The science of life : fully illustrated in tone and line and including many diagrams

BOOK 1

way.’ During mastication the food is kept

at the front and sides of the mouth ; when about to be swallowed, it is collected in a

Adenoid

Palate Epiglottis LowerJaw Backbone Vocal Cord Thyroid Gland Gullet Windpipe Fig. 27. The mouth, nose, and throat seen in section

as uf the head had been sawn into equal halves f quae display the parts concerned in swallowing. -

lump on the back of the tongue. There is then a rapid succession of events: first a pause in the masticatory movements, then a slight upward movement of the diaphragm, then a violent thrust upward and backward of the tongue, which shoots the bolus back into the oesophagus. As this last movement occurs the soft palate moves back to protect the nose-cavity, the cartilages of the larynx displace themselves so that the opening of the windpipe becomes a tightly closed slit, and a curious little flap of cartilage, the epiglottis, which normally sticks out into the throat, ducks smartly out of the way of the oncoming bolus. In this manner the food is flung into the oesophagus and at the same time it passes out of our ken. It leaves the mouth, which is under

the direct control of our conscious minds,

52

Fig. 28.

THE SCIENCE OF LIFE

GHAPTER 2

for those dark and mysterious regions of whose proceedings we are normally unaware.

The oesophagus (and this is also true of the rest of the digestive tube) is able to perform active movements automatically without any direction from the brain. Its muscles are always slightly contracted, and their contraction is maintained by the agency of a special local nervous system in its tissues. The presence of a bolus in any part of the oesophagus, large enough to stretch its wall, sets these muscles going. The part of the oesophagus immediately in advance of the bolus relaxes, becomes loose and flabby, while the part immediately behind the bolus contracts vigorously. As a result, the bolus is squeezed forward by the tightening muscle into the looser part. But as it moves forward it sets up the same changes; the part immediately behind it contracts and urges it on, while the part in front slackens to receive it. In this way the bolus is driven on by a ring of contraction. This process is called peristalsis. Inthe oesophagus, peristalsis is so powerful that a man can swallow perfectly well when he is standing on his head. He can and most animals do drink “* upwards,” and in cases of operation in this region the danger of an instrument being swallowed if once it gets into the gullet has to be provided for. Whatever the patient may wish in the matter, his oesophagus will follow the laws of its own being so soon as it gets a grip.

The stomach has a capacity of about three-and-a-half pints. It has two openings,

to

How food is squeezed along the digestive tube.

The walls of the latter tighten behind the mouthful and slacken in front of it.

that of the oesophagus, through which the mixture of food and saliva enters the stomach, and that of the duodenum (the first part of the intestine), through which the products of gastric digestion go out. ‘The wall of the