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The Source, or Origin, of the Slocd is the Chyle, which paiTing the Latlcals, is deliver'd into the Subclavian ; where, mixing with the Stood, they proceed together to the Right Ventricle of the Heart ; and there, being yet more intimately mix'd, they circulate together thro the whole Body; till after feveral Circulations, and Secretions, at the feveral Strainers of the Body, they are affimilated fo as to make one uniform compound Mais, which appears to be nothing elfe but Chyle, alter'd by the Artifice of Na- ture, and exalted into Stood ; there being no Appearance of any thing extraneous mix'd with the Liquor circu- lating in the Slood-Veffeh, but Chyle ; excepting what had been before feparated from it, for fome particular Purpo- fes, which being once ferved, it is returned to it again : unlefs, perhaps, it may receive fome Portion of Air in the Lungs. That there is Air mix'd in the Stood, and circulating with ir, is pall doubt 5 but, whether any more than was at firlt contain'd in the Food whereof the Chyle is form'd, is a Qucflion not yet decided. The principal Arguments urg'd for it, are, the Ncceftity of Refpirarion; which is accounted for on another Principle : and the florid Colour the Stood receives in the Lungs, and firlt fhews in the Vena 'Pulmo- nalis ; which is countenanced by an Experiment made with the red grumous Part of the Stood after Coagulation on Slood-lettwg ; for, upon turning the Under-Surface, which was betore black, upwards, and expofing it to the Air, by its Contact: therewith it acquires a florid Colour, like that of the Stood in the Vena 'Pubncnalis. But this Effect others account for from the extraordinary Agitation and Comminution of the Blood in the Lungs.
Indeed, Dr. Keill and fome others go further. Mr. Soyle having examin'd the Specifick Gravity of Stood, and found that of the Serum to be greater than that of Stood, in the Proportion of 1190 to 1040, i.e. nearly as 8 to 7, it follow'd, that the Cruor, or Slood-Globules, were fpecifically lighter than the Scrum, and that in a great degree ; which was further confirm'd by the Globules being fuftain'd in the Serum, both while circulating, and when let out. Hence it was conjc£tur'd, that thefe Globules were nothing elfe but thin Veficles fill'd with a fubtle aerial Subftance: And this Opinion was confirm'd from its being obferv'd, in view- ing the Circulation by aMicrofcope, that a S loo d-G lobule, in parting rhro a very narrow Veflel, wou'd change its Shape from a Globular to an Oval Form, and wou'd again recover its former Figure, as foon as it was got thro its narrow PalTage; which Appearance was naturally enough afcribed to the Elafticityof the included Air: And, from this Conjec- ture, were accounted for a great Number of the Phenomena of the Animal Oeconomy, particularly Dr. Keiths Theory of Mufcular Motion. Eut this Principle Dr. Jurin has exa- min'd, and appears to have overthrown. He made feveral Experiments, in fome of which the Cruor before fufpended a-top of the Serum, by its Adhefion to the Sides of the Porringer being cut ofK, and put in another VefTel of Se- rin??, immediately iunk. In others, the Cruor buoy'd up in the Serum, even without any Adhefion to the Sides of the Glafs, and merely by the Bubbles of Air adhering to its Surface, upon including it in a Receiver, and exhaufting the Air, the Bubbles burfting, the Cruor wou'd fink ; whence he concludes, the globular Part of the Slood to be heavier than the ferous : And, from other Experiments, he afcertains the Proportion of the Gravity of Slood to that of Serum, to be as 1054 to 1050 ; whence the Quantity of the Glo- bules being before fix'd at £ of the whole, the precife Gra- vity of the Globules beyond that of the Serum is eafily determined. The Slood-Globulcs, therefore, are not Vefi- cles fill'd with Air, or any other fluid Subftance lighter than Serum : Which is further confirm'd from this, that Slood- Globulcs are not found to dilate, or undergo any Alteration, in an exhaufted Receiver, when view'd thro a Microfcope ; whereas, were they fill'd with any elaftick Fluid, they would either burft, or at leaf! dilate into 70 or 80 times the Space.
As to the Heat of the Stood, Authors are exceedingly divided about the Caufe thereof: The Antients afcrib'd it to a vital Flame, or innate Heat lodg'd in the Heart, and thence communicated to the Slood. Dr. Willis imagines a kind of Accenfion in the Slood ; and thinks its Heat refults from its being, as it were, fet on fire, and perfevering in that State. Dr. Henjbaw folves it from an Ebullition con- sequent on the Mixture of two Fluids, fo diiTimilar as the Chyle and Slood. Others have recourfe to the chymical Principles of Alcali and Acid; others to the mutual Action of the Principles, or component Parts of he Slood, by means whereof an intcftine Motion, and by that means an inteltine Heat, or Incalefcence, is effected. Dr.tDrake, with more Rea- lon, attributes the Heat of the Slood to the Spring of the Air inclos'd together with it in the VelTels : For, Air being incloled in the Slood-Vejfels, will endeavour to expand it felt, and, confequenrly, if it have Force enough, will drive outwards the Parts of the Body that inclofe it ; by which means it caufes the Stood to beat againft the Sides of the VelTels, which having mufculous, contractile Coats, do in
their Turns comprefs it again, and fo caufe a reciprocal Jl-'.Jlits in the Slood, greater than the mere circulatory Mo- tion cou'd ; whence the Parts of the Solids, or containing VelTels, being put into a conftant Agitation, a Heat is pro- duced in both, which they mutually impart to each other. Lajlly, Dr. Soerbaave accounts for it from the Aclion of the Heart, and the Re-action of the Aorta : For, the Stood, driven by the Heart obliquely againli the Sides of the Aor- ta, prefles them, and fpends almoll its whole Momentum againft the Curvity thereof and is, by its Figure and Elas- ticity, prefs'd back again. Every Moment of Time, there- fore, each Particle of Stood acquires a new Motion, a new Nifus and Rotation : Hence follows a perpetual Attrition, Attenuation, rubbing oft" of Angles, and a Si- militude and Homogeneity of all the Parts ; and hence the Mafs derives its Fluidity, Heat, Divifion into Particles accommodated to all VelTels, PrefTure into the Lateral Tubes, &c.
Nor is the Caufe of the Rednefs of thsSloodlefs obfeurei The Chymifts account for it from the Exaltation of its Sul- phur; others from the Mixture of faline and fubacid Juices with fulphureous; and others from the Colour of the Heart. The French Philofophers attribute this Rcdnefs to the Smallnefs of the Size, and Roundnefs of the Figure of the Particles that compofe the Cruor ; notwithltanding, that red being the Colour, of all others, leaft refrangible, and the Globular Figure, of all others, molt refrangible, that Figure feems, ofall others, leaft apt to produce this Colour. Others fetch the Colour of the Shod from the Impregna- tion of the Air in the Lungs: For, that Air is difpos'd to produce fuch an Effefl, appears from the Experiment a- bove. But others, more referv'd, extend this Effect of the Air no further, than to account for the Difference of Red- nefs between the Venal and Arterial Slood ; fuppofing, that after its Colour has been heighten'd, and render'd more florid by the Mixture of the Air in the Lungs, it retains it pretty well in the Arteries ; but that circulating in the Bo- dy, and carried thro the Veins, the Air tranfpires, by de- grees, thro the Pores of the VelTels, and leaves the Liquor of a paler Dye. Soerbaave accounts for the Colour of the Stood from the fame Caufe as for its Heat, viz. from the Action of the Heart, and Re-action of the Sides of the Aorta, Sorelli, to afcertain the Caufe of the Rednefs, took a Parcel of the Cruor, after it had feparated itfelf as far as fpontaneoufly it wou'd from the Serum, and wafhing it frequently in Water, found it feparable into a vifcous, flippery Subftance, confifting of white, or colourlefs Fibres, (which rofe to the Surface of the Water, and there gather'd into a Skim, or coherent Pellicle of a reticular Texture) and a deep red Pouder, which precipitated pretty plenti- fully to the Bottom. Hence it appears, that the red Co- lour of the Slood is imparted to it by red tinging Particles, as in the common Cafe of Dyers. By examining the red Precipitate apart, and finding which of the Elements it confiired chiefly of, a Man who wou'd reafon about the Co- lour of the Stood from Principles of the Chymifts, might carry that Matter nearer an Iflue.
However, this red Colour, tho generally found in all ter- reftrial Animals, is not yet abfolutely neceflary and eflen- tial ; there being whole Species which have their circula- ting Liquor, or Stood, white and limpid : To which Dr. 'Drake adds an Inftance of a pure white Slood, like Milk, which he let out of the Median Vein of a Man, and which, when cold, did not feparate into a CrafTamenrum, as the red ufually does; nor yield a Skim, or Cream, or turn four upon keeping, as Milk does. Hi. Seal gives us another Inftance of the like kind ; and Dr. Lower adds a third, of a Perfon who bled fo long at the Nofe, till at laft the Broth he drank, flow'd, little alter'd, that Way as Slood.
From the Principles, or conftituent Parts of the Slood above-recited, varioufly combined and diftributed by the Circulatory Motion imprefs'd by the Heart, (fee Circula- tion,) and by the Ofcillatory, expanfive Morion of the in- terfpers'd Air, and the Re-action of the Contractile Vef- fels, flow all the Properties, and Operations of the Stood. From this Mixture of Elements, and their lax Composition, it becomes fufceptible of various Alterations and Imprefli- ons; the principal whereof are, Coagulation, which ufually attends it out of the Body, fometimes in it, and fcarce ever without an artificial Procurement, but always mortal : and 1)ijfolution, which is juft oppofite to the former, and con- fifts in fuch a Comminution of the Fibrous Parts of the Slood, as indifpofes it for a Separation of the Cruor from the Serum. This is frequently the Confequence of Malig- nant and Peftilential Fevers, £5c. and is likewife occafion'd by fome Kinds of Poifons. Thefe two contrary Affections of the Stood, Dr. 'Drake afcribes to the oppofite Kinds of Salts, Acids, and Volatile Alcalies. For tho, adds he, in a human Body no fincere Acid is found, nor cou'd it, indeed, be confiftent with Life ; yet it may, and does often enter the Stood fo compounded, as to bridle the Volatile, Alca- lious Salt of the Stood, and fo hinder the due Attenuation Ff and
