Two important purposes for the body
It is a source of materials for the growth and
replacement of tissue and a source of fuel for energy. And it is digestion that
bridges the gap between what we enjoy as food and what must enter the
bloodstream to supply the body with its needs. The digestive process, which
takes place in the 3D-foot tube called the alimentary canal, involves both
mechanical and chemical changes. It requires the squeezing and pushing of
muscles and the action of a series of digestive juices.
It is interesting; it
is understandable; and it is very much worth understanding, for understanding
can do much to avoid disturbances and prevent useless anxieties, some of them
stirred up by commercial interests in behalf of digestive aids and remedies
which, perhaps as often as not, are gulped at considerable expense by those who
have no real need for them.
START OF THE PROCESS
The alimentary canal has five
major divisions: (1) mouth; (2) esophagus, or gullet; (3) stomach; (4) small
intestine; and (5) colon, or large intestine. The mouth, though many people are
not aware of it, has an important role to play in digestion. Here, of course,
chunks of food are reduced to small particles as the tongue, itself a bundle of
muscles, rolls them around while the incisors and canine teeth in front cut and
tear and the molars in back grind.
In the mouth, too, during chewing, the first
digestive chemical change takes place. A p.li r of silliv.uy glands, known as
the parotid glands, lie in the sides of the face, in front of and slightly
below the ears. Saliva from these glands reaches the mouth through parotid
ducts which open on the inner surfaces of the cheeks opposite the second molar
teeth.
There are also a pair of sub-maxillary glands in the
angles of the lower jaws and a pair of sublingual glands under the tongue.
Ducts from these glands open into the floor of the mouth beneath the tongue.
Saliva mixes with food in the mouth and softens and lubricates it. Saliva, too,
contains a digestive enzyme, ptyalin, which acts to convert cooked starch to
sugar.
This is why, even as you are chewing them, bread and potatoes dissolve.
Ptyalin, however, is unable to reach and work on starch grains that are still
wrapped in their natural cellulose containers, and this is why starches should
be cooked before being eaten.
A word here about enzymes- since these is substances
very much involved in the whole process of digestion.
