Alimentary canal

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. The oral cavity has a number of teeth and a muscular tongue. Each tooth is embedded in a socket' of jaw bone. This type of attachment is called the codont.
Majority of mammals including human being forms two sets of teeth during their life, a set of temporary milk or deciduous teeth replaced by a set of permanent or, adult teeth. This type of dentition is called diphyodont. An adult human has 32 permanent teeth which are of four different types (Heterodont dentition), namely, 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 furmula which inhuman is 2123/2123/ The hard chewing surface of the teeth, made up of enamel, helps in the mastication of food. 

Differeritiation of teeth: Morphologically, teeth can be distinguished as homodont or heterodont.

(i) Homodont: When all the teeth are structurally and functionally similar, Examples - Vertebrates except metatherian and eutherian mammals.

(ii) Heterodont: When the teeth are different in structure and functions. They are distinguished into four types incisors, canines, premolars and molars. Examples - metatherian and eutherian mammals.

(a) lncisors: These are the front teeth borne by the premaxillae in upper jaw and tips of dentaries in lower jaw. They are single-rooted monocuspid and lung, curved and sharp-edged. They are adapted for cutting or cropping and biting.

(b) Canines: There is one pointed canine in each maxillary of upper jaw and each dentary of lower jaw next to the incisors. They are meant for piercing, tearing and offence and defence. They are single rooted and monocuspid.

(c) Premolars: They have one root (only in upper first PM two roots) and two cusps (bicuspid). They are meant for crushing, grinding and chewing.

(d) Molars: They have more than two roots (upper molars have three roots and lower molars. have two roots) and 4 cuspid.

Structure of teeth: Teeth is divided into three parts

(i) Root: Inner most, attached to the bone with help of cement (hyaluronic acid).

(ii) Neck: Middle, small, covered with gum. Gum provides strength to the teeth.

(iii) Apex or crown: External exposed part of teeth. Longest part, white in colour.  

The tongue is a freely movable muscular organ attached to the floor, of the oral cavity by the frenulum. The upper surface of the tongue has small projections called papillae, some of which bear taste buds.

Taste papillae are of following types –

(i) Circumvallate: Circular largest 8-12 in number, present in the posterior part of the tongue extending from one side to another. They possess taste buds. These are the largest of all the papillae.

(ii) Fungi form: Mushroom shaped (Fungi - shaped), numerous, present at the anterior margins and tip of the tongue. They have 200 taste buds.

(iii) Foliate: Leaf like flat, less 8-10 in number, present at the posterior margin of the tongue. They are absent in human and found in rabbit.

(iv) Filiform: Conical shaped, smallest and most numerous distributed throughout tongue. They are without taste buds.

Hence, in human taste is recognized with the help of circumvallate and fungiform taste papillae. In man the anterior end of tongue feels sweet taste, posterior part feel bitter taste, sides feel sour taste and a small part behind the anterior end feel salty taste.

The oral cavity leads into a short pharynx which serves as a common passage for food and air.

The oesophagus and the trachea (wind pipe) open into the pharynx.

A cartilaginous flap called epiglottis prevents the entry of food into the glottis - opening of the wind pipe during swallowing.

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. A muscular sphincter (gastro-oesophageal) regulates the opening of oesophagus into the stomach.

The stomach, located in the upper left portion of the abdominal cavity, has three major parts - a cardiac portion into which the oesophagus opens, a fundic region and a pyloric portion which opens into the first part of small intestine. 

Stomach of ruminants (cud-chewing mammals): The stomach of cattles have four parts, as rumen (paunch), reticulum (honeycomb), omasum (psalterium) and abomasum (rennet). Some authors believe that first three chambers are parts of the oesophagus, the fourth chamber is the real stomach secretes HCI and enzymes. The embryological studies have proved that all the chambers are parts of the real stomach. Camel and deer lack omasum. Reticulum is the smallest part and its cells are provided with water pockets for the storage of metabolic water.

In the rumen, food undergoes mechanical and chemical breakdown. Mechanical breakdown results from through churning brought about by muscular contractions and aided by cornified surface of villi. Chemical breakdown is caused by symbiotic microorganisms (bacteria and ciliates) that release enzyme cellulase, which act on cellulose and simplify it into short-chain fatty acids, such as acetic acid, butyric acid, propionic acid. This is called microbial digestion.

Small intestine is distinguishable into three regions, a 'U' shaped duodenum, a long coiled middle portion jejunum and a 'highly coiled ileum. The opening of the stomach into the duodenum is guarded by the pyloric sphincter ileum opens into the large intestine. It consists of caecum, colon and rectum. Caecum is a small blind sac which hosts some symbiotic micro-organisms. A narrow finger­like tubular projection, the vermiform appendix which is a vestigial organ, arises from the caecum. The caecum opens into the colon. 
Glands of small intestine: Various glands found in small intestine. Each gland has three types of cells:

(1) Undifferentiated epithelial cell

(2) Zymogenic cell (paneth cell) and

(3) Argentaffin (Enterochromaffin cell). 

and a descending part. The descending part opens into the rectum which opens out through the anus.

Colon posses two specialized structures as Taeniae coli (present in the middle of colon) and Haustra, (dilated sac-like or pockets like structures surrounding taeniae). Colon is concerned with absorption of water of undigested food, 5%, salts, vitamins etc. Hence concerned with faeces formation. Colon bacteria also synthesized vit. B₁₂ and K.

Rectum has strong sphincter muscle in its wall. The sphincter keeps the canal as well as anus, closed when not used for defecation. Anal canal connects rectum with anus and it is about 3 cm long. Anus is the terminal inferior opening, of alimentary canal, which is guarded by an internal involuntary sphincter and an external voluntary sphincter.

The wall of alimentary canal from oesophagus to rectum possesses four layers namely serosa, muscularis, sub-mucosa and mucosa.

Serosa is the metermost layer and is made up of a thin mesothelium (epithelium of visceral organs) with some connective tissues.

Muscularis is formed by smooth musdes usually arranged into an inner circular and an outer longitudinal layer. An oblique muscle layer may be 'present in some regions.

The submucosal layer is formed of loose connective tissues containing nerves, blood and lymph vessels. In duodenum, glands are also present in sub­mucosa.

The innermost layer lining the lumen of the alimentary canal is the mucosa. This layer 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 enormously. 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 also forms glands in the stomach (gastric glands) and crypts in between the bases of villi in the intestine (crypts of Lieberkuhn).

All the four layers show modifications in different parts of the alimentary canal. 

• DIGESTIVE GLANDS

The digestive glands associated with the alimentary canal include the salivary glands, the liver and the pancreas.

Saliva is mainly produced by three pairs of saliv glands, the parotids (cheek), the sub-maxillary sumandibular (lower jaw) and the sublinguals (bel the tongue). These glands situated just outside buccal cavity secrete salivary juice into the buccal cavity. 

The secretion of salivary glands is called saliva or salivary juice.

Salivary juice and its enzymes:-

Makes the medium slightly acidic for the action of its enzyme. 

Help in taste detection, deglutition, speaking 'etc 
Gastric glands: There are approximately 35 million of gastric glands present in human stomach gastric gland secretes gastric juice.

Gastric juice and its enzymes:

Inactivates the action of ptyalin.

Makes the medium acidic for the action of gastric enzymes.

HCI kills micro organisms.
HCI kills the living organism (prey etc.) if ingest.

Intestinal glands: Intestinal glands in mammals is a collective name for crypts of Lieberkuhn (secretes alkaline enzymatic juice) and Brunner's glands (secretes mucous), Intestinal glands secrete intestinal juice.

Intestinal juice and its enzymes:

Inhibits the action of gastric enzymes.

Makes the medium alkaline for the action of it's enzymes. 

Pancreas: Single, endodermal, flat, leaf-like yellowish, heterocrine (mixed) gland, present between the ascending and descending limb of duodenum and opens Into duodenum through pancreatic duct.

Exocrine: It 15 the major part (about 99%) of pancreas. The exocrine tissue of the pancreas consists of rounded lobules (acini) that secrete an alkaline pancreatic juice. The juice is carried by the main pancreatic duct, also called duct of Wirsung, into the duodenum through the hepatopancreatic ampulla (ampulla of vator). An accessory pancreatic duel, also named duct of Santorini, may sometimes lead directly into the duodenum.

Endocrine: Minor part (1% only) also called as is lets of Langerhans scattered in the exocrine part. It consist of four various type of cells, as a (A) cells, b (B) cells, d (D) cells and F or PP cells. a- ­cells secretes glucagon hormone, b-cells secretes insulin hormone and cells secrets somatostatin. The PP or F-cells secrete pancreatic polypeptid hormone to control somatostatin. The secretion passes directly into blood.

Liver is the largest gland of the body weighing about 1.2 to 1.5 kg in an adult human. It is situated in the abdominal cavity, just below the diaphragm and has two lobes. The hepatic lobules are the structural and functional units of liver containing hepatic cells arranged in the form of cords. Each lobule is covered by a thin connective tissue sheath called the Glisson's capsule. The bile secreted by the hepatic cells passes through the hepatic ducts and is stored and concentrated in a thin muscular sac called the gall bladder. Tile duct of gall bladder (cystic duct) along with the hepatic duct from the liver forms the common bile duct. 

The bile duct and the pancreatic duct open together into the duodenum as the common hepato­pancreatic duct which is guarded by a sphincter called the sphincter of Oddi.
 

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