The Main causes of Laughter

124. Laughter

Laughter consists in the blood, coming from the right cavity of the heart through the pulmonary vein, suddenly inflating the lungs repeatedly.

This causes the air they contain to forcefully exit through the windpipe, where it forms an inarticulate and loud sound.

As the lungs swell and this air exits, it pushes all the muscles of the diaphragm, chest, and throat, thereby causing movement in the facial muscles connected to them.

Laughter is this action of the face, accompanied by this loud sound.

125. Why does laughter not accompany the greatest Joys?

Laughter seems to be one of the principal signs of joy. But joy can only cause laughter when joy is moderate and mixed with some admiration or some hatred.

In extreme joy, the subject of that joy never leads to bursts of laughter.

Moreover, one cannot easily be induced to laugh by any other cause as when one is sad.

This is because in great joys, the lungs are always so full of blood that they cannot be further inflated repeatedly.

126. The main causes of laughter

I observe only two causes that suddenly inflate the lungs in this way.

1. The surprise of admiration

When combined with joy, this can open the heart’s openings so quickly that a large amount of blood entering suddenly through the right side via the vena cava becomes rarefied there, and passing through the pulmonary vein, inflates the lung.

2. The mixture of some liquid that increases the rarefaction of the blood

An example is the more flowing part of that which comes from the spleen.

This part of the blood, pushed toward the heart by a slight motion of hatred, aided by the surprise of admiration, and mingling there with blood coming from other parts of the body, which joy abundantly introduces, can cause this blood to expand much more than usual.

127. Its cause in Indignation

Laughter sometimes accompanies indignation. It is usually artificial and feigned.

But when it is natural, it seems to arise from the joy one has in seeing that one cannot be offended by the evil of which one is indignant, combined with the surprise of the novelty or unexpected encounter with this evil.

Therefore, joy, hatred, and admiration contribute to it.

However, it can also be produced without any joy, solely by the movement of aversion. This sends blood from the spleen to the heart, where it is rarefied and pushed into the lung, which it easily inflates when it finds it nearly empty.

Generally, anything that can suddenly inflate the lung in this manner causes the outward action of laughter, except when sadness changes it to sighs and cries accompanying tears.