405. It is then the venous system that is immediately related to inspiration, and the arterial to expiration. Each respiratory action exerts a specific influence over its own sanguiferous system, and the influence of the one action is the reverse of that of the other, as the two currents they work flow in opposite directions. The lungs, in inspiration, expand and receive the venous stream; in expiration, collapse and expel the arterial stream. The expansion of the lungs in inspiration is thus simultaneous with the dilatation of the heart: during the inspiratory action both organs receive their blood. The collapse of the lungs in expiration is simultaneous with the contraction of the heart: during the expiratory action both organs expel their blood. 406. We are thus enabled to form a clear and exact conception of the mechanism and action of both parts of this complicated function. Almost all the points connected with the systemic circulation were established upwards of three hundred years ago (279), but many points connected with the pulmonic circulation have been established only recently. Our knowledge of the phenomena of both, and of their mutual relation and dependence, has been slowly increasing, and is at length tolerably complete; and now that we understand the exact office and working of each, we see that the action of the one is not only in harmony with that of the other, but co-operates with it, and renders it perfect. 407. But although the main points relative to the influence of inspiration and expiration over the pulmonary circulation may be said to be universally admitted, still physiologists are not agreed as to the relative quantities of blood which are transmitted through the lungs during these different respiratory states. All are agreed that the state of inspiration is favourable to the passage of the blood through the lungs: some maintain that this expansion of the lungs in inspiration is essential to the pulmonary circulation. There is the like general consent that the state of expiration retards the flow of blood through the lungs; by many it is conceived that it completely stops the current. By these physiologists it is supposed that, during the action of expiration, the lungs are in a state of collapse; that they contain a comparatively small portion of air; that in this state the air vesicles are so compressed, and the pulmonary blood-vessels so coiled up, that the lungs are absolutely impermeable, and consequently, that when the blood arrives at the right chambers of the heart, it is incapable of making its way to the left. This, according to a prevalent theory, is the immediate cause of death in asphyxia, the state of the system induced by suspended respiration, as in drowning, hanging, and suffocation. Death takes place in this condition of the system, it is argued, because the circulation of the blood is arrested at the right side of the heart, cannot permeate the lungs, and consequently cannot