YOL. XXIII.] PHILOSOPHICAL TRANSACTIONS. 701 



whole progress through the lungs. The pulmonary vein, which empties itself 

 into the left venlricle of the heart, spreads itself on the aspera arteria and bron- 

 chia, in the s;ime manner as the artery does. The necessary consequence of 

 this disposition is, that this artery and vein being coextended with, and fa?tened 

 to, the bronchia, must needs suffer such alteration of superficial dimensions, as 

 the bronchia do in the elevation or depression of the costae. While the ribs are 

 in a state of depression, wliether before communication with the external air or 

 after, the annular cartilages of the bronchia shrink one into another, and by 

 that means their dimensions are exceedingly contracted. In conformity to this 

 condition of the bronchia, the pulmonary artery and vein likewise must either, 

 by means of their muscular coats, contract themselves to the same dimensions, 

 or lie in folds or corrugations; which is less probable. On the other hand, 

 when the ribs are elevated, and the diaphragm bears downward, the air rushing 

 into the lungs, shoots out the cartilaginous rings, and divaricates the branches 

 of the trachea, and by them extends and divaricates the several divisions of the 

 pulmonary artery and veins, and thereby lengthens and enlarges their cavities. 



This enlargement of their cavities is very considerable, not only on account 

 of the addition which they thence receive in length, but also on account of 

 their divarication. For when the ribs are depressed, and the lungs subside, the 

 blood-vessels are not only contracted, but their branches, which are exceedingly 

 numerous, approach each other, and lie in juxta-position, by which their cavities 

 are very much compressed and straitened; when the ribs are elevated, and the 

 lungs turgid with air, not only the fibres, by which their coats in the opposite 

 state were contracted, are extended, but those innumerable vessels, which lying 

 before in lines almost parallel on each other, compressed each other, making an 

 acute angle at their junctures, are divaricated and separated from each other, 

 and make an obtuse one, by which their channels are widened. Thus a pas- 

 sage is opened to the blood, from the right ventricle of the heart to the left, 

 through the lungs, to which it could not otherwise pass ; and the opposition 

 which the blood, contained in that ventricle, must otherwise necessarily have 

 made to its constriction, is taken off, and the systole thereby facilitated. 



Nor is that all: for the diastole being caused by the force of the blood rush- 

 ing into the ventricles, this ampliation and extension of the pulmonary artery is 

 a sort of check or counterpoise to it, and prevents an endeavour towards two 

 contrary actions at once, which must necessarily frustrate both. For the heart 

 being a springy, compressible body, whose proper action, which is contraction, 

 depends on the influx of certain fluids into its fibres or substance ; and besides, 

 containing a fluid in its ventricles, or great cavities, in one of which is the 

 mouth of this artery, the action of this vessel must in a great measure resemble 



