And this seems to me to be the cause of this spontaneous respiration of ours; for often the mind is occupied in discourse with others, or is entirely quiescent when the body is relaxed in sleep, but the respiration of air does not cease, though the will gives no cooperation to this end. Now I suppose, since the heart is surrounded by the lungs, and in the back part of its own structure is attached to them, moving that organ by its own dilatations and compressions, that the inhaling and exhaling157 of the air is brought about by the lungs: for as they are a lightly built and porous body, and have all their recesses opening at the base of the windpipe, when they contract and are compressed they necessarily force out by pressure the air that is left in their cavities; and, when they expand and open, draw the air, by their distention, into the void by suction.
This then is the cause of this involuntary respiration—the impossibility that the fiery element should remain at rest: for as the operation of motion is proper to heat, and we understand that the principle of heat is to be found in the heart, the continual motion going on in this organ produces the incessant inspiration and exhalation of the air through the lungs: wherefore also when the fiery element is unnaturally augmented, the breathing of those fevered subjects becomes more rapid, as though the heart were endeavouring to quench the flame implanted in it by more violent158 breathing.
But since our nature is poor and in need of supplies for its own maintenance from all quarters, it not only lacks air of its own, and the breath which excites heat, which it imports from without for the preservation of the living being, but the nourishment it finds to fill out the proportions of the body is an importation. Accordingly, it supplies the deficiency by food and drink, implanting in the body a certain faculty for appropriating that which it requires, and rejecting that which is superfluous, and for this purpose too the fire of the heart gives nature no small assistance.
For since, according to the account we have given, the heart which kindles by its warm breath the individual parts, is the most important of the vital organs, our Maker caused it to be operative with its efficacious power at all points, that no part of it might be left ineffectual or unprofitable for the regulation of the whole organism. Behind, therefore, it enters the lungs, and, by its continuous motion, drawing that organ to itself, it expands the passages to inhale the air, and compressing them again it brings about the expiration of the imprisoned air; while in front, attached to the space at the upper extremity of the stomach, it warms it and makes it respond by motion to its own activity, rousing it, not to inhale air, but to receive its appropriate food: for the entrances for breath and food are near one another, extending lengthwise one alongside the other, and are terminated in their upper extremity by the same boundary, so that their mouths are contiguous and the passages come to an end together in one mouth, from which the entrance of food is effected through the one, and that of the breath through the other.
Internally, however, the closeness of the connection of the passages is not maintained throughout; for the heart intervening between the base of the two, infuses in the one the powers for respiration, and in the other for nutriment. Now the fiery element is naturally inclined to seek for the material which serves as fuel, and this necessarily happens with regard to the receptacle of nourishment; for the more it becomes penetrated by fire through the neighbouring warmth, the more it draws to itself what nourishes the heat. And this sort of impulse we call appetite.
But if the organ which contains the food should obtain sufficient material, not even so does the activity of the fire become quiescent: but it produces a sort of melting of the material just as in a foundry, and, dissolving the solids, pours them out and transfers them, as it were from a funnel, to the neighbouring passages: then separating the coarser from the pure substance, it passes the fine part through certain channels to the entrance of the liver, and expels the sedimentary matter of the food to the wider passages of the bowels, and by turning it over in their manifold windings retains the food for a time in the intestines, lest if it were easily got rid of by a straight passage it might at once excite the animal again to appetite, and man, like the race of irrational animals, might never cease from this sort of occupation.
As we saw, however, that the liver has especial need of the cooperation of heat for the conversion of the fluids into blood, while this organ is in position distant from the heart (for it would, I imagine, have been impossible that, being one principle or root of the vital power, it should not be hampered by vicinity with another such principle), in order that the system may suffer no injury by the distance at which the heat-giving substance is placed, a muscular passage (and this, by those skilled in such matters, is called the artery) receives the heated air from the heart and conveys it to the liver, making its opening there somewhere beside the point at which the fluids enter, and, as it warms the moist substance by its heat, blends with the liquid something akin to fire, and makes the blood appear red with the fiery tint it produces.
Issuing thence again, certain twin channels, each enclosing its own current like a pipe, disperse air and
