The lungs are the ventilators in the system: they receive and let in the air; they sift it from all its gross particles, and extract the useful parts from this universal element of life; thus they give from every little vesicle, a due proportion of extracted air to the blood by the adjacent veins; and by the neighbouring arteries, again extract the foul air from the circulated blood, which by the mouth and nostrils is expelled again; at that instant a fresh quantity of air is drawn in again for a new supply. Thus by intervals, the lungs, like a perfect ventilatory machine, repeatedly, and without intermission, keep time with _systolic_ and _diastolic_ motions of the heart, in exchanging fresh and foul air for the support of this wonderful animal structure. This is the function of breathing _respiration_, namely _inspiration_, or the drawing in the fresh air; and _expiration_, the expelling the foul air. The heart may be considered as the grand perpetual pumping engine, constructed on principles, so as to have the least friction possible. This curious pump, _the heart_, sends the fluid of life, _the blood_, to all the parts, even the remotest in the whole machine. The _arteries_ are the canals of the conveyance; they are strong elastic tubes, whose fibres are of spiral direction, and receive from the heart a power of contraction; which is continued from the heart in the moment of the _systole_, even to the remotest and most minute ramifications; by which the blood is pushed along to the minutest extremities. This is immediately succeeded with a _diastolic_ reception of a fresh quantity of blood; so that on succession, the _diastole_ and _systole_ is perpetuated not only in the heart, but throughout all the arteries, in a friendly correspondence over the whole system: And this constitutes the _pulse_. The veins, however, are somewhat of a different texture from the arteries; whilst the arteries transport the blood by an elastic and pulse-like contraction, the veins on the contrary, receive it from them at the extremities, in an easy and imperceptible gradation; for whereas the fibres in the arteries are in a spiral direction, elastic and strong, the coat of the veins are tender, and the fibres more longitudinal. Hence, the wounds in the arteries and veins differ greatly; for whilst the arteries are for ever in a strong agitation, the veins perform their functions with less exercise; and therefore the wounds in arteries are always attended with danger, whilst the veins heal easy. From the same reason also, the arterial blood differs from the venal; for as the arterial blood is sent to all the Parts, as the grand nutriment, from whence the glands secrete their peculiar humours; the veins only transport it back again, to prepare it for a new rotation. Whence the blood in the arteries is of a high colour, and of a more alkalascent nature, than that in the veins.