From what has been said above respecting the history of opinions on the subject of respiration, it will appear that this doctrine amounts very nearly to the hypothesis of Priestley, that the use of respiration is to carry off phlogiston from the blood. To the same principle, al though still more indefinitely expressed, we might refer the recrementitious steams of Boyle, and even the fuligi nous excrement of Galen, and indeed he goes so far as to speak of respiration as a species of combustion, and describes the lungs as a kind of vent or chimney for the emission of vapour. He farther attributes the dark co lour of venous blood to the mixture of a black or sooty matter with it, and the change to scarlet to the discharge of this matter, so that certain individuals who have been disposed to exalt the knowledge of the ancients, have not hesitated to ascribe to Galen a very correct acquaintance with the modern doctrines of respiration. But, unfortu nately for the argument, Galen supposed this combustion to occur, not in the lungs, but in the heart, and conceived that a main object of respiration was to cool the blood, or, as he expresses it, to quench the fire of the heart.
There is still another indirect effect of respiration to be mentioned, the conversion of chyle into fibrin. It may be presumed that, as this change consists in taking from the blood a substance which has an undue proportion of car bon, a superabundance of this element will be left in the blood from which the fibrin has been secreted, and this blood will be carried to the lungs, where it will part with its excess of carbon. And here we are presented with another instance of the reciprocal action of the different functions upon each other. Arterial blood contains the elements of the muscular fibre ; when the blood has part ed with its fibrin, it becomes venalized, and now contains the matter which, by uniting with oxygen in the lungs, evolves caloric ; this gives energy to the muscles and nerves, and imparts to the whole machine its capability for action.
We have hitherto said little respecting the mechanical effects of respiration, although, until lately, these were what entirely engrossed the attention of physiologists. Boyle dwelt much upon the power of the lungs in destroy ing the spring of the air, and the same effect was insisted upon by \layow and his contemporaries. But since we are become better acquainted with the nature of the at mosphere, we know that a great part of what was formerly attributed to mechanical, really depends upon chemical causes, and that, when they supposed that the air lost its spring, and was, in this manner, rendered unfit for respi ration and combustion, it was owing to the conversion of part of its oxygen into carbonic acid, and frequently to the absorption of the acid thus formed. As they were igno rant of the chemical properties of the air, and of the changes which it experiences in passing through the lungs, they imputed all the effects of respiration to the changes of bulk in the thorax ; and from observing that, when re spiration ceased, the motion of the heart was also sus pended, they concluded that one principal object of respi ration was to expand the lungs, so as to permit the blood to arrive at the left side of the heart. Numerous experi ments were instituted to establish this position, and so lit tle were physiologists in the habit of making experiments, or of drawing correct deductions from them, that this opinion was adopted even by Hales and Haller. It had indeed been observed by Boyle, a century before, that the expansion of the lungs was of no avail unless the air was perpetually changed; but ingenious conjectures were al ways at hand to disguise the truth, and for a long time these prevailed over what we should now consider the ob vious inference from the facts.