Page 47 - Biology Form Two

Biology for Secondary Schools

During development, mammals acquire After the food has been chewed, it is
two sets of teeth, which are milk teeth rolled by the tongue into small balls
(temporary teeth) and permanent teeth. called boli (singular bolus). The teeth,
Most children develop a full set of saliva, and tongue play important roles
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primary teeth by the age of 3 years. A in rolling the food into the bolus. In this
set of complete milk teeth in human form, the food is ready to be swallowed
consists of twenty teeth, in which there via the pharynx and oesophagus.

are eight incisors, four canines, eight During swallowing, the food bolus is
premolars, and no molars. At the age of pressed against the roof of the mouth
6 years, the milk teeth start to weaken and pushed into the pharynx by the

and fall out. By the age of 6-12 years muscular action of the tongue. The
permanent teeth grow to replace the pharynx is the cavity behind the mouth,
milk teeth. By the adult age, all milk which is connected to the stomach by
teeth are completely replaced with the oesophagus. It is also connected
permanent teeth. to the nostrils or air pathways by
the nasal cavity. Furthermore, the
Teeth facilitate the chewing of food pharynx is connected to the ears by
in the mouth. Chewing is necessary the eustachian tubes and to the trachea
because it breaks down large particles by the epiglottis. Epiglottis is a flap of
of food into smaller particles and mixes cartilage that controls entry of air into
the food particles with saliva for easy the trachea. It opens to allow air to enter

swallowing. The presence of food in the the lungs through the trachea. When
mouth stimulates the salivary glands to eating food, the epiglottis closes the
release more saliva. Saliva contains trachea to prevent food from entering

a digestive enzyme called salivary the air passage. After the bolus has
amylase (ptyalin) that catalyses the entered the oesophagus, the epiglottis
break down of cooked starch into opens again to allow air to pass into
the trachea and proceeds down to the
maltose. Saliva in the mouth also helps lungs. From the pharynx, food goes
to moisten the mouth, tongue and lips. into the oesophagus. Figure 2.3 shows

This enables easy chewing and mixing the structure of the oral cavity and the
food in the mouth.
position of the pharynx in the throat.

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