On studying the transverse section of the oesophagus the following structures are visible.
ii. Below the fibrous coat, there is a layer of external longitudinal muscle, which, in transverse section is visible in the form of a large number of bundles.
iii. Circular muscles, forming the third layer, are in the form of long stripes.
iv. There is also a large inner longitudinal muscle.
v. Lying below the inner longitudinal muscle is situated a connective tissue - layer having mucous glands and blood vessels.
vi. Inner layer consists of muscularis mucosae.
vii. Innermost layer consists of stratified epithelium, having lumen on the inner side and the connective tissue on its outer side.
i. A transverse section of the stomach of Rabbit shows that it consists of four coats: (a) Serous, (b) muscular (c) submucous and (d) mucous coats.
ii. The serous coat is derived from peritoneum and is the outermost covering of the intestine. It is made up of an outer layer of squamous epithelium and an inner layer of connective tissue.
iii. Muscular coat consists of an outer layer of longitudinal muscle and an inner layer of circular unstriped muscle fibres. At the cardiac end of the stomach there is a third muscular coat, the fibres of which are situated in the inner oblique direction.
iv. The next inner layer is sub-mucosa, which is a layer of loose areolar connective tissue having many blood vessels.
v. Outside the mucous coat is a layercalled muscularis mucosae,which is a double muscular layer, having on the outside, longitudinal muscle fibres and in the innerside circular muscle fibres. Muscularis mucosae are highly developed in the stomach rather than in the intestine.
vi. Mucosa coat of the stomach consists of gastric glands closely packed together. The walls of the gastric glands are glandular,therefore, they produce gastric juice. When the transverse section of the stomach is through cardiac part, the gastric glands are called cardiac glands. These contain only mucus-
secreting cells. In the fundic part as described by some authors, there are three kinds of cells called
a) mucus-secreting cells,
b) Zymogen or pepsin secreting cells and
c) Oxyntic cells, which produce HCI.
i. Histologically the duodenum is also formed of four coats: serous, muscular, sub-mucous and mucous coats.
ii. However the mucous coat consists of columnar epithelial cells covering the inner surface of the intestine and a layer of connective tissue containing lymphoid tissue and numerous blood - vessels.
It has numerous finger - like projections called Villi. The function is to increase the absorptive surface. In the mucous coat between the villi are situated simple tubular glands called crypts of Lieberkuhn. Embedded in the mucous coat of the duodenum are small glands known as Brunner's glands, whose ducts pass inwards to open on the surface of the mucous coat. Each Brunner's gland is a convoluted tube lined with columnar epithelium. The crypts of Lieberkuhn and Brunner's gland produce the intestinal juice or saccus entericus.
The structure of the large intestine resembles in most respects with the other parts of duodenum, but there are neither villi nor Brunner's glands.
Histology of Liver
Liver is one of the most important digestive glands in the body.Histologically liver consists of groups of polygonal lobules. These are separated from one another by their membranes or septa possessing connective tissue, thus, forming Glisson's capsule.
Liver cells in the lobules are located so as to form radial hepatic cords between which blood-capillaries or sinusoids are situated. Branches of the hepatic portal vein are found around the lobules so as to form interlobular veins. In the centre of each lobule is an intralobular vein, which joins to form the hepatic vein. There are also some special types of cells called Kupffer cells, situated in the wall of the sinusoids. These cells serve to remove the bacteria from the liver.
Functions of Liver
The structure of liver and its role in digestion have already been dealt with. However, the functions of liver can be summarised as follows.
- As already mentioned, it secretes bile, which is a bitter alkaline fluid and is dark-green in colour due to the presence of decomposed products of haemoglobin of the old and worn-out red blood corpuscles. Bile has several salts both organic and inorganic as well as some waste substances. The functions of the bile are as follows:
a.Bile is responsible for emulsification of fats. Thus, fat is broken into minute droplets, which can remain suspendedin a fluid medium.
b.Bile contains bile salts and bile pigments.
c.The bile salts are bicarbonate, glycocholate, taurocholate of sodium.
d.Sodium bicarbonate neutralises the acidity of chyme.
e.Glycocholate and taurocholate of sodium break down fats
into smaller globules to form emulsion.
f.Bile pigments are bile verdin and bile rubin.
g.The green colour of bile juice is only due to the presence of bile pigments.
h.Bile salts are necessary for absorption of vitamin 'K' and other fat soluble vitamins.
- Liver helps the storage of vitamins 'A' and 'D' out of which the former is manufactured in the liver.
- Liver forms lymph and it is responsible for the destruction of worn-out red blood corpuscles. So it is haemolytic organ.
- It is the heat-producing centre of the body. The heat is well maintained in the body.
- Liver also produces fibrinogen, which is responsible for clotting of blood. Without clotting of blood, we cannot recover from injuries. Clotting is possible only by vitamin 'K' which reaches the liver where it is converted into prothrombin, responsible for clotting.
- Liver produces heparin, so as to prevent clotting and thus blocking the arteries of the heart and blood vessels of the brain.
- Ammonia, which is poisonous, is converted into urea, it is ultimately removed by the kidneys.
- It changes amino-acids into albumins, which regulate the balance of salt water without which we cannot survive.
- Liver removes the excess of the sugar by changing it into glycogen, which splits up and is used during the days of food scarcity.
- Kupffer cells found in the liver capture, engulf and destroy germs of diseases.
- Liver is a storage organ of iron and copper.
Thus the liver is of great importance to the animal as it serves as a digestive gland, as a storage organ and helps removal of excretory waste products.