teeth). There are present four kinds of teeth— incisors, canines, premolars and molars.

Incisors:

They are usually specialized for cutting.

Canines:

They lie immediately behind the incisors. They are also used for cutting the food.

Premolars and molars:

These are called cheek teeth which are broad, strong crush­ing teeth. Third molars in human being are called wisdom teeth. The latter are vesti­gial in human beings.

(b) Number:

The milk or deciduous or temporary teeth are 20 in number; 10 each in the upper jaw and in the lower jaw. The milk teeth begin to erupt when the child is about 6 months old and should all be present by the end of 24 months. The permanent teeth begin to replace the milk teeth in the 6th year of age. These teeth are 32 and usu­ally complete by 18-25 years.

(c) Dental Formulae:

Milk teeth of man include 8 incisors, 4 canines and 8 molars (premolars are absent). Molars of milk teeth are shed off and premolars of permanent teeth take their place. The permanent teeth are 8 incisors, 4 canines, 8 premolars and 12 molars. Thus 12 teeth (8 premolars and 4 molars) are monophyodont (teeth which grow only once in life). Dental Formulae of milk teeth and permanent teeth of human are given below.

212/212 x 2 = 20

2123/2123 x 2 = 32

Milk teeth Permanent Teeth:

The dental formula gives half of the total number of teeth. This is doubled to determine the full number.

(d) Structure:

A typical tooth consists of three regions; crown— the part which projects above the gums, the neck— the part which is surrounded by gum and the root— the part which is embedded in the bone.

The incisors and canines have one root, the upper first premolars have two roots and the upper second premolars and lower premolars usually have only one root. The upper molars have three roots and the lower molars have two roots.

A human tooth consists of the following parts:

Enamel:

It is the hardest substance of the human body. It covers the dentin in the crown.

Dentin:

It has numerous fine canaliculi that pass radially from the pulp cavity towards the enamel.

Cement:

It covers the root of the tooth.

Periodon­tal Ligament:

It is made up of collagen fibres and covers the cement. It fixes the tooth in its socket.

Pulp Cavity:

Dentin encloses the pulp cavity that contains a mass of cells, blood vessels and nerves which constitute the pulp. Narrow extensions of the pulp cavity called root canals, run through the root of the tooth.

Apart from the connective tissue cells of the pulp and of the periodontal membrane and the cementocytes in cement, there are two main types of cells. These are dentine forming odon­toblasts and enamel forming ameloblasts.

II. Pharynx (Throat):

It is divided for descriptive purposes into three parts; the nasopharynx, oropharynx and laryngopharynx.

(i) The nasopharynx (nasal part of the pharynx) lies behind the nasal cavities, above the soft palate. The Eustachian tube (also called auditory tube) connects nasopharynx with the middle ear.

(ii) The oropharynx (oral part of the pharynx) lies behind the oral cavity (buccal cavity). The nasopharynx and oral cavity open into the oropharynx which is a com­mon passage for both food and air.

(iii) The laryngopharynx (laryngeal part of the pharynx), is the most inferior portion of the pharynx. It leads into the oesophagus behind and into the larynx in front.

Function:

The pharynx is a common passage for food and air.

Waldeyer’s Ring:

The lymphatic tissues of the pharynx and oral cavity are arranged in a ring like manner, which are collectively called Waldeyer’s ring (- Waldeyer’s lymphatic ring).

The ring mainly consists of the following:

(i) Pharyngeal Tonsil is attached to pharynx. In children pharyngeal tonsil may become enlarged and is then referred to as the adenoids. The resulting swelling may be a cause of obstruction to normal breathing.

(ii) Tubal Tonsils are situated around the Eustachian tube.

(iii) Palatine Tonsils are attached to the palate. The palatine tonsils are often in­fected (tonsillitis) leading to sore throat. Such enlarged tonsils may become a focus of infection and their surgical removal (ton­sillectomy) becomes necessary.

(iv) Lingual Tonsil is attached to pharyngeal part of the tongue.

All these lymphoid tissues are active in production of immunoglobin. A which forms an important part of our immune system.

III. Oesophagus:

The human oesophagus or food pipe is about 25 cm long. It lies behind the trachea and the heart. It comprises three parts: cervical part in the neck, thoracic part in the thorax and abdominal part in the abdomen. The oesophagus passes through the diaphragm and opens into the stomach.

Function:

The oesophagus transfers food from the pharynx to the stomach.

IV. Stomach (= Gaster):

It is the wid­est organ of the alimentary canal. The stomach is J-shaped organ. The lesser cur­vature lies on the posterior surface of the stomach. The greater curvature is on the anterior surface of the stomach.

The fold of peritoneum which attaches the stomach to the posterior abdominal wall extends be­yond the greater curvature. This is called the greater omentum which stores fat. The stomach has four parts: cardiac part, fundus, body and pyloric part.

(i) Cardiac Part (= cardia):

It is so called because it is present near the heart. The gastro esophageal sphincter (= car­diac sphincter) lies in the opening between oesophagus and stomach. It is not a true valve. It is a functional sphincter.

(ii) Fundus:

It is commonly filled with air or gas.

(iii) Body:

It is the main part of the stomach.

(iv) Pyloric Part (Pylorus):

It is the posterior part of the stomach.

The pyloric part is divided into the pyloric antrum and the pyloric canal. The latter opens into the duodenum. The pyloric sphincter guards the opening between the stomach and the duodenum and periodically permits partially digested food to leave the stomach and enter the duodenum.

Functions of the Stomach:

It stores food for some time. It churns and breaks up food and mixes the pieces with gastric juice. Partial digestion of food (proteins and fats) takes place here. It produces Castle’s intrinsic factor (a glycoprotein) which is necessary for the absorption of vitamin B₁₂ to be absorbed in the intestine.

It secretes pro-enzymes— pepsi­nogen and pro-rennin and enzymes gastric lipase and gastric amylase. It also secretes gastrin (hormone). Alcohol, aspirin, some lipid-soluble drags, moderate amounts of sugar and water are absorbed by the stomach wall.

V. Small Intestine:

It is so named because it has small diameter. Length is correlated with the height of the individual but not with weight. It is the longest part of the alimentary canal. It is about 6.25 metres long. It comprises three parts; duodenum, jejunum and ileum.

(i) Duodenum:

It is so called because it is about as long as the breadth of 12 fingers. It is about 25 cm long and is the shortest, widest part of the small intestine. It is somewhat С-shaped. The hepatopancreatic ampulla (ampulla of Vater) opens into the duodenum. This ampulla receives both bile duct from the liver and main pancreatic duct from the pancreas. Iron is mainly absorbed in the duodenum.

(ii) Jejunum:

It has a diameter of about 4 cm. Its wall is thick. It is redder and more vascular. It is the middle part of the small intestine and is about 2.5 metres long.

(iii) Ileum:

It has a diameter of 3.5 cm. Its wall is thinner than that of the jejunum. It is the longest part of small intestine and is about 3.5 metres long. Both the jejunum and ileum are greatly coiled. They are suspended by mesentery.

Small nodules of lymphatic tissue can be seen along the entire length of the small intestine. In some places, particularly along the ileum, these nodules are clustered together in groups called Peyer’s patches or lymph nodules.

Peyer’s patches are a distinguishing characteristic of the ileum, which produce lymphocytes (type of WBCs). Finger-like projec­tions of the mucosa, the villi are present in the small intestine. Villi are absent over the Peyer’s patches.

The villi increase the surface of the small intestine. Each villus is covered with epithelium and contains a lymph capillary (lacteal) and blood capillaries. The entire small intestine has circular folds of the mucous membrane, the plicae circulares (‘Valves’ of Kerkring). These folds are more prominent in the jejunum. They further increase the absorptive surface considerably.

Functions of the small intestine:

The small intestine completes digestion of proteins, carbohydrates, fats and nucleic acids. It absorbs nutrients into the blood and lymph. It secretes certain hormones such as cholecystokinin, secretin, enterogastrone, duocrinin, enterocrinin and villikinin. It also secretes digestive enzymes.

VI. Large Intestine:

Its diameter is larger than that of the small intestine. Hence it is so named. It is about 1.5 metres long and is divisible into three parts; caecum, colon and rectum.

(i) Caecum and vermiform appendix:

The caecum is a pouch-like structure which is about 6 centimetres. The vermiform appendix (commonly called the appendix) is an outgrowth of the caecum.

It is a slightly coiled blind tube of about 8 centimetres long. Its wall contains prominent lymphoid tissue. Appendix is thought to be vestigial. The inflam­mation of vermiform appendix is called appendicitis. The caecum and appendix are well developed in herbivorous mammals like rabbits.

(ii) Colon:

The caecum leads into the colon, which is divided into four regions; the ascending, transverse, descending and sigmoid colon (pelvic colon is its former name). Ascending colon is the shortest part of the colon. The colon has three longitudinal bands called taeniae coli and small pouches called haustra (sing, haustrum).

(iii) Rectum:

The sigmoid colon opens into the rectum. The rectum comprises the last 20 centimetres of the digestive tract and terminates in the 2-centimetre long anal canal. The opening of the anal canal is called anus.

The anus has an internal anal sphincter composed of smooth muscle fibres and an external anal sphincter comprised of striped (voluntary) muscle fibres. Structures formed due to enlargements of veins of anal columns in anal canal as well as anus are called haemorrhoids or piles.

Functions of the large intestine:

The chief functions of the large intestine are the absorption of water and the elimination of solid wastes. However, moderate quantities of vitamin К and vitamin В complex are manufactured by bacteria in the large intestine.

Histology of Human Gut (Alimentary Canal):

The wall of alimentary canal consists of four basic layers.

From the outer surface inward to the lumen (cavity) the layers are as follows:

1. Visceral peritoneum (= Serosa):

It is made up of squamous epithelium and areolar connective tissue. It is continuous with the mesentery. Since the oesophagus lies outside the coelom, its outer wall is not covered by peritoneum (serosa) but by an irregular coat of dense elastic fibrous connective tissue called adventitia external ( = external adventitia).

2. Muscularis (Muscular coat):

It is composed of outer longitudinal and inner cir­cular muscle fibres. In the stomach an additional layer of oblique muscle layer is found inner to the circular muscle fibres.

These muscle fibres are un-striped (smooth). In between the longitudinal and circular muscle fibres there is a network of nerve cells and parasym­pathetic nerve fibres, called the Auerbach’s plexus (= myenteric plexus). The Auerback s plexus controls peristalsis.

3. Sub-mucosa:

It consists of loose connective tissue richly supplied with blood and lymphatic vessels and in some areas with glands. Another network of nerve cells and sympathetic nerve fibres, called Meissner’s plexus (= sub-mucosal plexus) is present be­tween the muscular coat and the mucosa. This plexus controls the secretion of intestinal juice.

4. Mucosa (= Mucous membrane):

It is the innermost layer lining the lumen of the alimentary canal. It is so named because it secretes mucus to lubricate the inner lining of the gut. This layer forms irregular folds (rugae) in the stomach.

Mucosa is composed of three layers:

(i) The muscular is mucosa consists of outer longitudinal and inner circular muscle fibres, both are un-striped.

(ii) The lamina propria consists of loose connective tissue, blood vessels, glands and some lymphoid tissue.

(iii) The epithelium forms gastric glands in stomach, and villi and intestinal glands in small intestine.

In upper one third of the oesophagus both Auerbach and Meissner’s plexuses are absent.

Part # 2. Digestive Glands

I. Salivary Glands (Fig. 16.10):

Salivary glands discharge their secretion into the oral cavity. In man, the salivary glands are three pairs— parotid, sublingual and submandibulor glands,

(i) Parotid glands. These are the largest salivary glands which are situated near the ears. Their ducts open into the oral cavity near the upper second molars. The duct of parotid gland is called Stenson’s duct,

(ii) Sublingual glands. These are smallest salivary glands which are located beneath the tongue and their ducts called sublingual ducts or ducts of Rivinus which open into the floor of the oral cavity,

(iii) Submandibular (also called sub maxillary) glands.

These are me­dium sized salivary glands which are located at the angles of the lower jaw. Their ducts open into the oral cavity near the lower central incisors.

The duct of submandibular gland is called Wharton’s duct. The parotid salivary glands secrete much of salivary amylase or a-amylase (= ptyalin). Sub-lingual and sub-mandibular salivary glands secrete salivary amylase and mucus. Salivary amylase is absent in herbivores.

The disease mumps is a viral infection that may involve one or both parotid salivary glands. The fluids secreted by the salivary glands constitute saliva. Saliva is slightly acidic (pH 6.8). About 1,000-1500 ml of saliva is secreted per day.

Saliva is a mixture of water and electrolytes (Na⁺, K⁺, CI^–, HC0₃^– ), derived from blood plasma, mucus and serous fluids (watery constituent of saliva), and salivary amylase or ptyalin (enzyme) and lysozyme (antibacterial agent). Ions of thyocyanate are also present in the saliva.

II. Gastric Glands (Fig. 16.11):

These are numerous microscopic, tubular glands formed by the epithelium of the stom­ach. Gastric glands have three major types of cells.

(i) Chief cells or Peptic cells (= Zymogenic cells) are usually basal in location and secrete gastric digestive enzymes as pro-enzymes or zymogens; pepsinogen and pro-rennin.

The chief cells also produce small amount of gastric amylase and gastric lipase. Gastric amylase action is inhibited by the highly acid condition. Gastric lipase con­tributes little to digestion of fat. Pro-rennin is secreted in young mammals. It is not secreted in adult mammals.

(ii) Oxyntic cells (= Parietal cells) are large and are most numerous on the side walls of the gastric glands. They are called oxyntic cells because they stain strongly with eosin. They are called parietal cells as they lie against the basement membrane. They secrete hydrochloric acid and Castle intrinsic factor.

(iii) Mucous cells (= Goblet cells) are present throughout the epithelium and secrete mucus.

The secretions of these cells form gastric juice with pH 1.5-2.5 (very acidic). Infant’s gastric juice pH is 5.0. About 2,000-3,000 ml of gastric juice is secreted per day. The gastric juice contains two pro-enzymes— pepsinogen (pro-pepsin) and pro-rennin, and enzymes gastric lipase and gastric amylase, and mucus and hydrochloric acid.

The epithelium of gastric glands also has the following two types of cells:

(i) Endocrine cells are usually present in the basal parts of the gastric glands. These are argentaffin cells and Gastrin cells (= G-cells). Argentaffin cells produce serotonin (its precursor is 5-hydroxytryptamine, 5-HT), somatostatin and histamine. Gastrin Cells (= G-cells) are present in the pyloric region and secrete and store the hormone gastrin.

Serotonin is a vasoconstrictor and stimulates the smooth muscles. Somatostatin suppresses the release of hormones from the digestive tract. Histamine dilates the walls of blood vessels. Gastrin stimulates the gastric glands to release the gastric juice.

(ii) Stem cells are undifferentiated cells that are also present in the epithelium of the gastric glands. They multiply and replace other cells. They increase in number when the gastric epithelium is damaged (e.g., when there is a gastric ulcer) and play an important role in healing.

III. Liver (= Hepar):

It is the largest gland of the body. The liver lies in the upper right side of the abdominal cavity just below the diaphragm. It is heavier in males than females. In males it generally weighs 1.4-1.8 Kg and in females 1.2-1.5 Kg.

The liver is divided into two main lobes— right and left lobes separated by the falciform ligament. The latter is a membrane that is continuous with the peritoneum. The right lobe of the liver is further differentiated into right lobe proper, a quadrate lobe and a caudate lobe on the posterior surface.

Internally, the structural and functional units of liver are the hepatic lobules containing hepatic cells arranged in the form of cords. Each lobule is covered by a thin connective tissue sheath called the Glisson’s capsule. Glisson’s capsule is the characteristic feature of mammalian liver. The mammalian liver also contains Kupffer cells that are phagocytic cells and eat worn out WBCs, RBCs and bacteria.

Fat storage cells are also present. The plates of liver cells are separated from the endothelial lining of the sinusoid by a narrow perisinusoidal space of Disse. Some fat cells may also be seen in the space of Disse. Blood vessels and bile ductules present in the portal canals are surrounded by a narrow space of Mall.

Bile is secreted by the liver cells (hepatocytes). Bile enters bile canaliculi or bile capillaries (a net work of tubular spaces between the liver cells). The bile canaliculi empty into small Hering’s canals walled by cuboidal epithelium. These canals pour bile into interlobular bile duct (=bile ductule) walled by columnar epithelium.

Gall Bladder:

A pear shaped sac like structure is attached to the posterior surface of the liver by connective tissue. It stores bile secreted by the liver. Rat and horse do not have gall bladder.

Ducts:

The right and left hepatic ducts join to form the common hepatic duct. The latter joins the cystic duct which arises from the gall bladder. The cystic duct and common hepatic duct join to form bile duct which passes downwards posteriorly to join the main pancreatic duct to form the hepatopancreatic ampulla (= ampulla of Vater).

The ampulla opens into the duodenum. The opening is guarded by the sphincter of Oddi. The sphincter of Boyden surrounds the opening of the bile duct before it is joined with the pancreatic duct.

Blood Supply (Fig. 16.15):

Blood enters the liver from two sources. From the hepatic artery it gets oxygenated blood and from the hepatic portal vein it receives deoxygenated blood. Blood in the hepatic artery comes from the aorta. Blood in the hepatic portal vein comes directly from the intestine containing newly absorbed nutrients. The hepatic portal vein also brings blood from the spleen to the liver. Liver has high power of regeneration.

Functions of Bile:

Bile is a watery greenish fluid mixture containing bile pigments, bile salts, cholesterol and phospholipids.

Bile serves the following functions:

(i) Neutralization of HCI:

Its sodium bicarbonate neutralizes HC1 of chyme (semi-fluid food that comes from the stomach).

(ii) Emulsification:

Sodium glycocholate and sodium taurocholate break the large fat droplets into the smaller ones. This process is called emulsification.

(iii) Absorption of fat and fat-soluble vitamins:

Its salts help in the absorption of fat (fatty acids and glycerol) and fat-soluble vitamins (A, D, E and K) in the small intestine.

(iv) Excretion:

Bile pigments (bilirubin and biliverdin) are excretory products.

(v) Prevention of Decomposition:

Bile is alkaline hence it prevents the decomposition of food by preventing the growth of bacteria on it.

(vi) Stimulation of Peristalsis:

Bile increases peristalsis of the intestine.

(vii) Activation of Lipase:

Bile contains no enzyme but activates the enzyme lipase.

Obstruction of the hepatic or bile duct by gall stones or due to other causes is common. Jaundice occurring as a result of such obstruction is called obstructive jaundice. In this disease the bile is absorbed into the blood instead of going to the duodenum and cause yellowing of eyes and skin.

IV. Pancreas (Fig. 16.12 & 16.16):

The pancreas is soft, lobulated, greyish- pink gland which weighs about 60 grams. It is about 2.5 centimetres wide and 12 to 15 centimetres long, located posterior to the stomach in the abdominal cavity.

External Structure of Pancreas:

The Pancreas comprises the head, neck, body and tail. The head lies in the curve of the duodenum, the neck follows the head, the body behind the stomach and the tail reaches the spleen lying in front of the left kidney.

The main pancreatic duct (= duct of Wirsung) is formed from smaller ducts within the pancreas. The main pancreatic duct opens into the hepatopancreatic ampulla (= ampulla of Yater). An accessory pancreatic duct (= duct of Santorini) is also present in the pancreas and opens directly into the duodenum.

Internal Structure of Pancreas:

It consists of two parts: exocrine part and endocrine part.

(i) Exocrine part:

The exocrine part of the pancreas consists of rounded lobules (acini) that secrete an alkaline pancreatic juice with pH 8.4. About 500-800 ml of pancre­atic juice is secreted per day. The pancreatic juice is carried by the main pancreatic duct into the duodenum through the hepatopancreatic ampulla.

The accessory pancreatic duct di­rectly pours the pancreatic juice into the duodenum. The pancreatic juice contains sodium bicarbonate, three pro-enzymes; trypsinogen, chymotrypsinogen and procarboxypeptidase and some enzymes such as elastase, pancreatic a-amylase, DNase, RNase and pancre­atic lipase. The pancreatic juice helps in the digestion of starch, proteins, fats and nucleic acids.

(ii) Endocrine part:

The endocrine part of the pancreas consists of groups of islets of Langerhans. The human pancreas has about one million islets. They are most numerous in the tail of the pancreas. Each islet of Langerhans consists of the following types of cells which secrete hormones to be passed into the circulating blood.

(a) Alpha cells (= α-cells):

These cells are more numerous towards the periphery of the islet and constitute 15% of the islet of Langerhans. They produce glucagon hormone which converts glycogen into glucose in the liver. Thus glucagon is diabetogenic hormone.

(b) Beta cells (= β-cells):

These cells are more numerous towards the middle of the islet and constitute 65% of the islet of Langerhans. They produce insulin hormone which converts glucose into glycogen in the liver and muscles. Deficiency of insulin causes dia­betes mellitus.

(c) Delta cells (= δ-cells):

These cells are also found towards the periphery of the islet of Langerhans and constitute 5% of the islet of Langerhans. They secrete somatostatin (SS) hormone which inhibits the secretion of glucagon by alpha cells and secretion of insulin by beta cells. This hormone also slows absorption of nutrients from the gasrointestinal tract.

Somatostatin secreted by argentaffin cells of gastric and intestinal glands suppresses the release of hormones from the digestive tract. Somatostatin is also secreted by the hypo­thalamus of the brain where it inhibits the release of growth hormone (somatotropin) by the ante­rior lobe of pituitary gland. That is why it is also called growth inhibitory hormone.

(d) Pancreatic polypeptide cells (= PP cells or F-cells):

Apart from the three main types of cells described above, the PP cells are also present in the pancreas, which constitute 15% of the Islet of Langerhans. These cells secrete pancre­atic polypeptide (PP) which inhibits the release of pancreatic juice. Thus the pancreas performs two main func­tions i.e., secretion of pancreatic juice which contains digestive enzymes and production of hor­mones.

V. Intestinal Glands (Fig. 16.17):

These are formed by the surface epithelium of the small intestine. These are of two types: crypts of Lieberkuhn and Brunner’s glands.

(i) The crypts of Lieberkuhn are simple, tubular structures which occur through­out the small intestine between the villi. They secrete digestive enzymes and mucus. The mucus is secreted by the goblet cells (= mucous cells) whereas water and electrolytes are secreted by enterocytes present on the intestinal crypts. These crypts have at the base paneth cells and argentaffin cells.

(a) Paneth cells are found particularly in the duodenum. These cells are present in the bottom of crypts of Lieberkuhn. These cells are rich in zinc and contain acidophilic granules. The function of these cells is not certain but there is evidence that they secrete lysozyme (antibacterial substance). Paneth cells are also capable of phagocytosis.

(b) Argentaffin cells synthesize secretin hormone and 5-hydroxytryptamine (5-HT).

(ii) The Brunner’s glands are found only in the duodenum and are located in the submucosa. They secrete a little enzyme and mucus. The mucus protects the duodenal wall from getting digested. Digestion of most of nutrients takes place in the duodenum under the action of various enzymes. The Brunner’s glands open into the crypts of Lieberkuhn.

The secretion of intestinal glands is called intestinal juice or succus entericus with pH 7.8. About 2,000-3,000 ml of intestinal juice is secreted per day. The intestinal juice contains many enzymes— maltase, isomaltase, sucrase, lactase, α- dextrinase, enterokinase, aminopeptidases, dipeptidases, nucleotidases, nucleosidases and intestinal lipase.

In addition to the glands mentioned above the entire alimentary canal has mucous glands that produce mucus. The mucus lubricates the digestive tract and food. Human digestive system has many accessory organs. Tongue, salivary glands, liver, gall bladder and pancreas are some important human accessory digestive organs.

Swallowing or Deglutition (Fig. 16.18):

The food is tasted in the oral cavity and mixed with saliva. Tongue manipulates food during chewing and mixing with saliva. This collection of food, the bolus (mass of food) is then pushed inward through the pharynx into the oesophagus.

This process is called swallowing or deglutition. Swallowing involves coordinated activity of tongue, soft palate, pharynx and oesophagus.

Swallowing is conveniently divided into three stages:

(i) The Voluntary stage:

The tongue blocks the mouth. The bolus is forced to move from the oral cavity into the pharynx (oropharynx). This represents the volun­tary stage of swallowing.

(ii) The Pharyngeal stage:

With the passage of the bolus into the pharynx, the invol­untary pharyngeal stage of swallowing begins. The palate closes off the nose and the epiglottis seals off the glottis of larynx. Thus breathing is temporarily interrupted. The bolus is passed from the pharynx into the oesophagus.

(iii) The Oesophageal stage:

This also represents the involuntary stage of swallowing. The bolus passes through the laryngopharynx and enters the oesophagus in 1 to 2 seconds. The respiratory passage then reopens and breathing resumes. Swallowing is controlled by a swallowing centre located in the medulla oblongata and lower pons varolii of the brain.

Peristalsis:

During the oesophageal phase of swallowing, food is pushed through the oesophagus by involuntary muscular movements called peristalsis.

Peristalsis is produced by involuntary contraction of circular muscles in the oesophagus lying just above and around the top of the bolus and simultaneous contraction of the longitudinal muscles lying around the bottom of and just below the bolus.

Contraction of the longitudinal muscles shortens the lower part of the oesophagus, pushing its walls out­ward so that it can receive the bolus. After this circular muscles of the oesophagus relax. The contractions are repeated in a wave that moves down the oesophagus, pushing the food towards the stomach. There is least peristaltic movement in the rectum of human being.