Page:Observations on Man 1834.djvu/187

 virtuous, religious attention to the duty of the time and place. To which must be added great abstinence in diet, and bodily labour, if required.

Section VII
OF OTHER MOTIONS, AUTOMATIC AND VOLUNTARY, NOT CONSIDERED IN THE FOREGOING SECTIONS OF THIS CHAPTER.



I observe,

First, that the motion of the heart is constant, more equable than any other in the body, and cannot be supposed to proceed from impressions made on the neighbouring parts; which things agree well together upon the supposition of the doctrine of vibrations: for thus it cannot proceed from the first or fourth source of motory vibrations; and if it proceed from the second and third, it ought to be constant, and nearly equable.

Secondly, We are not to inquire, how the heart is first put into motion in the foetus, but only how its ordinary motions may be continued, by means of vibrations descending from the brain, after they are once begun.

Now, for this purpose, let us suppose, that the auricles, with the beginnings of the pulmonary artery and aorta, have finished their contractions; and that the ventricles and coronary vessels are full. It follows, that the ventricles will now be excited to contraction by three causes; viz. by the vibrations which have continued to descend freely into their fibres ever since their last contraction was finished, by their distention from fulness, and by the motion and impulse of the blood in the coronary vessels: but from the moment that the ventricles begin to contract, the auricles and arteries will begin to fill, and will be fullest at the instant when their contraction is finished. The auricles and arteries must therefore begin to contract at this instant, from descending vibrations and distention, and the ventricles to be relaxed and filled, the vibrations in their fibres being checked by their past contraction. Let these actions be completed, and the heart will be in the circumstances first supposed; i.e. the ventricles will be ready for a fresh contraction, and so on alternately, as long as life continues.

Thirdly, The vibrations which keep up the heart’s motion, are nearly allied to those arising from heat; for the second source of