the act of respiring or breathing the air.

Respiration constitutes one of those functions which are properly termed vital, as being essential to life; for to live and to breathe are in fact synonymous terms. It consists in an alternate contraction and dilatation of the thorax, by first inspiring air into the lungs, and then expelling it from them in expiration.

It will perhaps be easy to distinguish and point out the several phenomena of respiration; but to explain their physical cause will be attended with difficulty: for it will naturally be inquired, how the lungs, when emptied of their air, and contracted by expiration, become again inflated, they themselves being perfectly passive?—How the ribs are elevated in opposition to their own natural situation? and why the diaphragm is contracted downwards towards the abdomen? Were we to assert, that the air, by forcing its way into the cavity of the lungs, dilated them, and consequently elevated the ribs and pressed down the diaphragm, we should speak erroneously. What induces the first inspiration it is not easy to ascertain: but after an animal has once respired, it would seem likely that the blood, after expiration, finding its passage through the lungs obstructed, becomes a stimulus, which induces the intercostal muscles and the diaphragm to contract, and enlarge the cavity of the thorax, in consequence perhaps of a certain nervous influence, which we will not here attempt to explain. The air then rushes into the lungs; every branch of the bronchial tubes, and all the cellular spaces into which they open, become fully dilated; and the pulmonary vessels being equally distended, the blood flows thro' them with ease. But as the stimulus which first occasioned this dilatation ceases to operate, the muscles gradually contract, the diaphragm rises upwards again and diminishes the cavity of the chest, the ribs return to their former state, and as the air passes out in expiration, the lungs gradually collapse, and a resistance to the passage of the blood again takes place. But the heart continuing to receive and expel the blood, the pulmonary artery begins again to be distended, the stimulus is renewed, and the same process is repeated, and continues to be repeated, in a regular succession during life: for though the muscles of respiration, having a mixed motion, are... Respiration (unlike the heart) in some measure dependent on the will, yet no human being, after having once respired, can live many moments without it. — In an attempt to hold one's breath, the blood soon begins to distend the veins, which are unable to empty their contents into the heart; and we are able only during a very little time to resist the stimulus to inspiration. In drowning, the circulation seems to be stopped upon this principle; and in hanging, the pressure made on the jugular veins may co-operate with the stoppage of respiration in bringing on death.

Boerhaave takes the principal uses of respiration to be the further preparation of the chyle, its more accurate mixture with the blood, and its conversion into a nutritious juice proper to repair the decays of the body. Other authors take a great use of respiration to be, by the neighborhood of the cold nitrous air, to cool the blood coming reeking hot out of the right ventricle of the heart through the lungs, and to act as a refrigeratory; others assert one grand use of respiration to be the throwing off the fuliginous vapors of the blood, along with the expelled air; and for inspiration they assert, that it conveys a nitro-serial ferment to the blood, to which the animal-spirits and all muscular motion are owing. But Dr Thurston rejects all these as being the principal uses of respiration; and from the experiments of Dr Croon, Dr Hooke, and others, made before the Royal Society, he shows the principal use of respiration to be that of moving or passing the blood from the right to the left ventricle of the heart, and so to effect circulation; whence it is that persons hanged, drowned, or strangled, so suddenly die, viz., because the circulation of the blood is stopped; and for the same reason it is that animals die so speedily in the air-pump. This use of respiration Dr Drake not only confirms, but carries farther, making it the true cause of the diaffole of the heart, which neither Borelli, Dr Lower, nor Mr Cowper, had well accounted for.

From experiments made on dogs and other animals, Dr Hales shows, that without respiration the blood would soon turn putrid and pestilential; and indeed the only animal exempted from the necessity of respiration is a fetus.

With regard to the force of respiration, the last mentioned author observes, that though a man, by a peculiar action of his mouth and tongue, may suck mercury 22 inches, and some men 27 or 28 high; yet he found from experience, that by the bare inspiriting action of the diaphragm and dilating thorax, he himself could scarcely raise the mercury two inches, at which time the diaphragm must act with a force equal to the weight of a cylinder of mercury, whose base is commensurate to the area of the diaphragm, and its height two inches, whereby the diaphragm must at the same time sustain a weight equal to many pounds; neither are its counteracting muscles, those of the abdomen, able to exert a greater force.

With regard to the quantity of moisture carried off by respiration, the Doctor, from an experiment on wood-ashes, estimates that quantity to be equal to 17 grains in 50 respirations; whence there will proportionably be 408 grains evaporated or breathed off in 1200 respirations, being the number in an hour, and thence in 24 hours 9792 grains, or 1.39 pounds; which supposing the surface of the lungs to be 41,635 square inches, then the quantity evaporated from that inward surface will be \( \frac{1}{10} \) part of an inch deep.

From the violent and fatal effects of very noxious vapors on the respiration and life of animals, the Doctor shows how the respiration is proportionably incommoded when the air is loaded with lesser degrees of vapors, which vapors do in some measure clog and lower the air's elasticity, which it best regains by having these vapors dispelled by the ventilating motion of the free open air, that is best rendered wholesome by the agitation of winds; thus what we call a close warm air, such as has been long confined in a room, without having the vapors in it carried off by communicating with the open air, is apt to give us more or less uneasiness in proportion to the quantity of vapors which are floating in it. And thus many of those who have weak lungs, but can breathe very well in the fresh country air, are greatly incommoded in their breathing when they come into large cities, where the air is full of fuliginous vapors; and even the most robust and healthy, in changing from a city to a country air, find an exhilarating pleasure arising from a more free and kindly inspiration, whereby the lungs being less loaded with condensing air and vapors, and thereby the vehicles more dilated with a clearer and more elastic air, a freer course is thereby given to the blood, and probably a purer air mixed with it.

Dr Priestley has shown, that one of the great uses of respiration is to carry off the phlogistic or putrid particles from animal-bodies, by which they are prevented from putrefying while alive, to which, without respiration, they would be as liable as though they were dead. See Air, no 46; Blood, no 29, et seq., and Putrefaction, passim.

Respiration of Fishes. See Fish, no 5 & 6.