The organs more immediately necessary to life are, the HEART, which conveys to every part of the body that fluid, without a constant supply and change of which, vitality must be speedily exhausted; and the LUNGS, by whose functions this essential fluid undergoes those unknown changes, from the action of the atmosphere, which adapt it for the performance of the important duties to which we have alluded. In conformity with these views, the functions of the heart, and their connection with those of the lungs and brain, very naturally present themselves as the first objects of physiological inquiry; and there is certainly no discovery in modern times more interesting in its relations, and at the same time so useful in practical application, as that which has determined the nature of the connections between the functions of respiration and the motions of the heart; and shewn why the cessation of the former should occasion the destruction of the latter. The existence of this mysterious connection constituted a subject of interest and inquiry in the more remote ages, and it will not be unprofitable to take a review of the different theories which have been proposed for its explanation. Until the celebrated experiment[13] of _Hook_, it was supposed that the heart’s motion was maintained by the alternate contraction and dilatation of the lungs in the act of breathing; but the extraordinary philosopher above mentioned decided this point by exposing the thorax of a dog, and separating the pleura extensively from the external surface of the lungs, and then, by means of a pair of double bellows, keeping up a constant stream of air through the air cells; by this contrivance respiration was duly performed, while the lungs remained motionless, and yet it was found that the vigour of the heart’s action was not in the least impaired; whereas, if the theory which _Hook_ undertook to refute, had been founded in truth, the heart, under such circumstances, must necessarily have become quiescent. _Mr. Hunter_[14] supposed the existence of a sympathy, or association, between the motions of the heart and lungs; and the same opinion appears to have been entertained by _Dr. Currie_[15]; _Dr. Darwin_[16] deduced the existence of this immediate connection from that general law of the animal œconomy, by which motions that are frequently repeated in succession acquire the power of recurring in the same order, independently of the original exciting cause; “it is thus,” says he, “that by the stimulus of the blood in the right chamber of the heart the lungs are induced to expand themselves.” _Dr. Bostock_[17], however, has very satisfactorily opposed this hypothesis, by observing that in the fœtus the heart commences its contractions immediately upon its formation, while the lungs remain perfectly at rest; and that when the animal leaves the uterus, the motion of the lungs commences, but the periods of the contraction of the diaphragm bear no determinate ratio to