The ordinary motions of the heart appear to arise from the second and third classes of motory vibrations, mentioned in the last proposition; and it is remarkable, that its motions are found to be, in general, and cæteris paribus, stronger or weaker, as the sum total of these two classes is greater or less. The systole and diastole succeed each other, from the causes assigned in the seventeenth proposition. We are to conceive, however, that both the influx of the venal blood into the ventricles, and of the arterial into the coronary vessels, have a considerable share in bringing on the systole, in the way of distention and irritation.
May we not conjecture, from that experiment of Dr. Hook’s, in which he kept a dog alive, by a mere continued stream of fresh air passing through the lungs, without any such alternate motion of the chest as takes place in common respiration, that one principal use of the air, which is an electric per se, in respiration, is to restore to the blood, as it passes through the lungs, that electricity which it has lost in circulating through the body? For, upon this supposition, the blood which arrives at the left ventricle, will, in cessations of respiration, and also where foul air is respired, want its due electricity; whence, according to Prop. XVI. the muscles, and especially the heart, will want one of the principal requisites for contraction. However, convulsive motions may ensue after a syncope, from the fifth class of motory vibrations.
It is remarkable here, that the hearts of frogs, vipers, and several other such animals, as can live in great degrees of cold, and without respiration, continue to beat, as has been taken notice of above, for a long time after they are taken out of their bodies. We must therefore suppose, that the fibres of their hearts, and the blood globules which remain in them, are endued with an electric, or other attractive virtue, of a more durable kind than the fibres and blood globules of the more perfect animals; also, that this virtue may be put into action by a less degree of heat. All which is very agreeable to the other circumstances of their economy.
Respiration and crying are excited in the new-born child from the cold handling of the midwife, and other vivid sensations impressed immediately upon its coming into the world. These vivid sensations put the whole system of muscles, or at least those of the trunk and larynx, into action at once, as far as their mutual antagonism will permit, the stronger set of conspiring muscles overpowering the weaker for a certain short time, and then, after their force is exhausted, according to Prop. XVII. giving way for a shorter time to the weaker. But this alternate action of the muscles of the trunk and larynx will be an imperfect kind of respiration, with crying, as may be easily seen from the disposition of the muscles. Respiration is afterwards kept up, partly by the propensity of the muscles to alternate action, explained Prop. XVII.; partly, perhaps, by the power of habit, i.e. association;
