Now as a soft and yielding substance, if it had no assistance from the hard parts, would certainly have, like molluscs, neither motion nor articulation, nature accordingly mingles in the body the hardness of the bones, and uniting these by close connection one to another, and knitting their joints together by means of the sinews, thus plants around them the flesh which receives sensations, furnished with a somewhat harder and more highly-strung surface than it would otherwise have had.
While resting, then, the whole weight of the body on this substance of the bones, as on some columns that carry a mass of building, she did not implant the bone undivided through the whole structure: for in that case man would have remained without motion or activity, if he had been so constructed, just like a tree that stands on one spot without either the alternate motion of legs to advance its motion or the service of hands to minister to the conveniences of life: but now we see that she contrived that the instrument should be rendered capable of walking and working by this device, after she had implanted in the body, by the determining spirit which extends through the nerves, the impulse and power for motion. And hence is produced the service of the hands, so varied and multiform, and answering to every thought. Hence are produced, as though by some mechanical contrivance, the turnings of the neck, and the bending and raising of the head, and the action of the chin, and the separation of the eyelids, that takes place with a thought, and the movements of the other joints, by the tightening or relaxation of certain nerves. And the power that extends through these exhibits a sort of independent impulse, working with the spirit of its will by a sort of natural management, in each particular part; but the root of all, and the principle of the motions of the nerves, is found in the nervous tissue that surrounds the brain.
We consider, then, that we need not spend more time in inquiring in which of the vital members such a thing resides, when the energy of motion is shown to be here. But that the brain contributes to life in a special degree is shown clearly by the result of the opposite conditions: for if the tissue surrounding it receives any wound or lesion, death immediately follows the injury, nature being unable to endure the hurt even for a moment; just as, when a foundation is withdrawn, the whole building collapses with the part; and that member, from an injury to which the destruction of the whole living being clearly follows, may properly be acknowledged to contain the cause of life.
But as furthermore in those who have ceased to live, when the heat that is implanted in our nature is quenched, that which has become dead grows cold, we hence recognize the vital cause also in heat: for we must of necessity acknowledge that the living being subsists by the presence of that, which failing, the condition of death supervenes. And of such a force we understand the heart to be as it were the fountainhead and principle, as from it pipe-like passages, growing one from another in many ramifications, diffuse in the whole body the warm and fiery spirit.
And since some nourishment must needs also be provided by nature for the element of heat—for it is not possible that the fire should last by itself, without being nourished by its proper food—therefore the channels of the blood, issuing from the liver as from a fountainhead, accompany the warm spirit everywhere in its way throughout the body, that the one may not by isolation from the other become a disease and destroy the constitution. Let this instruct those who go beyond the bounds of fairness, as they learn from nature that covetousness is a disease that breeds destruction.
But since the Divinity alone is free from needs, while human poverty requires external aid for its own subsistence, nature therefore, in addition to those three powers by which we said that the whole body is regulated, brings in imported matter from without, introducing by different entrances that which is suitable to those powers.
For to the fount of the blood, which is the liver, she furnishes its supply by food: for that which from time to time is imported in this way prepares the springs of blood to issue from the liver, as the snow on the mountain by its own moisture increases the springs in the low ground, forcing its own fluid deep down to the veins below.
The breath in the heart is supplied by means of the neighbouring organ, which is called the lungs, and is a receptacle for air, drawing the breath from without through the windpipe inserted in it, which extends to the mouth. The heart being placed in the midst of this organ (and itself also moving incessantly in imitation of the action of the ever-moving fire), draws to itself, somewhat as the bellows do in the forges, a supply from the adjacent air, filling its recesses by dilatation, and while it fans its own fiery element, breathes upon the adjoining tubes; and this
