What is Stomach
Single oval,elongated,unilobed present within abdominal cavity below diaphragm.It consists of three parts as cardiac/fundic(anterior), corpus/body(middle,chief part) and pyloric(posterior part) in human, whereas in rabbit stomach is bilobed and consists of three parts as cardiac(Anterior),fundic(middl ,chief part) and pylorus(posterior).Two types of valves are present in the stomach viz. Cardiac sphincter valve between oesophagus and stomach and pyloric sphincter valve between stomach and duodenum.In new born baby cardiac sphincter is much less developed that is why regurgitation of gastric content is very comman .Inner surface of stomach is raised into numerous longitudinal folds called gastric rugae. In case of ruminant mammals (cud chewing mammals) oesophagus consists of only skeletal or voluntary muscles.
Outermost layer is serosa.Muscular layer is three layered with outer longitudinal,middle circular and inner oblique.Muscles are involutionary and unstriated.Epithelial lining is made up of simple columnar epithelial cells and specialized cells present in the gastric glands. The nomenclature of gastric glands is according to the parts of the stomach.Various tyeps of gastric glands and the cells present in them are as follows.
Cardiac gastric glands in rabbit and human.Cells present are mucous neck cells secreting mucous.
Fundic gastric/Main gastric glands in rabbit and corpus in human has at least four distinct types of cells.
(a) Peptic or zymogenic or chief or central cells:
Secretes two digestive proenzymes pepsinogen and prorennin.
(b)Oxyntic or parietal cells:
Secretes HCI and castl's intrinsic factor required for the absorption of vitamin B12 .Hyperacidity is abnormally high degree of acidity due to the secretion of large quantity of HCI i.e.,gastric juice.
(c)Mucous neck cells:
Secretes alkaline mucous.
(d)Argentaffin cells or Kultchitsky or enterochromaffin cells:
Responsible for the secretion of vasoconstrictor seratonin.
Pyloric gastric glands in rabbit and human-cells are mucous neck cells secreting mucous and some cell, called 'gastrin' or 'G' cells, secretes a hormone, named gastrin, which increases the motility of gastric wall and stimulates gastric glands for active secretion.
Function of stomach
(i)Storage of food.
(ii)Trituration or churning of food to mix with gastric juice.
(iii)Functions of gastric juice(discussed along with gastric juice).
Stomach of ruminants(cud-chewing mammals):
The stomach of cattles have four parts,as remen(paunch), reticulum(honeycomb), omasum(psalterium) and abomasum(nennet).Some authors believe that first three chambers are parts of the oesophagus,the fourth chamber is the real stomach secreting 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.