THE celebrated Dr. BOERHAAVE in his In­stitutiones Medicae, lays it down as a fun­damental Principle, that the Blood, in passing through the Lungs, undergoes a Degree of Attrition considerably greater than in any other Part of the Body. This Attrition, according to his System, eminently increases the Heat of the Blood, at the same Time that it unites the ele­mentary Parts of it more intimately, and makes the whole Mass more dense and compact.

I am so far from assenting to this Theory, that I cannot but apprehend it is abundantly con­futed by the Texture and Appearance of the Parts themselves. Is it to be supposed, for Instance, that the right Ventricle of the Heart, which is thin­ner and apparently weaker than the left, can com­municate a greater Impulse to the Blood? This however is one of the first Requisites in producing a greater Degree of Attrition. There is still a more remarkable Difference between the Pulmonary Ar­tery, and the Aorta; the one, being loose and flaccid; the other, of so dense and compact a Na­ture, as to resemble, in some Measure, a Cartilage. [Page 4]If we go on to examine the Texture of the Parts, through which these Circulations are respectively performed, and observe how tender and spungy the Lungs are, in Comparison of the Muscular Parts of the Body, we cannot avoid coming to the same Conclusion; unless we would suppose that Nature made the stronger Substance to resist the weaker Impression: which surely would be inter­preting its Meaning very perversely.

Neither is it material to say that the Blood may suffer considerable Attrition by the alternate Motion of the Lungs in Respiration; either when expanded by the Air, or compressed by the Thorax. For, Action and Reaction being equal, it makes no Dif­ference in the Effect, whether the Lungs be every Moment pressed upon the Blood, or the Stream of Blood forced against the Lungs. Should we there­fore admit the Sum of the Attritions produced by these two Causes in the Lungs, to be greater than that which subsists in the corresponding Arteries of the Body, the same Objection would still remain, in regard to the Structure of the Parts. It would still be impossible to comprehend how the Author of Nature could proportion his Work so ill, as to make the Power of Resistance greatest, where the Impression was least: to give an unnecessary Degree of Strength to one System of Arteries, or an insuffi­cient one to the other.

The Difficulties that attend this Theory are not more extraordinary, than the Proofs, by which it is supported, are defective. In regard to the Velo­city of the two Circulations, Dr. HALLER expresses himself thus: ‘Cum sanguinis tantum per pulmones transeat in dato tempore, quantum per universum corpus humanum, sequitur, ut sanguis per pulmonem in eadem ratione fluat celeriùs, quâ ipse pulmo minor est. (In Boerhaav. Praelection. Acad. Vol. II. p. 169.)’ It is hard to say whether the Doctor is more unfor­tunate here in his Premisses, or his Conclusion. In the first Place, that equal Quantities of Blood pass in equal Times through the Lungs, and through the larger Circuit of the Body, is asserted, as far as I can inform myself, without any manner of Foun­dation either in Reason or Experiment. And se­condly, admitting it to be true, the Consequence would be just the contrary of what the Doctor supposes; namely, that the Motion of the Blood through the Lungs must be slower, and in the ve­ry same Proportion too, that he assigns for its being quicker. In a late Edition of the Commentary on Boerhaave's Institutions, I find the following Note, by which it appears that the Doctor begins to sus­pect at least, the Justness and Force of his Reasoning: ‘Haec omnia expedita sunt. Unum superest an velo­ciùs trajicitur per pulmones Sanguis? Videtur etiam [Page 6]lentiùs trajici, quia sanguis per corpus trajectus viam multò majorem eodem tempore absolvit,—& video hanc rationem fusè & ornatè proponi a Cl. Krugero in Physiologiâ Germanicè scriptâ. Vol. II. p. 230. Ed. Gotting. 1745.’

It has indeed been observed by Dr. Hales (Hae­mastat. p. 67.) that in the Lungs of a Frog, the Blood moves with greater Velocity than in any other Part of the Body. We are not however to make any general Inferences from this Phaenome­non, which seems to depend upon the peculiar Structure of the Frog and some other small Ani­mals, the Heart of which, having only one Ventri­cle, must of Necessity give the same Impulse to all the circulating Blood. We cannot, I say, conclude from Instances of this Kind, that the relative Ve­locity of the two Circulations is exactly the same in larger Animals, where the Heart, having two Ventricles, may possibly impel the Blood with dif­ferent Degrees of Force.

But to consider this Experiment in another Light. It appears that the Power exerted at the Origin of the pulmonary Artery and the Aorta being equal, the Blood moves with greater Velocity through the Lungs, than through any other Part of the Body: what are we to conclude from this, [Page 7]but that the Blood-Vessels of the Lungs are larger, allowing the Blood to pass through them with greater Freedom? Which being once admitted, Dr. Boerhaave's Theory is directly overthrown, in the very Instance that Dr. Haller refers to in Sup­port of it (Vol. II. p. 230.) and still more clearly with respect to larger Animals, where the Force impelling the Blood into the Lungs is with good Reason supposed to be less than that which throws it into the Aorta.