For as the respiration goes in and out the inward fire attached thereto follows it; and whenever in its constant oscillations this fire enters in through the belly 79a and lays hold on the meats and drinks, it dissolves them, and dividing them into small particles it disperses them through the outlets by which it passes and draws them off to the veins, like water drawn into channels from a spring; and thus it causes the streams of the veins to flow through the body as through a pipe.

Once again let us consider the process of respiration, and the causes in virtue of which it has come to be such as it now is. 79b This, then, is the way of it. Inasmuch as no void exists¹ into which any of the moving bodies could enter, while the breath from us moves outwards, what follows is plain to everyone— namely, that the breath does not enter a void but pushes the adjacent body from its seat; and the body thus displaced drives out in turn the next; and by this law of necessity every such body is driven round towards the seat from which the breath went out and enters therein, filling it up and following the breath; and all this takes place as one simultaneous process, like a revolving wheel, because that no void exists. 79c Wherefore the region of the chest and that of the lungs when they let out the breath become filled again by the air surrounding the body, which filters in through the porous flesh and circulates round. And again, when the air is repelled and passes out through the body it pushes the inspired air round and in by way of the passages of the mouth and of the nostrils. The originating cause 79d of these processes we must assume to be this. Every living creature has its inward parts round the blood and the veins extremely hot, as it were a fount of fire residing within it; and this region we have, in fact, likened to the envelope of the fish-weel, saying that all that was extended at its middle was woven of fire, whereas all the other and outward parts were of air. Now we must agree that heat, by Nature’s law, goes out into its own region to its kindred substance; and inasmuch as there are two outlets, the one out by way of the body, 79e the other by way of the mouth and the nose, whenever the fire rushes in one direction it propels the air round to the other, and the air which is thus propelled round becomes heated by streaming into the fire, whereas the air which passes out becomes cooled. And as the heat changes its situation and the particles about the other outlet become hotter, the hotter body in its turn tends in that direction, and moving towards its own substance propels round the air which is at the former outlet; and thus the air, by continually undergoing and transmitting the same affections, causes inspiration and expiration to come about as a result of this double process, as it were a wheel that oscillates backwards and forwards.

Moreover, we must trace out in this way the causes of the phenomena connected with medical cupping-glasses, 80a and the causes of deglutition, and of projectiles, whether discharged aloft or flying over the surface of the earth; and the causes also of all the sounds² which because of their quickness or slowness seem shrill or deep, and the movement of which is at one time discordant because of the irregularity of the motion they cause within us, and at another time concordant because of its regularity. For the slower sounds overtake the motions of the earlier and quicker sounds when the latter begin to stop 80b and have already fallen to a speed similar to that with which the slower sounds collide with them afterwards and move them; and when the slower overtake the quicker sounds they do not perturb them by imposing upon them a different motion, but they attach to them the beginning of a slower motion in accord with that which was quicker but is tending to cease; and thus from shrill and deep they blend one single sensation, furnishing pleasure thereby to the unintelligent, and to the intelligent that intellectual delight³ which is caused by the imitation of the divine harmony⁴ manifested in mortal motions.

Furthermore, as regards all flowings of waters, and fallings 80c of thunderbolts, and the marvels concerning the attraction of electron⁵ and of the Heraclean stone⁶—not one of all these ever possesses any real power of attraction; but the fact that there is no void, and that these bodies propel themselves round one into another, and that according as they separate or unite they all exchange places and proceed severally each to its own region,—it is by means of these complex and reciprocal processes that such marvels are wrought, as will be evident to him who investigates them properly. 80d

Moreover, the process of respiration—with which our account commenced—came about, as we previously stated, in this manner and by these means. The fire divides the foods, and rises through the body following after the breath; and as it rises, with the breath it fills the veins from the belly by drawing into them from thence the divided particles. And it is owing to this that in all living creatures the streams of nutriment course in this way through the whole body. And inasmuch as these nutritive particles are freshly divided 80e and derived from kindred substances,—some from fruits, and some from cereals, which God planted for us for the express purpose of serving as food,⁷—they get all varieties of colors because of their commingling, but red is the color that runs through them most of all, it being a natural product of the action of the fire in dividing the liquid food and imprinting itself thereon.⁸

Wherefore the color of the stream which flows through the body acquired an appearance such as we have described; and this stream we call “blood,” which is the nutriment of the flesh 81a and of the whole body, each part drawing therefrom supplies of fluid and filling up the room of the evacuated matter. And the processes of filling and evacuating take place just as the motion of everything in the Universe takes place, namely, according to the law that every kindred substance moves towards its kind. For the bodies which surround us without are always dissolving us and sending off and distributing to each species of substance what is akin thereto; while the blood-particles, again, being minced up within us and surrounded by the structure of each creature as by a Heaven, are compelled to copy the motion of the whole; 81b hence, when each of the particles that are divided up inside moves towards its kin, it fills up again the emptied place. And when what passes out is more than the inflow every creature decays, but when less, it increases. Now when the structure of the whole creature is new, inasmuch as the triangles which form its elements⁹ are still fresh, and as it were straight from the stocks, it keeps them firmly interlocked one with another, and the whole mass of it is of a soft composition, 81c seeing that it is newly produced from marrow and nourished on milk; and as the triangles contained therein, which have invaded it from without and go to form the meats and drinks, are older and weaker than its own, it divides and overcomes them with its own new triangles, and thus renders the creature large by feeding it on many similar substances. But when the root of the triangles¹⁰ grows slack owing to their having fought many fights during long periods, 81d they are no longer able to divide the entering triangles of the food and assimilate them to themselves, but are themselves easily divided by those which enter from without; and in this condition every animal is overpowered and decays; and this process is named “old age.” And finally, when the bonds of the triangles in the marrow which have been fitly framed together no longer resist the strain but fall asunder, they let slip in turn the bonds of the soul, and it, when thus naturally set loose, flies out gladly; 81e for whereas every process which is contrary to nature is painful, that which takes place naturally is pleasurable. So too, in like manner, the death which occurs in consequence of disease or by wounds is painful and violent, but that which follows on old age and constitutes a natural end is the least grievous of deaths and is accompanied by more of pleasure than of pain.

The origin of disease is plain, of course, to everybody. 82a For seeing that there are four elements of which the body is compacted,—earth, fire, water and air,—when, contrary to nature, there occurs either an excess or a deficiency of these elements, or a transference thereof from their native region to an alien region; or again, seeing that fire and the rest have each more than one variety, every time that the body admits an inappropriate variety, then these and all similar occurrences bring about internal disorders and disease. For when any one element suffers a change of condition that is contrary to nature, all its particles that formerly were being cooled 82b become heated, and the dry presently become moist, and the light heavy, and they undergo every variety of change in every respect. For, as we maintain, it is only the addition or subtraction of the same substance from the same substance in the same order and in the same manner and in due proportion which will allow the latter to remain safe and sound in its sameness with itself. But whatsoever oversteps any of these conditions in its going out or its coming in will produce alterations of every variety and countless diseases and corruptions. 82c

Again, in the structures which are naturally secondary¹¹ in order of construction, there is a second class of diseases to be noted by him who has a mind to take cognizance of them. For inasmuch as marrow and bone and flesh and sinew are compacted from the elements,—and blood also is formed from the same constituents, although in a different way,— most of the other maladies come about like those previously described, but the most severe of them have dangerous results for the reason following: whenever the production of these secondary substances proceeds in the reverse direction, then they are corrupted. For in the order of nature flesh and sinews arise from blood,¹² 82d the sinew from the fibrine because of its kindred quality, and flesh from the coagulated substance which coagulates on its separation from the fibrine; and further, the substance which is derived from the sinews and flesh, being viscid and oily,¹³ not only glues the flesh to the substance of the bones but also feeds and increases the bone itself which encloses the marrow, while that which is formed of the purest kind of triangles, very smooth and very oily, filters through 82e the density of the bones, and, as it oozes and drips from the bones, moistens the marrow. Now when each of these substances is produced in this order, health as a rule results; but if in the reverse order, disease. For whenever the flesh is decomposed and sends its decomposed matter back again into the veins, then, uniting with the air, the blood in the veins, which is large in volume and of every variety, is diversified by colors and bitter flavors, as well as by sharp and saline properties, and contains bile and serum and phlegm of every sort.