Let us now turn to the Exercises and to Harvey's own account of the cause of the heart-beat. The first passage to be quoted begins with a few sentences which have been introduced previously, but which form a necessary cue for the statement we are to study. Harvey says to Riolanus:-- "For the present I will only say and set forth without demonstration--by good leave of the learned and with due respect to the ancients--that the heart, as the beginning, author, source, and origin of everything in the body and the first cause of life, should be held to include the veins and all the arteries and also the contained blood; just as the brain, including all its nerves and sensory organs and spinal marrow, is the one adequate organ of sensation, as the phrase is. If by the word 'heart,' however, only the body of the heart be meant with its ventricles and auricles, I do not believe that it is the manufacturer of the blood; nor that the blood possesses vigor, faculty, reason,[168] motion, or heat, as the gift of the heart. Moreover, I judge the cause of diastole and expansion not to be the same as that of systole and contraction, either in the arteries, or in the auricles or the ventricles of the heart; but that part of the pulse which is called diastole has another cause, different from the systole, and always and everywhere must precede every systole; I judge the first cause of expansion to be the innate heat and expansion to occur first in the blood itself, gradually thinned and swelling up like matters in fermentation, and to be extinguished last in the same; and I accept Aristotle's parallel with pottage or milk with this proviso, that the rising or falling of the blood is not brought about by vapors, or exhalations, or spirits, excited into some vaporous or aërial form, and is caused, not by an external agent, but by an internal principle, and is regulated by nature.