The digestive system consists of the alimentary canal along
which the food passes after eating to where the residual wastes are
eliminated from the body, together with the liver and the pancreas.
The digestive system is responsible for the ingestion of food, it's
breakdown into its constituent nutrients and their absorption into
the blood stream, and the elimination of wastes from that
process.
The liver produces bile and is associated with the metabolism of
nutrients together with a number of other functions. The main
function of the pancreas is the production of digestive enzymes and
special compounds called hormones.
The alimentary canal
The alimentary canal is a long tube like organ starting at the
beak at the head and ending with the vent or cloaca in the
abdominal region. Generally the alimentary canal has layers of
muscle running lengthwise and around it and is lined with mucous
membranes. Glands producing important digestive juices are found in
different locations of the canal. The nutrients from the food,
after digestion are absorbed through the wall of the alimentary
canal into the circulatory system for transport to the liver or
other parts of the body. The waste remaining is eliminated from the
body via the cloaca or vent.
Roof and floor of the chicken mouthMouth structure
In the fowl the lips and cheeks are replaced by the beak - an
area of dense and horny skin lying over the mandible and incisive
bones that serve as the bony foundation. There are no teeth. The so
called egg tooth found on the end of the beak of newly hatched
chickens is an aid to their escape from the egg at hatching and
disappears after a day or two. The hard palate, forming the roof of
the mouth, presents a long, narrow median slit that communicates
with the nasal cavity (median - along the middle). The hard palate
has five transverse rows of backwardly pointing, hard, conical
papillae. Numerous ducts of the salivary glands pierce the hard
palate to release their secretions into the mouth cavity.
Salivary glands
A thick layer of stratified squamous epithelium covers the free
surface. The salivary glands run the whole length of the hard
palate, the groups of glands merging to form one mass of glandular
tissue under the epithelium. Lymphoid tissue is found in most
glands.
The salivary glands are:
1.Maxillary - in the roof of the mouth
2.Palatine - on either side of the nasal opening in the roof of
the mouth
3.Apheno-pteryoid glands - in the roof of the pharynx on each
side of the common opening for the eustachian tubes (the eustachian
tubes connect the middle ear to the mouth and their function is to
equalise the air pressure on each side of the tympanic membrane in
the ear)
4.Anterior sub-mandible glands - in the angle formed by the
union of the upper and lower beaks or mandibles
5.Posterior sub-mandibular glands
6.Lingual glands - in the tongue
7.Crico-arytenoid glands - around the glottis
8.A small gland in the angle of the mouth
Pharaynx and tongue
The pharynx is continuous with or follows the mouth. The
combined cavity of the mouth and the pharynx is often referred to
as the oropharynx. The common opening for the two eustachian tubes
is located in the middle of its dorsal wall (roof). The tongue is
long and pointed and conforms to the shape of the beak in which it
operates. The epithelium of the tongue is thick and horny,
especially towards the tip. A transverse row of simple, large and
horny papillae with their tips directed towards the rear of the
mouth cavity are located on the posterior end. The hyoid bone
provides the framework to support the tongue. The entoglossal bone
extends longitudinally in the median plane. small patches of
lymphatic tissue are located throughout the corium. Mucous glands
are located in the tongue with short ducts directed towards the
rear. Some but not others believe that there are taste buds located
on the tongue. In any case the sense of taste appears to be very
weak if at all present.
The mouth has two major functions:
1.To pick up the food particles - this skill is modified by beak
trimming. Birds with normal beaks become very adept at the
manipulation of food particles thus leading to significant food
wastage and one effect of beak trimming is to reduce this
wastage.
2.To direct the food into the oesophagus - as part of the bird's
eating behaviour.
Oesophagus, crop and proventriculus
The oesophagus is wide and is capable of being significantly
stretched. It connects the mouth region to the crop in close assoc
iation with the trachea. The crop is a large dilation of the
oesophagus located just prior to the oesophagus entering the
thoracic cavity. The crop provides the capacity to hold food for
some time before further digestion commences. This capacity enables
the bird to take its food as "meals" at time intervals but permits
continuous digestion. Inside the thoracic cavity, the oesophagus
enters or becomes the proventriculus - a very glandular part of the
digestive tract often called the glandular stomach.
The wall of the oesophagus is composed of four layers of tissue
the innermost being mucous membrane. The mucous membrane is an
important barrier to the entry of microbes and the mucous it
produces is a lubricant that aids the passage of the food along the
alimentary canal. The structure below the crop is similar to that
above except there is less lymphoid tissue below the crop. The crop
structure is similar to that of the oesophagus except there are no
glands present in fowls. Ducks and geese have glands in the crop
mucous membranes. In pigeons the surface cells of the crop slough
off during brooding to form pigeon's milk - used to feed the baby
pigeons in the nest.
Proventriculus
The glandular stomach or proventriculus is relatively small and
tubular. The wall is very thick and is composed of five layers:
1.Outer serous membrane.
2.Muscle layer composed of three separate layers:
Two thin longitudinal layers.
Thick circular layer.
3.Layer of areolar tissue containing blood and lymph
vessels.
4.Thick layer composed mainly of glandular tissue.
5.Mucous membrane.
The glands form the greater part of the thickness of the organ.
Simple single glands group to form lobules each of which converges
into a common cavity near the surface. The cavities converge to
form a common duct that leads to the surface through the apex of a
small papilla (see Fig. 2). These glands produce a number of juices
or enzymes that are used in the digestion or breaking down of food
into it's constituent nutrients (see Table 1). The mucous membrane
is raised into folds and between these folds are numerous simple
tubular glands that produce hydrochloric acid as well as lymphoid
tissue.
Gizzard
The muscular stomach or gizzard is located immediately
succeeding the proventriculus. It is placed partly between the
lobes and partly behind the left lobe of the liver. It has a
flattened, rounded shape somewhat like a convex lens, with one side
slightly larger than the other. Each surface is covered by a
glistening layer of tendinous tissue - thicker at the centre and
becoming thinner towards the edges. Under this outer layer very
powerful masses of red muscle are located. The inner surface is
lined with a creamy coloured, thick, horny tissue raised in ridges.
The gizzard almost always contains quantities of hard objects such
as gravel or other grit that aids in the disintegration of food -
the primary function of the gizzard.
The entrance from the proventriculus and the exit to the
duodenum are close together and dorsal in location. The gizzard
consists of a number of layers of tissues in some of which straight
tubular glands are located. The innermost layer is a strong,
flexible skin able to withstand the potentially damaging effects of
the muscular action grinding the food often in the presence of
stones or other insoluble material. The glands of the gizzard
produce a liquid, keratinised material that passes to the surface
of the horny lining where it hardens to replace tissue worn away by
the grinding action of the organ.
The small intestine
The small intestine begins at the exit from the gizzard and ends
at the junction of the small intestine, caeca and colon. It is
relatively long and has a constant diameter. Of the three parts of
the mammalian small intestine - the duodenum, jejunum and ileum,
only the duodenum can be easily distinguished in the fowl. There is
no clear demarcation between the jejunum and ileum and the small
intestine appears as one long tube. Much of the digestion of the
food and all of the absorption of the nutrients takes place in the
small intestine and hence its structure is quite important. The
structure is as follows:
1.Serosa - a serous membrane on the outside of the
intestine.
2.A layer of longitudinal muscle - fibres run along the length
of the intestine.
3.A layer of circular muscle - three times as thick as the
longitudinal muscle. Located between the two muscle layers are:
Blood vessels.
Lymph vessels.
A network of nerve fibres.
4.An ill-defined sub-mucosa - the areolar of the oesophagus.
5.Mucous membrane consisting of:
A thick muscularis mucosae of longitudinal and circular
muscle.
Corium - many glands, lymphoid tissue, muscle fibres and a
variety of free cells.
Epithelium or surface.
The small intestine has a number of very important
functions:
1.Produces a number of enzymes involved in the digestion
process
2.Site of much of the digestion of the food
3.Site of much of the absorption of food
Villi
When a piece of the small intestine is immersed in water it
takes on a very velvety appearance because of the presence of villi
- long flattened, fingerlike projections extending into the lumen
(inside) of the intestine like flexible fingers. The villi are very
actively involved in the absorption process. A single layer of
columnar epithelium together with goblet cells covers the lining.
The goblet cells secrete mucous. Permanent folds in the mucous
membrane called the "valves of kerkring" are located at the
proximal end (closest to the front) of the duodenum.
A lacteal (lymph vessels), capillaries, bundles of plain muscle
fibres, nerves and other tissues and cells occupy the core of the
villus. The villi have the function of providing a vastly increased
surface area for the more efficient absorption of the nutrients.
The efficiency of the absorption is influenced by the surface area
available for the nutrients to move through - the more villi the
better the absorption. They also provide a means of concentrating
the nutrients collection ability once they have moved through the
intestine wall.
Duodenum
After the duodenum the small intestine forms a coil and is
suspended from the dorsal wall of the abdominal wall by a thin
membrane - the mesentery. This membrane carries the blood vessels
associated with the intestine. The duodenum starts at the gizzard
and forms an elongated loop about 20 centimetres long. The pancreas
lies between the arms of the loop and being attached to each arm of
the duodenum actually holds the two arms together.
Lymphoid tissue in the duodenum is very plentiful and is usually
located in the corium. The lymphoid tissue collects the lymph - the
lymph vessels transport a special fluid other than blood that is
found in the spaces between cells and tissues till it passes into
the blood system. Bile ducts from the gall bladder attached to the
liver and two to three pancreatic ducts enter the small intestine
by a common papilla at the caudal end (closest to the rear) of the
duodenum. The pancreas, a very important organ in the process of
digesting food, is located closely associated with the duodenum
being attached to each side of the duodenal loop and lying between
the two arms.
Jejunum and the ileum
The jejunum and the ileum, together about 120 cm long commence
at the caudal end of the duodenum where the bile and the pancreatic
duct papilla is located and terminates at the ileo-caecal-colic
junction. This junction is where the small intestine, the two caeca
and the colon all meet. This portion of the small intestine is
similar in structure to the duodenum except that:
1.It is suspended in the mesentery
2.The villi are shorter
3.There is less lymphoid tissue
Meckel's Diverticulum is a constant feature about half way along
the small intestine appearing as a small projection on the outer
surface of the small intestine. This projection is where the yolk
sac was attached during the development of the embryo.
Large intestine
The large intestine is very short and does not differ to any
extent from the calibre of the small intestine. It runs in nearly a
straight line below the vertebrae and ends at the cloaca. Sometimes
this section is referred to as the colon and the rectum - the
rectum being the terminal part. The bursa of fabricius is located
immediately above the cloaca of young birds but disappears when the
birds have reached approximately one year old.
Caeca
The two caeca or blind pouches are about 16-18 centimetres long
in the adult. They extend along the line of the small intestine
towards the liver and are closely attached to the small intestine
along their length by the mesentery. Each caecum has three main
parts:
1.A narrow base with thick walls arising at the
ileo-colic-caecal junction
2.Middle part with thin walls
3.The wide blind apex with fairly thick walls
The structure of the caeca is as follows:
1.Serous membrane
2.Outer longitudinal muscle
3.Circular muscle
4.Inner longitudinal muscle forming the muscularis mucosae of
the mucous membrane
The cloacaCloaca
The large intestine terminates in the front part of the cloaca.
The cloaca is a tubular cavity opening to the exterior of the body
and is common to the digestive and urogenital tract. The structure
of the cloaca is very similar to that of the intestine except that
the muscularis mucosa disappears near the vent. It divides into
three chambers, each separated by a constriction not readily
defined:
1.The copradaeum - a continuation of the colon-rectum
2.The urodaeum - middle part into which the ureters and genital
ducts open
3.The proctodaeum - opens to the exterior of the vent. Birds
less than one year old have a dorsal opening leading into the
blind, rounded sac - the bursa of fabricius
Liver
The liver is a bi-lobed organ that lies ventrally (below) and
posterior (in rear of) to the heart and is closely associated with
the proventriculus and the spleen. The right side lobe is the
larger. The liver is dark brown or chocolate in colour except for
the first 10-14 days when it may be quite pale due to the
absorption of lipids (fats) from the yolk as an embryo. It weighs
approximately 50 grams. The capsule or glissosis, the membrane
covering the organ, is thinner than that of mammals.
The gall bladder lies on the right lobe beneath the spleen. Two
bile ducts emerge from the right lobe. One of these originates from
the gall bladder and the second provides a direct connection from
the liver to the small intestine. A system of ducts connects the
right and left lobes.
There are a number of functions that the liver carries out:
1.Bile formation - consisting of bile, various pigments and bile
salts. Bile is involved in the digestion of fats to fatty acids and
glycerol
2.The metabolism of:
carbohydrate.
lipids.
protein.
3.Production and destruction of blood cells.
4.Synthesis of plasma proteins and fibrinogen (associated with
blood clotting).
5.Storage of glycogen, fat and fat-soluble vitamins e.g. vitamin
A.
6.Detoxification of certain substances (detoxify - destroy the
poisonous effect).
The liver cells have a high rate of destruction and a good
regenerative capacity (re-growth ability). Notwithstanding this, in
the normal animal, much of the organ is in reserve and can be
removed or destroyed without causing undue stress.
Blood supply and drainage
There are two blood supply systems - one originating from the
coelic artery for the normal maintenance of the liver as an organ
and the second, called the hepatic portal system, transports the
nutrients from the small intestine after absorption to the liver.
This latter system enters the liver via two veins - one for each
the right and left lobes. The two blood supply systems join
together inside of the organ.
The liver is drained via the hepatic veins into the posterior
vena cava (hepatic - to do with the liver; vena cava - one of the
main veins that enters the heart). The liver has a network of
sinusoids (empty holes in the tissues as in a sponge). The hepatic
portal system, the capillaries of the arterial blood supply and the
hepatic veins are in close association with each other in these
sinusoids.
Bile
The liver consists of a series of two cell thick sheets of
tissue with a sinusoid on either side of the sheet. Bile, the
product of the liver is made by the cells. The blood vessels, when
they enter these sinusoids become closely associated with them to
provide for the easy transfer of material from one system to
another. Minute canals called canaliculi that have the task of
collecting and transporting the bile are associated with the cells
in the tissue sheets. These canals eventually join together to form
the bile ducts - one going directly to the intestine and one to the
gall bladder before it connects to the small intestine.
Pancreas
This organ has three lobes occupying the space between the two
arms of the duodenal loop. Two or three ducts pass the secretions
of this organ into the distal end of the duodenum via papillae
common with the ducts from the gall bladder and the liver. The
structure is similar to that of the pancreas of mammals and
consists of special secreting tissue for pancreatic juice as well
as other groups of cells called the "islets of langerhan". These
are mainly associated with the production of hormones. In poultry
the cells of the islets of langerhan are less defined than those in
mammals. The functions of the pancreas are:
Produce pancreatic juice - a mixture of digestive enzymes.
Produce the hormones insulin and glucogen that are involved in
the metabolism of carbohydrate