Our most immediate interest in the foregoing passage lies in this: that on the very same page, with the few clear simple words which tell for the first time of Harvey's facts and proofs, he has briefly written down conjectures as to the meaning of the circulation. These are as strikingly put as certain jottings are obscure which deal on a neighboring page with some possible meanings of the heart-beat.[109] In neither group of conjectures do the functions of the lungs play a part; but the discoverer asks himself whether it be not to revisit the heat of the heart that the whole mass of the blood circles back to the chest in its Harveian course! More than thirty-two years after the date of Harvey's note-book Harvey wrote to Riolanus:-- "There are some who consider that as no impulsion of nutriment is required for the nutrition of plants, their particles attracting little by little whatever they need to replace what they have lost, so in animals there is no need of any impulsion, the vegetative faculty in both working alike. But there is a difference. In animals a perpetual flow of warmth is required to cherish the members, to keep them alive by the aid of vivifying heat, and to restore parts injured from without. It is not merely nutrition that needs to be provided for."[110] In the first Exercise to Riolanus Harvey had touched also upon the use of the circulation, interweaving this doctrine of heat with the doctrine of respiration as he then held it, in a passage the last part of which I have quoted already. Quoted more fully he says:-- "And this, indeed, is the principal use and end of the circulation, for which the blood revolves with perpetual influence in its ceaseless course and is driven along its circuit: namely, that all the parts in dependence upon the blood may be kept alive by the primary innate heat and in their state of vital and vegetative being, and may perform all their functions; whilst, to use the language of physiologists, they are sustained and actuated by the inflowing heat and vital spirits. Thus by the aid of two extremes, viz.: cold and heat, is the temperature of the animal body retained at its mean. For as the air inspired tempers the too great heat of the blood in the lungs and center of the body and effects the expulsion of suffocating fumes, so in its turn does the hot blood, thrown through the arteries into the entire body, cherish and nourish and keep alive all the extremities, preventing extinction due to the power of external cold."[111]