UNIT-1
INTRODUCTION - Digestion and Absorption
Process of conversion of complex food substances to simple absorbable forms is called digestion.
When the process of digestion occurs within the cell in the food vacuole it is called intracellular
digestion. E.g., protozoa, porifera, coelenterata and free living platyhelminthes.
When the process of digestion occurs outside the cell it is called extracellular digestion. E.g.,
Coelenterates and phylum platyhelminthes to phylum chordata.
DIGESTIVE SYSTEM
Digestion in vertebrates occurs in the digestive tract or alimentary canal.
The various parts involved in digestion can be broadly divided into two groups -
Digestive tract or alimentary canal
Accessory digestive glands
ALIMENTARY CANAL
The alimentary canal is a long coiled tube having muscular wall & glandular epithelium extending
from mouth to anus.
The organs comprising the alimentary canal includes mouth and pharynx (called buccopharyngeal
cavity), oesophagous, stomach and intestine.
MOUTH
Mouth consists of vestibule and buccal or oral cavity.
Vestibule is a space which is bounded externally by lips, cheeks and internally by the gums and
teeth.
Buccal or oral cavity is bounded by palate above (dorsally), throat with tongue below and jaws with
teeth on the sides.
Palate forms the roof of buccal cavity and is differentiated into anterior hard palate and posterior
soft palate.
Hard palate has uvula as a free hanging flap which closes the internal nares during swallowing of
food bolus.
Soft palate bears transverse palatine rugae supported by bones for holding food during mastication.
The tongue is a freely movable muscular organ attached to the floor of the oral cavity by the
frenulum.
, Tongue not only helps in ingestion, mastication and deglutition of food but also help in voice
production and tasting of food.
The upper surface of the tongue has small projections called papillae, some of which bear taste buds.
Teeth are hard structures meant for tearing cutting, crushing and holding food.
Each tooth is embedded in a socket of jaw bone. This type of attachment is called thecodont.
Majority of mammals including human being forms two sets of teeth during their life, a set of
temporary milk or deciduous teeth replaced by a set of permanent or adult teeth. This type of
dentition is called diphyodont.
An adult human has 32 permanent teeth which are of four different types (heterodont dentition) -
incisors (I), canine (C), premolars (PM) and molars (M).
Arrangement of teeth in each half of the upper and lower jaw in the order I, C, PM, M is represented
by a dental formula which in human is .
Dental formula
Morphologically, teeth can be distinguished as homodont or heterodont.
Homodont : When all the teeth are structurally and functionally similar. E.g., Vertebrates except
metatherian and eutherian mammals.
Heterodont : When the teeth are different in structure and functions. They are distinguished into four
types - incisors, canines, premolars and molars. E.g., Metatherian and eutherian mammals.
, o Incisors (8) : These are the front, long, curved and possess sharp-edged teeth. They are
adapted for cutting or cropping and biting.
o Canines (4) : There is one pointed or (dagger shaped) canine in each maxillary of upper jaw
and each dentary of lower jaw next to the incisors. They are meant for piercing, tearing and
offence and defence. These are poorly developed in man.
o Premolars (8) : They are meant for crushing, grinding and chewing. They are also called
cheek teeth.
o Molars (12) : They are also called cheek teeth. They are specialized for crushing and
grinding the food.
Enamel represents the hardest substance of the body. It is completely acellular and avascular and non
regenerable. It helps in mastication of food.
The element that hardens the tooth enamel is fluorine.
Pharynx:
The oral cavity leads into a short pharynx which serves as a common passage for food and air.
The oesophagus and the trachea (windpipe) open into the pharynx.
A cartilaginous flap (called epiglottis) prevents the entry of food into the glottis – opening of the
windpipe – during swallowing.
OESOPHAGUS
The oesophagus is a thin, long tube (25 cm) which extends posteriorly passing through the neck,
thorax and diaphragm and leads to a ‘J’ shaped bag like structure called stomach.
A muscular sphincter (gastro-oesophageal) regulates the opening of the oesophagus into the stomach.
STOMACH
Stomach is the most distensible and widest organ of the alimentary canal.
Empty stomach possesses folds called gastric rugae, which disappears when the stomach is distended
with food.
Loss of gastric rugae are one of the earliest sign of stomach cancer.
The stomach, located in the upper left portion of the abdominal cavity, has three major parts – a
cardiac portion into which the oesophagus opens, a fundic region and a pyloric portion, which opens
into the first part of the small intestine.
Cardiac part are so called because it is present near the heart. It is broad at upper part with cardiac
sphincter which prevents regurgitation of food.
Fonds is commonly filled with air and gases.
INTESTINE
Intestine is responsible for most of the digestion and absorption of food and usually formation of dry faeces.
INTRODUCTION - Digestion and Absorption
Process of conversion of complex food substances to simple absorbable forms is called digestion.
When the process of digestion occurs within the cell in the food vacuole it is called intracellular
digestion. E.g., protozoa, porifera, coelenterata and free living platyhelminthes.
When the process of digestion occurs outside the cell it is called extracellular digestion. E.g.,
Coelenterates and phylum platyhelminthes to phylum chordata.
DIGESTIVE SYSTEM
Digestion in vertebrates occurs in the digestive tract or alimentary canal.
The various parts involved in digestion can be broadly divided into two groups -
Digestive tract or alimentary canal
Accessory digestive glands
ALIMENTARY CANAL
The alimentary canal is a long coiled tube having muscular wall & glandular epithelium extending
from mouth to anus.
The organs comprising the alimentary canal includes mouth and pharynx (called buccopharyngeal
cavity), oesophagous, stomach and intestine.
MOUTH
Mouth consists of vestibule and buccal or oral cavity.
Vestibule is a space which is bounded externally by lips, cheeks and internally by the gums and
teeth.
Buccal or oral cavity is bounded by palate above (dorsally), throat with tongue below and jaws with
teeth on the sides.
Palate forms the roof of buccal cavity and is differentiated into anterior hard palate and posterior
soft palate.
Hard palate has uvula as a free hanging flap which closes the internal nares during swallowing of
food bolus.
Soft palate bears transverse palatine rugae supported by bones for holding food during mastication.
The tongue is a freely movable muscular organ attached to the floor of the oral cavity by the
frenulum.
, Tongue not only helps in ingestion, mastication and deglutition of food but also help in voice
production and tasting of food.
The upper surface of the tongue has small projections called papillae, some of which bear taste buds.
Teeth are hard structures meant for tearing cutting, crushing and holding food.
Each tooth is embedded in a socket of jaw bone. This type of attachment is called thecodont.
Majority of mammals including human being forms two sets of teeth during their life, a set of
temporary milk or deciduous teeth replaced by a set of permanent or adult teeth. This type of
dentition is called diphyodont.
An adult human has 32 permanent teeth which are of four different types (heterodont dentition) -
incisors (I), canine (C), premolars (PM) and molars (M).
Arrangement of teeth in each half of the upper and lower jaw in the order I, C, PM, M is represented
by a dental formula which in human is .
Dental formula
Morphologically, teeth can be distinguished as homodont or heterodont.
Homodont : When all the teeth are structurally and functionally similar. E.g., Vertebrates except
metatherian and eutherian mammals.
Heterodont : When the teeth are different in structure and functions. They are distinguished into four
types - incisors, canines, premolars and molars. E.g., Metatherian and eutherian mammals.
, o Incisors (8) : These are the front, long, curved and possess sharp-edged teeth. They are
adapted for cutting or cropping and biting.
o Canines (4) : There is one pointed or (dagger shaped) canine in each maxillary of upper jaw
and each dentary of lower jaw next to the incisors. They are meant for piercing, tearing and
offence and defence. These are poorly developed in man.
o Premolars (8) : They are meant for crushing, grinding and chewing. They are also called
cheek teeth.
o Molars (12) : They are also called cheek teeth. They are specialized for crushing and
grinding the food.
Enamel represents the hardest substance of the body. It is completely acellular and avascular and non
regenerable. It helps in mastication of food.
The element that hardens the tooth enamel is fluorine.
Pharynx:
The oral cavity leads into a short pharynx which serves as a common passage for food and air.
The oesophagus and the trachea (windpipe) open into the pharynx.
A cartilaginous flap (called epiglottis) prevents the entry of food into the glottis – opening of the
windpipe – during swallowing.
OESOPHAGUS
The oesophagus is a thin, long tube (25 cm) which extends posteriorly passing through the neck,
thorax and diaphragm and leads to a ‘J’ shaped bag like structure called stomach.
A muscular sphincter (gastro-oesophageal) regulates the opening of the oesophagus into the stomach.
STOMACH
Stomach is the most distensible and widest organ of the alimentary canal.
Empty stomach possesses folds called gastric rugae, which disappears when the stomach is distended
with food.
Loss of gastric rugae are one of the earliest sign of stomach cancer.
The stomach, located in the upper left portion of the abdominal cavity, has three major parts – a
cardiac portion into which the oesophagus opens, a fundic region and a pyloric portion, which opens
into the first part of the small intestine.
Cardiac part are so called because it is present near the heart. It is broad at upper part with cardiac
sphincter which prevents regurgitation of food.
Fonds is commonly filled with air and gases.
INTESTINE
Intestine is responsible for most of the digestion and absorption of food and usually formation of dry faeces.
