Digestive system Lecture.pdf

Digestive system
4.1 Modifications in relation to feeding habits:
length and surface area, internal folds and supplementary diverticulae, ruminants stomach
4.2 Dentition in mammals

DIGESTIVE SYSTEM
• Organisms depend on nutrients to maintain
metabolic needs.
• All animals are consumers (a.k.a.
heterotrophs), meaning they do not have
the ability to manufacture their own food;
therefore, they must consume other
organisms as food.
• The primary purpose of the digestive
system is to break down the consumed
foods and transfer nutritional components
to the body’s cells for use in production of
the body’s fuel and ATP as well as to
manufacture other biomolecules.

There are three categories animals can be placed into based on their diets: herbivore, carnivore, and
omnivore.
Herbivores consume plant matter, whereas the primary diet of a carnivore is other animals. Omnivores
consume both plant and animal matter.
In vertebrates, the structures that vary between organisms based on diet often include differences in
dentition and gut length.
Herbivore
Herbivores consume algae or plant matter such as seeds, leaves, and fruits.
Because some of these materials are low in easily accessible energy, herbivores have evolved two
alternatives for releasing the nutrients: foregut and hindgut gastric fermentation.
Gastric fermentation utilizes bacteria that breakdown the hard to digest cellulose, the plant cell wall’s
primary component.
Herbivore teeth are usually broad and flat and are used to grind plant matter. The lower incisors and
canines are modified for biting off vegetation, and herbivores often lack upper incisors and canine.

Gastrointestinal tracts of foregut-fermenting mammalian herbivores. In animals that use foregut (a.k.a. pre-gastric)
fermentation, the stomach is modified into four chambers/compartments where the first chamber is the rumen. This rumen
provides a place for the bacterial breakdown of food. Ruminants regurgitate the partially digested mass from their rumen,
known as “cud,” and continue to chew the plant matter to break it down further. Examples of foregut ruminants include cows,
sheep, camels, and deer.

Gastrointestinal tracts of small hindgut-fermenting mammalian herbivores. In those animals that utilize hindgut fermentation
(a.k.a. post-gastric) such as rabbits, rhinos, and horses, the microbial digestion occurs in the large intestine (colon) and/or a large
cecum. These organisms are known as monogastric animals, because they lack the multi-chambered stomachs of the ruminants.
The post-gastric fermentation process is less efficient (20% – 65% fiber digestion) than pre-gastric fermentation (52% - 80%);
therefore, some monogastric animals practice coprophagy (the consumption of feces) to increase the absorption from nutrients of
the food that has already passed through their system. Because this process is not efficient, monogastric herbivores have to
consume large amounts of food to meet their nutritional needs, sometimes spending up to 16 hours per day grazing

Gastrointestinal tracts of mammalian carnivores. Carnivores consume other organisms. Because meat is easily digested
compared to plant material, the digestive system of a carnivore is typically shorter than an herbivore of comparable size. In
carnivores, the caecum is sometimes reduced and may be partially replaced by the appendix. Generally, carnivores have
pointed incisors and canines designed for killing prey and ripping muscle. The premolars and molars are designed to crush
food.

Omnivore Omnivores consume both plant and animal matter. The length of their digestive system more closely
resembles that of an herbivore as compared to a carnivore. However, omnivores lack the fermenting vats found in
herbivores. Examples of omnivores include humans, pigs, and bears. Omnivore dentition is relatively unspecialized as
compared to herbivores and carnivores. They contain teeth designed for biting (incisors), tearing (canines), grinding
(premolars), and crushing (molars).

Comparisons of digestive tract anatomy.
• It can be seen that the human digestive tract is
relatively small.
• Compared with that in the pig, an omnivore that is
often regarded as a model for humans, the human
large intestine is much reduced.
• The dog intestine is capacious but relatively short.
• The human large intestine is also small compared
with anthropoid apes, here illustrated by the
orangutan.
• The kangaroo, a nonruminant foregut fermenter,
has a large sacculated stomach, whereas the
hindgut fermenter, the horse, has a capacious,
multicompartment large bowel.
• The koala, which consumes only leaves that are
rich in tannins and volatile oils, has an extensive
large intestine and reduced small intestine.

Stomach is straight in cyclostomes, gar, Belone, etc., and
spindle-shaped in Proteus, Necturus, some lizards and snakes. In
turtles and tortoises, it is a wide curved tube, and in
elasmobranchs the stomach is J-shaped. In crocodiles and birds,
the stomach has two parts, a proventriculus with gastric glands,
and a highly muscular gizzard, which represents the pyloric
region and has a hard, cornified lining for grinding food.
In amphibians and reptiles, the stomach remains simple and
straight or gently curved, but in crocodile, it is rounded and very
muscular.
The stomach of birds is in two parts, the anterior part derived
from the fundus, called proventriculus, is very glandular and
produces digestive enzymes. The posterior part corresponds to
the pyloric region and is called ventricular or gizzard. It may be
exceedingly muscular for grinding coarse food. Sometimes with
the aid of pebbles (called gastrolith) eaten by birds.
Proventriculus and ventriculus are least distinct in carnivorous
birds and most sharply demarcated in granivorous species.
The stomach of mammals shows the greatest modifications. It
may be simple sac-like as in man or completely
compartmentalized as in ruminants.
STOMACH

In ruminant herbivores like cows, sheep and antelopes the stomach is highly modified to act as a
“fermentation vat”. It is divided into four parts. The largest part is called the rumen. In the cow it occupies
the entire left half of the abdominal cavity and can hold up to 270 litres. The reticulum is much smaller
and has a honeycomb of raised folds on its inner surface. In the camel the reticulum is further modified to
store water. The next part is called the omasum with a folded inner surface. Camels have no omasum.
The final compartment is called the abomasum. This is the ‘true’ stomach where muscular walls churn
the food and gastric juice is secreted
Ruminant Stomach

Ruminants swallow the grass they graze almost without chewing and it passes down the oesophagus to the
rumen and reticulum. Here liquid is added and the muscular walls churn the food. These chambers provide the
main fermentation vat of the ruminant stomach. Here bacteria and single-celled animals start to act on the
cellulose plant cell walls. These organisms break down the cellulose to smaller molecules that are absorbed to
provide the cow or sheep with energy. In the process, the gases methane and carbon dioxide are produced.
These cause the “burps” you may hear cows and sheep making.
Not only do the micro-organisms break down the cellulose but they also produce the vitamins E, B and K for
use by the animal. Their digested bodies provide the ruminant with the majority of its protein requirements.
In the wild grazing is a dangerous activity as it exposes the herbivore to predators. They crop the grass as
quickly as possible and then when the animal is in a safer place the food in the rumen can be regurgitated to
be chewed at the animal’s leisure. This is ‘chewing the cud’ or rumination. The finely ground food may be
returned to the rumen for further work by the microorganisms or, if the particles are small enough, it will pass
down a special groove in the wall of the oesophagus straight into the omasum. Here the contents are kneaded
and water is absorbed before they pass to the abomasum. The abomasum acts as a “proper” stomach and
gastric juice is secreted to digest the protein.

Many modifications are found in the intestine of vertebrates. The length of the intestine is related to the feeding habits
of the animal. It is relatively short in carnivorous forms and long in herbivores.
Cyclostomes: - Their intestine is straight. At its posterior end, it enlarges slightly to form a rectum, which terminates in
an anus. The longitudinal fold the typhlosole, which takes a somewhat spiral course, projects into the cavity of the
intestine.
Fishes: - In elasmobranchs, the small intestine is shorter than the stomach. It is wide, straight, and contains a well-
developed spiral valve. Spiral valves are also found in the small intestines of chimeras, Dipnoi, Latimeria, and others.
In elasmobranch, a large intestine, a short passageway between the small intestine and anus, bends slightly and then
opens into the cloaca.
A long slender, rectal gland connects to the intestine through a duct near the point where small and large intestines join.
In Dipnoi a cloacal caecum is present. In most of the fishes, the true cloaca is present but when no cloaca exists, the
posterior part of the large intestine is known as the rectum.
INTESTINE

longitudinal fold the typhlosole

Amphibians: - In caecilians, the intestine is not differentiated into large and smaller regions and shows a slight degree of
coiling. In salamander greater degree of coiling is evident and in anurans, this tendency is much more marked. In urodeles and
anurans, the large intestine is short, straight, and marked off from the small intestine. It opens into a cloaca.
A ventricular diverticulum of amphibian cloaca gives rise to the urinary bladder frequently on the ileocolic valve is present
between small and large intestines. Villi first becomes evident in certain members of this class; circular folds are also present in
the small intestine of some amphibians.
Reptiles: - The reptilian small intestine is elongated, coiled, and of fairly uniform diameter. The large intestine is generally of
greater diameter and opens into a cloaca. An ileocolic valve is located at the junction of the small and large intestine and this
point, except in crocodiles, a colic caecum arises. Reptiles are the first vertebrates to have true colic caeca.
Birds: - A tendency toward greater length is evident in the small intestine in this group. The large intestine is straight, relatively
short. And terminates in a cloaca. A colic caecum is lacking in parrots, woodpecker, and others; but most birds have one or two
such structures. In certain birds, the colic caeca attain a very large size and the walls may even bear villi. The relatively
enormous caecum of the ostrich contains a spiral fold not found in other birds.

Mammals: -
The intestine of mammals is more elaborately developed than those of other vertebrates. The coiled small intestine is
made up of three regions – duodenum, jejunum, and ileum. A pouch-like structure, known as Meckel’s diverticulum is
sometimes found projecting from the ileum. It represents a remnant of embryonic yolk stalk which has failed to
degenerate in a normal manner. It is found in about 2 % of all human adults and may give rise to serious
complications.
The average length of the small intestine of men is 22.5 feet however there is much variation. The large intestine or
colon is much shorter than the small intestine but of considerably greater diameter colon terminates in the rectum and
opens to the outside through the anus.
Among mammals, monotremes possess cloaca.
In herbivores mammal’s intestine is very long and maybe 20 to 28 times the length of the body.
In the cow average length is 165 feet and, in the horse, approximately 95 feet.
Carnivorous forms have an intestine only 5 to 6 times the length of the body.

Increase in surface area / internal folds
After the digestion is completed, foodstuffs are to be absorbed in the anterior part of the intestine. This requires an
extensive surface area, which is achieved by coiling of the gut, by circular folds in the mucosal lining (absent in
many small vertebrates), by finger-like microscopic villi, which are packed 10 to 40 / mm2 over the lining, and
finally by microvilli crowded 200,000/mm2 on the exposed surface of the epithelial cells, where they, and the
carbohydrate coat they support, form a brush border. The folds villi and microvilli of the digestive tube can increase
its surface 600 fold.

Internal folds in
Fishes: - in some members of every major group of fishes, there is a type of intestine seemingly primitive for the
vertebrates – the spiral intestine. In addition to minor epithelial folds, the surface area is greatly increased by the
presence of a spiral valve.
This is a fold of epithelium and connective tissue extending from one end of the intestine to the other. The
attachment of which in typical form twists numerous times in a spiral manner along the walls of the gut.
The development of the spiral fold greatly increases the internal area of the gut, and also its functional length,
because the food must follow a long course down around the twists of the spiral staircase to reach the end of the
seemingly short intestine.
Such spiral intestine is not found in higher vertebrates instead the intestine is a slender tube that is much more
elongated. In birds, the length of its coils is eight times that of the whole body. Some are found in mammals where
the intestine is completely coiled with a length of that averages seven to eight times that of the body, and is several
times greater than this figure in some large herbivores; Eg – the intestine of an ox may be more than 50m long!

Supplementary diverticula: -
 Coeca of the digestive tract is blind diverticula that may
occur anywhere from the oesophagus to the colon. They
increase the surface area of the tract.
 Primitively their function may have been merely to increase
the surface of the gut, now they function variously for
storage, fermentation, or vitamin concentration.
 Tetrapods usually have one or two pouch-like diverticula at
the juncture between small and large intestines. These are
called colic caeca.
 In men, the distal end of the caecum has degenerated and the
remnant of it is represented by the appendix.
 Reptiles are the first vertebrates to have true colic coeca. In
certain birds (duck, turkey, ostrich, etc.) the colic coeca
attain a very large size and the walls may even bear villi.
The relatively enormous caecum of the ostrich contains a
spiral fold not found in other birds. The crop sac of birds is
an oesophageal caecum.

Ray-finned fishes are distinctive for another structure that
increases the surface of the intestine; adjacent to the stomach
the intestine develops diverticula called pyloric caeca. Most
fishes have scores or hundreds of caeca, but some have few and
several have none. Histologically they resemble the adjacent
intestine.

TYPES OF TEETH AND DENTAL FORMULA IN MAMMALS.
It is believed that both placoid scales of elasmobranchs and true teeth of vertebrates have been derived from the
bony dermal plates of extinct ostracoderm. True teeth of vertebrates are generally built on the same fundamental
plan. The entire set of teeth of an organism may have a similar shape or the various teeth may be different
concerning their shape and function. When all the teeth of an organism are alike it is said to be homodont dentition.
Most vertebrates other than mammals have homodont dentition.
TYPES OF TEETH IN MAMMALS: -
In Echidna teeth are absent both in the embryonic and adult stage. In platypus, true teeth are present only during the
embryonic stage. These disappear in the adult stage and only epidermal teeth are present in platypus in the adult
stage.
Mammals like whales and ant-eaters have no teeth in the adult stage. However, certain anteaters and armadillos
possess imperfect teeth that lack root and enamel.
4.2 Dentition in mammals

TYPES OF TEETH IN MAMMALS: -
In Echidna teeth are absent both in the embryonic and adult stage. In platypus, true teeth are present only during the
embryonic stage. These disappear in the adult stage and only epidermal teeth are present in platypus in the adult stage.
Mammals like whales and ant-eaters have no teeth in the adult stage. However, certain anteaters and armadillos possess
imperfect teeth that lack root and enamel.
Most mammals have diphyodont dentition wherein the milk teeth are replaced by permanent teeth early in life. It is
interesting to note that milk teeth are lost even before birth in certain bats and guinea pigs. There are mammals such as
platypus, toothed whale, sloths, and where monophydont dentition prevails.
An intermediate example is that of marsupials where all the milk teeth except the last premolars are retained.
Among mammals, only certain cetaceans have homodont dentition. Eg:- toothed whales. The number of teeth may
range from t2 to 200. Certain carnivores belonging to suborder, pinnipeds also show a tendency towards homodont
dentition. As far as different types of teeth are concerned most of the mammals show heterodont dentition.
The teeth are differentiated into incisors, canines, premolars, and molars.
The number of teeth in each species is fixed and it is of taxonomic importance.
Placental mammals may show a maximum number of 44 teeth in heterodont dentition.
Scientists have devised a so-called dental formula for each species to explain the number of different types of teeth in
each half of the upper and lower jaw.

S. No. Name of
animals
Incisors Canines Premolars Molars Total
1. Rabbit 2
1
0
0
3
2
3
3
28
2. Rat 1
1
0
0
0
0
3
3
16
3. Horse 3
3
1
1
4
4
3
3
44
4. Squirrel 1
1
0
0
2
1
3
3
22
5. Mole 3
3
1
1
4
4
3
3
44
6. Dog 3
3
1
1
4
4
2
3
42
7. Cat 3
3
1
1
3
2
1
1
30
8. Cow 0
3
0
1
3
3
3
3
32
9. Man 2
2
1
1
2
2
3
3
32
The dental formula in different animals:-

Carnassial teeth
It is interesting to note that the shape of molar and
premolar teeth varies in different mammals depending
upon their feeding habits. For instance, terrestrial
carnivores possess sharp cutting crowns in their cheek
teeth. Such teeth are known as secodont.
Carnassial teeth which are, in fact, a premolar in the
upper jaw and molar in the lower jaw, are specifically
developed for streaming flesh.

The flattened cheek teeth with their small tubercles in man are known as bunodont type. These are used for grinding
the food.
There may be present, vertical, crescent-shaped folds of hard enamel enclosing soft areas of dentine in the cheek teeth
of ruminants like horses. Such spaces are always filled with cement. This condition is known as the selenodont
condition.
In elephants, there is found an intricate folding of the enamel and dentine to form transverse ridges in the enormous
grinding teeth. Such a condition is called lophodont condition.
Bunodont

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