OF THE RESPIRATORY VOLUMES. - FOP 1500 years, from the time of Galen to Robert Boyle, naturalists, physicians, and philoso phers disputed the simple operation of draw ing air into the thorax. There were three explanations given :—First, " That by the dila tations of the chest the contiguous air is thrust away, and that, pressing upon the next air to it, and so onwards, the propulsion is continued till the air be driven into the lungs, and so dilate them." Secondly, That the chest is like a pair of common bellows, " which comes therefore to be filled because it was dilated." Thirdly, That the lungs are like a bladder " which is therefore dilated because it is filled." The great philosopher Boyle adopts the bel lows' action viz., that the lungs are filled with air, because the chest is dilated, and that with 'out the motions of the thorax they could not be filled. " Indeed," says Boyle, " the dia phragm forms the principal instrument of ordinary and gentle respiration, although to restrained respiration, if I may so call it, the intercostal muscles, and perhaps some others, may be allowed eminently to concur." * Co temporary with Boyle, we find Richard Lower t (1667) correctly understanding the respiratory action; he makes a dog breathe like a broken-winded horse, by dividing the phrenic nerve. What Boyle and Lower demon strated, every one now believes without dispute; yet it took 100 years' disputation, through a number of unfounded hypothetical and contra dictory speculations, before the truths which Boyle and Lower promulgated were received. As late as the eighteenth century, little more than one hundred years ago (A. D. 1737), it was stated in the Gulstonian Lectures before the Royal College of Physicians that there was air between the pleur t,— a con dition which we now know is almost instant death. The first great epoch in the history of respiration was at the time of Harvey (1628), when he published his first work on the circulation of the blood, though at this time he did not stand commended for his discovery ; for most persons opposed it ; others said it was old ; and the epithet " cir culator," in its Latin invidious signification, was applied to him. We know respiration depends upon the weight of the air; and at a very remote period air was known to possess the quality of weight. Aristotle and
other ancient philosophers expressly speak of the weight of the air. The process of re spiration is attributed by an ancient writer to the pressure of the atmosphere forcing air into the lungs.* Galileo was therefore fully aware that the atmosphere possessed this pro perty ; yet when his attention was so immedi ately directed to one of the most striking effects of it, he did not see its connection with respira tion. It was reserved for his pupil, Torricelli, to discover (1643) the true law of atmo spheric pressure ; and as we can find no phi losophical reason assigned, prior to this date, why air enters the lungs in inspiration, we may date this as a first step in the advance of knowledge upon our subject. Nevertheless, no less an authority than Swammerdam adopted, for upwards of twenty years after this, the unphilosophical reasoning of Des cartes, that the air was forced into the lungs by its increased density around the breast, oc casioned by the dilatations of the thorax, in consequence of the elevation of the ribs.
In 1667 some attention was paid to respira tion being maintained by distinct volumes of air ; for Hook kept a dog alive with common bellows by artificial respiration.t Fabricius, in the beginning of the 17th century, cor rectly explained the action of the diaphragm. Borelli is the earliest physiologist (1679) who established an experimental inquiry into the quantity of air received by a single inspiration.* Jurin improves upon Borelli. About this time (1708) Dr. James Keile made some correct measurements of the volume of air breathed.' Then followed Dr. Hales, who threw more light upon the doctrine of air, the power of respiration, and the power of the heart, than all his predecessors ; yet he was quite ignorant of the use of respiration ; and at this period (1733) really very little was known upon the subject. In 1757 and fol lowing years, Black, Rutherford, Lavoisier, Priestley, and Scheele, the chemists of the age, threw light upon the matter by discover ing the composition of the atmosphere, and consequently the composition of respired air. It is since the time of Black that the most valuable mass of our knowledge upon respir ation has been discovered.