Spectrum Biology - September 2016

Caeca : The Intestinal Cellulose Digesting Tool

 In ruminants, microorganisms attack the food

before gastric digestion. Whilst in the typical

non-ruminant herbivores, microbial action on

cellulose occurs after digestion. Rabbits, horses

and rats digest cellulose by maintaining a

population of microorganisms in their unusually

large caecum. It is a blind pouch that extends from

the colon.

 Therefore, a few non-ruminant herbivores, such as

mice and rabbits, eat some of their own faeces to

process the remaining material in them, such as

vitamins.

The Human Digestive System

The digestive systems of human and most of the

mammals have the mechanical and chemical ability

to process many kinds of foods.

General Plan of the Human Canal

The structure of the alimentary canal follows a consistent

pattern from the level of the oesophagus onwards. Some

modifications are found in their structures, which are associated

with special functions.

The walls of the alimentary canal are formed by four layers of

tissue:

(i) Adventitia or outer covering (ii) Muscle layer

(iii) Submucus layer (iv) Mucosa-lining

The retroperitoneal space (retroperitoneum) is the anatomical

space in the abdominal cavity behind the peritoneum. Organs

are retroperitoneal if they have peritoneum on their anterior

side.

Ascending

colon

Ileum

Cecum

Appendix

Extensive caeca of a non-ruminant,

herbivore such as a rabbit

The Gut Wall and its Glands

Submucosa

Connective tissue, contains

blood and lymph vessels.

Glandular epithelium

secretes mucus, which

lubricates and

protects mucosa

Connective tissue

with blood and lymph vessels

Thin layer of smooth muscles

(muscularis is mucosa)

Mucosa

Lumen of gut External muscle

Inner circular muscle and outer

longitudinal muscle; coordinated

contraction of muscle causes waves

of movement (peristalsis) which

propel food along the gut.

Mesentery

Suspends gut in the abdomen cavity,

carries blood vessels, nerves and lymphatics. Serous coat

and muscles

Submucosa

Mucosa

Outer

covering

(serosa)

of fibrous

connective

tissue.

1

2

3

Glands of the gut are of three types:

1. Glands outside the gut
   Salivary gland (secretes saliva
   into buccal cavity).
   Liver (secretes bile to gall
   bladder).
   Pancreas (secretes
   pancreatic juice into
   duodenum).


2. Glands in the submucosa
   Brunner's gland (secretes
   mucus into duodenum).


3. Glands in the mucosa
   Gastric gland (secretes gastric
   juice into stomach)
   Crypt of Lieberkuhn at base of
   villus (source of epithelial cells)

The muscularis externa of gut wall is the

layer of circular and longitudinal smooth

muscle fibres arranged with OLIC (Outer

Longitudinal Inner Circular) arrangement. A

network or plexus of neurons and nerve

fibres is present in between the two layers.

This plexus having nerve cells and

parasympathetic fibres is called aurebach’s

or myenteric plexus. The submucosa layer

also contains another plexus of nerve cells

and sympathetic nerve fibres called

missiner’s plexus or submucosal plexus.

Diaphragm Liver

Aorta

Lesser omentum

Foramen of Winslow

Pancreas

(Retroperitoneal structure)

Duodenum

(Retroperitoneal structure)

Mesentery

Transverse colon

Greater omentum

Pelvic colon

(Retroperitoneal structure)

Rectum

(Retroperitoneal structure

Stomach

Lesser sac

Small

intestine

Uterus

(Retroperitoenal structure)

Bladder

(Retroperitoenal structure)

The peritoneum and its association with the abdominal organs of

the digestive system and the pelvic organs viewed from the side.