In what manner blisters may lessen the fever and frequency of the pulse attending internal inflammations, I have elsewhere endeavoured to explain[32]; and shall only add here, that in the cases above recited, where the quick pulse and feverishness proceeded more from a pituitous infarction than a true inflammation of the lungs, blisters, by relieving this organ, in some measure, of the load of humours oppressing it, would render the circulation through its vessels freer, and consequently lessen the quickness of the pulse, and other feverish symptoms. It may not, however, be improper briefly to point out the reason, why blisters, which have been observed to be remarkably efficacious, even when early applied, in pleurisies[33], are less so in true peripneumonies. This difference, I imagine, may be accounted for from there being no immediate communication between the pulmonary vessels and those of the sides and back, to which the blisters are applied; whereas the _pleura_, and intercostal muscles, are furnished with blood-vessels from the intercostal arteries, which also supply the teguments of the _thorax_: so that while a greater flow of serous humours, and also indeed of red blood, is derived into the vessels of the external parts, to which the vesicatories are applied, the force of the fluids in the vessels of the inflamed _pleura_, or intercostal muscles, must be considerably lessened. Further, as the intercostal muscles and _pleura_ are, as well as the teguments of the _thorax_, supplied with nerves from the _true_ intercostals, blisters applied to the back and sides may perhaps, on this account also, have a greater effect in relieving inflammations there than in the lungs, which have nerves from the eighth pair, and from the _intercostals_ improperly so called. Edinburgh, May 23d, 1757. _Extract of a Letter from Dr._ Whytt, _Professor of Medicine in the University of_ Edinburgh, _and F. R. S. to Dr._ Pringle, _F.R.S._ Edinburgh, 10 Nov. 1757. WHAT you remark with regard to blisters being freely used by the physicians at London, in the cases mentioned in the paper I last sent you, is very just, and indeed what I knew; but altho’ their efficacy in such circumstances is now generally acknowleged both in England and Scotland, yet I do not remember that their remarkable quality in lessening the quickness of the pulse has been particularly attended to. This, therefore, I thought it might not be amiss to ascertain by a few careful observations.