The heart is a muscular body, situated in the breast or chest, somewhat between the lobes of the lungs, and formed by nature to be the principal organ for the circulation of the blood and other fluids. Its structure is wonderful, and of a most curious piece of mechanism. It has two principal cavities, called _ventricles_, which are separated length-ways by a wall, and distinguished into the right and left.[4] Footnote 4: They might with equal propriety, be termed _anterior_, or foremost, and _posterior_ or hindermost. Each of these cavities or _ventricles_, have particular valves, call’d ears, or _auricles_; which perform the function somewhat similar to the suckers of a pump: By these contrivances the heart receives the blood from the veins, and expels it again by the arteries, to every part of the human body; in a manner that I shall hereafter describe. The lungs are organs immediately concerned in the circulation of the blood also; their function is to receive the air, to purify it, to circulate, and to distribute it in due proportion in the sanguineous mass, and to extract and expel that part of air already made use of; and thus act the part of ventilators in the animal œconomy. They are divided into two lobes. These lobes are a continuation of little bladders, that have an immediate continuation with the wind-pipe, _aspera arteria_. The wind-pipe from the larynx down to the very lungs, is protected externally with semicircular gristles, in order to keep it always open, and so shelter it from external injury; when it enters the lungs it spreads itself into numberless branches like a tree, and sends ramifications into every little air-bladder in the lungs. All along these ramifications and the air-bladders, are arteries, veins, and lymphatic vessels, which have an immediate communication with the heart, by means of the pulmonary arteries and veins. Now at the instant of _inspiration_, these little bladders in the lungs are filled with air, which infuses itself by the small arteries into the whole mass of blood; and at the _expiration_ again, the air which has been made use of, and drawn from the neighbouring little veins, is again expelled by the wind-pipe through the mouth: And thus is _respiration_ performed by alternate motions of the lungs; receiving the fresh, and alternately expelling the foul air again; thence justly may they be deemed the ventilators of the human _automaton_, and a principal organ of life, in the animal creation in general. The upper part of the wind-pipe, which is in the _fauces_ of the mouth, is called the _larynx_. This is the organ of voice. In this animal mechanism is contained, all the variety of formation requisite to every kind of sound that can be performed upon any instrument whatever. Its parts are chiefly of a gristly substance, at the same time exquisitely delicate, with nerves and glands; from whence it is subject to diseases on the slightest indisposition.