AN ENQUIRY Into the EXILITY of the VESSELS In a Human Body, &c.

THE Structure of the Ani⯑mal Body, whether we con⯑ſider the Size, Situation and Uſes of its larger and more complex Parts, with relation to each other, and their Subſervience to the whole, or by diving deeper into the ſecret and []more myſterious Parts of Nature, we ſcrutinize the ſeveral Parts of which theſe are compoſed, exhibits to our View ſuch an amazing Scene of un⯑bounded Power and Wiſdom, as in⯑finitely exceeds all the Contrivance of the reſt of the viſible Creation, how great the Bodies, or aſtoniſhing ſoever their Motions and Revolutions may upon a ſtrict Enquiry appear to us.

But tho' this Poſition is ſufficiently evident from the Structure of every Organ in the Animal Body, yet it is in no Part ſo ſingularly aſtoniſhing, as in the Growth of Animals, from their firſt original Stamina in the Ani⯑malcule, [3]till they arrive at their ap⯑pointed Size.

That every Veſſel and Fibre in the Body is contained in Miniature in an Animalcule, few, I believe, will diſ⯑pute, it being abſolutely impoſſible, that mere Matter and Motion, with⯑out its being conducted by the Veſſels proper to an Animal, ſhould produce an Animal at all; and ſtill more ab⯑ſurd, if there can be Degrees in Ab⯑ſurdity, to ſuppoſe ſuch a Cauſe ca⯑pable of producing that Variety of Species, in their regular Order, which, we ſee, is conſtantly kept up thro' ſo great a Series of Years and Genera⯑tions; but that inſtead of a Calf, a Cow might as naturally produce a Lion, Camel, or any other Creature, [4]as frequently as its own proper Spe⯑cies.

Beſides, could the Parts or Limbs, or even the moſt minute Organ be produced by the Motion of the Fluids, without their being conducted by the Veſſels of the Organ itſelf already latent in the Stamina, what ſhould hinder, why any Part taken away by Amputation ſhould not again be perfectly formed and reſtored, ſince all the adductory Veſſels continue in their former State, and are capable of ſupplying the ſame Fluids as be⯑fore? Conſequently ſince the moſt minute Organ cannot be ſupplied by any other than the forementioned Method, can any thing be ſo abſurd, as to ſuppoſe that all the regular Or⯑der [5]of Veſſels neceſſary to compoſe the whole Bodily Machine ſhould be pro⯑duced by ſuch a Cauſe, as is utterly incapable of forming the leaſt Part imaginable in a regular Manner? This is no leſs exemplified in the Seeds or proper Embryos of Plants. Thus the Seed of the Fir-tree, whoſe Bulk with all its Integuments ſcarce equals the Head of the ſmalleſt Pin, has its Fibres and Veſſels ſo enlarged by the Addition of freſh Particles, as to grow to an enormous Size, without ever de⯑viating (in whatever Soil it grows) from its own Species into any other kind of Tree. Which can be owing to no other Cauſe than the original Stamina latent in its Seed; which not only ſeparate, but conduct the Parti⯑cles proper for its Nouriſhment, each [6]to its proper Place in a manner pecu⯑liar to itſelf, thereby diſtinguiſhing it from all other Vegetables of what kind ſoever. But a Branch once lop⯑ped off can no more be reſtored by the Gardener's Art, than an amputa⯑ted Limb by the Surgeon's: 'Tis true, indeed, new Buds may be thruſt forth by the redundant Nouriſhment, both in the adjacent and more remote Parts, to compenſate in ſome meaſure this Defect: but whether this Mutila⯑tion of the Plant be of its Summit, or of a lateral Branch, the Extremity muſt ſhrivel and dry up, and will bud and bloſſom no more.

If therefore mere Matter, altho' ſuppoſed to be in Motion, muſt be thus conducted by the Veſſels of an [7]Animal, it muſt be conducted by the Animal itſelf, no Application of dif⯑ferent Forces ab extra to the Veſſels (was there any ſuch thing, as it is evi⯑dent from the Structure of the Uterus, there is not) being capable of regula⯑ting and directing ſuch an innumera⯑ble Variety of Motions, or indeed of contributing to its Operations, any farther than ſupplying it with a pro⯑per Nidus, and furniſhing it with Materials ready prepared for the Pur⯑poſe, as ſufficiently appears, not only from the Obſervations of the accurate Malpighi, Bellini, Redi, Merian and others, but diſcovers itſelf to our Sen⯑ſes, in the Generation of innumerable other Species of Animals, ſome depo⯑ſiting their Eggs on Vegetables, or in the Earth, to be brought to Light by [8]the genial Warmth of the Sun, others in Dunghills, and other putrefying Subſtances, and others in Streams of Water, where it is impoſſible for any ſuch Force to be applied.

Hence then Nutrition muſt conſiſt not only in the mere Diſtenſion, but in the Appoſition of new Particles into ſuch Parts of the Animal Body, as by the Enlargement of the Pores, and Vacuities in the Solids, are fitted to receive them, as I have elſewhere ſhewn by Experiment ^(a): And, in⯑deed, was it otherwiſe, the Animal would have a Power of making its own Fibres and Veſſels, and conſe⯑quently of producing itſelf; nay the [9]Circulation of the Fluids muſt be per⯑formed in Veſſels, before the Veſſels themſelves, thro' which it is to be conveyed, have a Being; which are ſuch Abſurdities, as I ſhall not give myſelf the Trouble of confuting.

That theſe new Particles by adhe⯑ring firmly to the original Stamina in⯑creaſe the Bulk of the Animal, till ſuch time as it arrives at the Perfec⯑tion its Structure is capable of, needs no Proof; but that the Strength of the original Animal owes its gradual Improvement to the ſame Cauſe, is likewiſe evident from the common Practice of ſeveral of our mechanic Workmen, who by ſteeping and ſatu⯑rating porous Woods, or the like light and brittle Subſtances in glutinous [10]Decoctions, vaſtly increaſe their na⯑tural Strength, without any Addition to their external Bulk, which would be ſtill farther increaſed, was that alſo enlarged by a regular Diſtenſion of Parts, as in the Caſe of Animal Bodies.

Hence then it will follow, that theſe adventitious Particles, which by their Coheſion with the original Sta⯑mina, increaſe the Bulk and Strength of the Body, and thereby enable it to perform the proper Offices, for which it was created, are not really any Part of the Body itſelf, but extraneous to it, actuated and regulated intirely by it, and conſequently contribute no farther to its exerciſing the Powers, it is deſigned to exert when perfect, than would be performed by a ſtrong [11]Lever acting with ſuch a Power, as would be impoſſible for a weak one to ſuſtain, tho' in all other Reſpects the Caſe might be exactly parallel.

Nor is this leſs true with regard to the Senſations, than Strength of the Body. A proper Tenſion in the nervous Fibres being as requiſite in this Caſe, as Strength in thoſe deſign⯑ed for Muſcular Motion, as appears from the pernicious Effects of too great Laxity, as well as Rigidity, in ſeveral Diſeaſes, the one rendering the Sen⯑ſations ſo languid and ſlow thro' their weak Vibrations, as not to affect the Animal with ſufficient Force and Vi⯑vacity; whilſt the other, thro' too great Stiffneſs and Inflexibility, ſcarce allows them to vibrate at all, as is the [12]Caſe in old Age; or laſtly by being too ſpringy and elaſtic, and thence exciting too ſtrong and frequent Vi⯑brations in ſuch as are younger, ei⯑ther renders their Senſations painful and uneaſy, or ſo quick in their Suc⯑ceſſions to each other, as to prove uſeleſs, nay often prejudicial to the Animal, as is the Caſe in Phrenſies, Maniacal Perſons, and the like.

How far the Strength of the Body depends upon the adventitious Parti⯑cles derived to it by Nutrition has been explained above, and it is hence no leſs evident, that they cannot poſ⯑ſibly contribute farther to its Senſa⯑tions, than as they add ſuch a Degree of Firmneſs, as may prevent any ex⯑ternal Species from acting upon the [13]nervous Fibrils ſo forcibly, as to in⯑jure them; whilſt at the ſame time by their intimate Adheſion with the original ſenſitive Organs, they cannot act upon the one, without exciting a proper Degree of Motion in the other; and conſequently the real Senſitive Part of an Animal can contain no greater Quantity of Matter, than is included in the Nervous Parts of the Animalcule.

This then being the State of an Animal when arrived at its Perfection, how amazing muſt it be to conſider, how ſmall and weak Organs do really actuate the whole Machine! But that this may more fully appear, and alſo afford us ſome determinate Idea of their extream Tenuity, I ſhall endea⯑vour [14]to reduce ſuch Microſcopical Ob⯑ſervations as are of beſt Credit to a cer⯑tain Standard; that by comparing them with each other, and reducing them to Numbers, our Notions of them may not be altogether ſo vague and indeterminate as at preſent they are.

Leeuwenhoek, that curious and di⯑ligent Inquirer into thoſe Works of Nature, which till his Time had by their Minuteneſs eſcaped all Diſcove⯑ry, has by his repeated Microſcopical Obſervations on the Semen Maſculi⯑num of Animals, not only ſhewn, that it abounds with Animalcula proper to each Species, but that, according to his Judgment, a thouſand of them would ſcarce be equal in Thickneſs [15]to a Grain of Sand ^(a); and conſe⯑quently that a cubical Inch would contain a thouſand Millions of Mil⯑lions of theſe little Beings. Profeſſor Keil indeed, in his elegant Inquiry into the actual Diviſibility of the Particles of Matter, has from a Principle of Dioptrics computed their Magnitude to be ſomewhat greater than that juſt now mention⯑ed; but he has however demon⯑ſtrably ſhewn, that the Length of a ſingle Animalculum cannot poſſi⯑bly exceed the 3 (divisor)100000^th Part of an Inch ^(b). That our preſent Enquiry may therefore be wholly confined within the Limits of Truth, rather than proceed upon probable Conjec⯑ture, [16]we will make Choice of this latter Eſtimate, and for the further Conveniency of Calculation, take it for granted, that when the Foot is decimally divided, even ſuch a Cubi⯑cal Inch will exceed the Bulk of a ſingle Animalculum, in no greater a Proportion, than that of a thouſand Millions of Millions to 27.

Now the ſpecific Weight of the various Parts of an Animal Body re⯑duced to a Medium amounts to a lit⯑tle more than the Weight of an equal Bulk of Water; if therefore we ſuppoſe one of theſe Animalcula to be to Wa⯑ter, as the other Parts of young Ani⯑mals are, (and greater it cannot be, as appears from what I have elſewhere demonſtrated with relation to the in⯑creaſed [17]ſpecific Gravity in the Arte⯑ries of aged Animals ^(a), it follows, that as a Cubical Inch of Water is equivalent to 0.5271 parts of an Ounce Troy, the Weight of a ſingle Animalcule will be equal to the 1 (divisor)140391450739^th part of a Grain nearly.

But by the Experiments of the in⯑genious Dr. Keil it appears, that in a Man of 12 Stone-weight, the Fluids are to the Solids when taken at a Me⯑dium, at the leaſt in the Proportion of 8 to 3 ^(b), excluſive of thoſe, which by the Help of Fire and a Chymical Proceſs may be extracted without the Deſtruction of a ſingle Fibre, which amounts to no ſmall Quantity. It has [18]alſo been farther ſhewn by Experi⯑ment, that the Veſſels in young Ani⯑mals bear a much leſs Proportion to their Cavities, than the like Veſſels in old ones of the ſame Species bear to theirs ^(a); conſequently, ſince the Denſity of any Body is as the Weight of the Body directly, and its Magni⯑tude inverſely, we ſhall find, by com⯑paring the foregoing Proportions of the Fluids and Solids together in each different State of the Animal, as well before its Birth, as after it has arrived at full Maturity, that all the Stamina, from which ſo noble a Being as Man himſelf was at firſt derived, could not contain ſo much ſolid Matter as would be equal in Bulk, to that of a Quan⯑tity of Water, of no greater a Weight [19]than the 1 (divisor)92408129934910602442073752000^th part of a Grain.

If the preceding Exility of the Solids in general be ſo ſurpizingly ſmall, That of the Senſitive Parts alone, could their Relation to the reſt be once adjuſted, would doubtleſs, to ſpeak in the Language of the Mathe⯑maticians, appear a mere Infiniteſimal. For even Leeuwenhoek has declared in expreſs Words, that ‘"Hae Cerebri Fibrillae, tenuibus puto amiciuntur Membranulis, quae ob inſignem Exilitatem nunquam ſeſe nuda⯑bunt conſpectui noſtro: Quod niſi ſtatuamus, quo pacto Fibrillarum Diſparationem vel Diſtinctionem Oculis aſſequeremur? ^(a)"’ To [20]which he might have added our Intel⯑lectual Faculties alſo, it being ſcarce poſſible to conceive Fibres always growing and encreaſing in Bulk, ſhould be kept ſeparate, and perform their various Operations, but muſt ſoon prove immoveable, without be⯑ing divided from each other by theſe ſlender Partitions.

This is not only evident from the Nature of the Fibres, but demonſtra⯑ble from Fact in the larger Parts even of full-grown Animals; which, when contiguous and deprived of their proper Integuments, never fail to ad⯑here and unite firmly to each other, as is ſufficiently known to the Prac⯑titioners in Surgery. How much ſooner then would this Adhaeſion be [21]produced in growing Animals, where the Fibres are not only more ſupple, and thence more eaſily united, but if deprived of theſe Membranes, would daily as they increaſe in Bulk, preſs more ſtrongly againſt each other?

The Writers in Geometry have laid it down as an eſtabliſhed Maxim, that ubicunque deficit Modulus, in aeternum latebit Menſura; on which account, tho' we may reſt ſatisfied, that the Subtility of the ſenſitive Parts in an Animal Body muſt neceſ⯑ſarily ſo far ſurpaſs the Extent of hu⯑man Abilities, as to put their Bulk beyond a Poſſibility of being ever de⯑termined to a Mathematical Exact⯑neſs; yet even the Nerves themſelves, thoſe delicate Inſtruments of all our [22]Knowledge, may be brought under ſuch an Examination, as to afford us a much clearer Idea of their exqui⯑ſite Minuteneſs, than what can be obtained from thoſe conjectural De⯑ſcriptions, the Anatomiſts have hi⯑therto given us concerning it.

For it is evident, that all the Mem⯑branes of an Animal Body have Muſ⯑cular Fibres, and that even the great⯑eſt Number of them are principally made up of ſuch; conſequently we could not err very far from the Truth, were we to eſtimate the Bulk of the Membranous Fibres in the ſame Pro⯑portion with the Muſcular ones. But we ſhall examine the Bulk and Pro⯑portion they bear to each other a lit⯑tle more diſtinctly.

Now Leeuwenhoek in his Obſerva⯑tions on the Muſcular Fibres has ſhewn, that ſome of them were equal to the 1/9th, others to the 1/16th part of a Hair ^(a); conſequently ſuppoſing each Fibre to be the Length of a Decimal Inch, there will be contained in ſuch a Cubic Inch ſeven Millions two hundred ninety thouſand of the for⯑mer, and twenty-three Millions forty thouſand of the latter Kind. But theſe Muſcular Fibres are by later Obſervations found to be really ſmall Muſcles, conſiſting of Parts ſimilar to the large one, which they conſti⯑tute, and terminating in Tendons after the ſame Manner as the former; and conſequently the Fibres, of which [24]the more lax and cavernous Parts of the Muſcle are compoſed, muſt be much ſmaller than the muſcular Fi⯑bres rendered viſible by his Glaſſes. Whether theſe be the ſame individual Fibres, which conſtitute the Tendon, by being more cloſely compacted in their Parts, I ſhall not undertake to determine; but this we learn from the forecited Author, that a hundred of theſe exceed not the Bulk of a Hair ^(a), and conſequently nine hun⯑dred Millions of them will be con⯑tained in the forementioned Cubic Inch.

But theſe are far from being the ſmalleſt Fibres in the Body. For that the pellucid Membranes conſiſt [25]of Fibres of ſtill ſmaller Dimenſions, decreaſing probably to the 1 (divisor)300^th part of a Hair, or upwards, is evident from the Structure of thoſe fine Capillary Arteries, which are deſtined to con⯑vey a Fluid to them. For the whole Diameter of ſuch an extream Artery does not exceed the 1 (divisor)1620^th part of an Inch ^(a) that is, the 1 (divisor)5th of a Hair nearly, and yet the Coat of this Veſ⯑ſel is made up of at leaſt three diſtinct Species of Membranes, each of them furniſhed with Veſſels peculiar to it⯑ſelf. And farther, if we conſider, that the tranſverſe Section of every Veſſel is of a circular Form, we muſt neceſſarily allow, that the Longitudi⯑nal Fibres muſt be much ſmaller than any of thoſe abovementioned; other⯑wiſe [26]inſtead of a Circle, a Polygon would be produced, as is evident from Geometry.

Now even theſe are bulky Sub⯑ſtances, when compared with the nervous Fibrils deſtined to Senſation, as appears from thoſe which form the Retina. For theſe Fibres were manifeſtly before their Expanſion into this Membrane collected within the Bulk of what is ſtyled by Anatomiſts the Optic Nerve, and are, as the in⯑genious Profeſſor Monro aſſures us, ſo extreamly ſmall, that it is demon⯑ſtrable, they cannot exceed the Thick⯑neſs of the 1 (divisor)32400th part of a Hair ^(a). If therefore we reject the odd four hundred in the foregoing Fraction, [27]and ſuppoſe, that no more than 32000 of theſe Nervous Fibrils are neceſſary to make up a Thickneſs equal to that abovementioned, and alſo ſet the Muſcular, Tendinous, and Membra⯑nous Fibres to be at a Medium, of no greater a Thickneſs than the 1 (divisor)200th Part of a Hair, (which, from what has been juſt now explained above, is apparently a very large Allowance with reſpect to the Subtility of the Fibres in general) it will then follow, that the Thickneſs of one of theſe Fibrils in the Retina will be to that of ſuch Fibres, as conſtitute the other Membranes and Organs of the Body, in the proportion of 1 to 160. But we have already ſhewn, that all the Sta⯑mina in an Animalcule could not con⯑tain ſo much ſolid Matter, as is equal [28]in Bulk to a Particle of Water weigh⯑ing the 1 (divisor)92408129934910602442073752000^th part of a Grain; conſequently, if we ſuppoſe, that a Nervous Fibril does accompany every individual Fibre in the Animal Syſtem, and compare the Proportions they bear to each other as given above, we ſhall find, that the whole ſenſitive Parts in the Animalcule taken together could not amount to a greater Bulk than that of the 1 (divisor)3764060396563654679984851840619241^th part of a cubical Inch, or that all the ſolid Matter of the Nerves, by which the Senſations in the human Syſtem are actually communicated to the Mind, cannot poſſibly weigh the 1 (divisor)14877708919520606993173874072000^th part of a Grain.

That the ſtupendous Expanſion and Action of the Fibres and Veſſels of an Animalcule up to that of a full grown Man ſhould be owing to an Entity like this, a mere fluxionary Increment with reſpect to Quantity and Magnitude, will, I doubt not, to many appear impoſſible, to all incom⯑prehenſible; but are not all other of the Creator's Works equally ſo? Or is this more than infinite Power act⯑ing on Matter infinitely diviſible is capable of, whoſe Particles muſt be ſtill higher ſubtiliſed, and ſpun into Fibres inconceivably more minute in the Bodies of Inſects, than is here ſpe⯑cified of the human, as they are all furniſhed with Arteries, Veins, Nerves, Muſcles, Bones, and Tendons ſuitable [30]to their various Structures, and Man⯑ner of ſubſiſting.

The incomparable Mr. Locke, ſpeaking of the Identity of the ſame Perſon, reſolves it wholly into its Con⯑ſciouſneſs, even though the whole Subſtance, material or immaterial, ſhould be intirely changed. Many Phyſicians have alſo been of opinion, and ſome even laid it down as a Maxim, that the whole Subſtance of the BOdy, during the Length of an ordinary Life, is ſeveral times ſo in⯑tirely changed, as to conſiſt wholly of new Particles, of the ſame Nature and Kind as the preceeding. How far theſe Opinions may agree with what is taught by Divine Revelation, where this very Sameneſs, in its [31]ſtricteſt Acceptation, ſeems to be ex⯑preſsly declared, is not my province to determine. But this we may plainly deduce from what has been premiſed, that the whole Subſtance of the Body, except theſe original Stamina of the Animalcule, may be many times changed, and yet the real Body continue the ſame, and be poſ⯑ſeſſed of the ſame perſonal Identity, with regard both to Body and Mind, at the End of the longeſt Life, as it was at the inſtant of its Birth; nay even in the Animalcule itſelf.

That theſe original Stamina ſhould all remain unhurt, and fit to perform their ſeveral Functions to the End of Life, is a Poſition not to be admitted; but that without a competent Num⯑ber [32]of them ſo remaining, the Animal will decline both in its vital and in⯑tellectual Faculties, notwithſtanding the Body be repleniſhed with a ſuf⯑ficient Number of homogeneous Par⯑ticles to ſupply the Place of ſuch Parts as are abraded and worn away, or otherwiſe hindered from performing their natural Functions, ſeems equally certain; both from the Decay of our intellectual Faculties, and the gradual Loſs of Vigour in the Body, even whilſt the Force of the vital Organs ſeems no ways impaired. Conſe⯑quently ſomething more muſt be ne⯑ceſſary, than the mere Application of homogeneous Matter to ſupply this Demand. But that can be no other, than that particular Texture, with which they were originally formed, [33]and by which they become capable of tranſmitting the Effects of exter⯑nal Agents to the Mind.

It may poſſibly be objected, that all Matter being equally incapable of Senſation, no Reaſon can poſſibly be aſſigned, why ſuch homogeneous Matter, as ſupplies the conſtant Loſs, the Body ſuſtains by the Attrition of its Parts on each other, and ſerves for its Accretion, Strength, and Nutri⯑tion, may not be equally capable of conveying our Senſations to the Mind, as the original Stamina above men⯑tioned. But if we may reaſon from what enſues in the larger Parts, which are compounded of theſe very Sta⯑mina, upon their being wounded, or broken with Loſs of Subſtance, 'tis [34]evident, that though by the Addi⯑tion of homogeneous Matter to the different Species of Veſſels, the Body to Appearance, may have received no great Detriment, and be ſufficient for the principal Purpoſes of Life; yet are ſuch Parts of very dull and ob⯑ſcure Senſation, if they may be ſaid to enjoy it in any other manner, than by their Union with the original Sta⯑mina. So that could we ſuppoſe theſe Stamina quite deſtroyed, and the Body ſo far changed, as to conſiſt only of ſuch homogeneous Matter, though with Veſſels in Form and Bulk ſimi⯑lar to the former, yet theſe united by a Texture different from that of the original Fibres; 'tis evident from what enſues upon a like Change in the Fibres conſtituting the larger [35]Parts, that ſuch a Perſon, if he had either Senſitive or Intellectual Facul⯑ties at all, muſt have them in a man⯑ner vaſtly different from thoſe of the former. And conſequently this Same⯑neſs or Identity muſt remain in a great Meaſure fixed and unalterable in the Body from its Birth to its Diſſolution.

Nor is this Difference in the ori⯑ginal Stamina confined in its Effects to the fibrous Parts of the Body only, but communicates them to the circu⯑lating Juices themſelves, diſtinguiſh⯑ing thereby the different Kinds of Fleſh of each Species of Animals from each other, even where they are ſu⯑ſtained by the ſame Kind of Food. Otherwiſe it would happen in Caſes of this Kind, that the Taſte, Com⯑plexion, [36]Odour, and Coheſion of the Fleſh would of Neceſſity be nearly the ſame in all.

The ſame would be the Conſe⯑quence with regard to the Glands of the ſame Body, which would ſepa⯑rate nearly the ſame Kind of Juice, and conſequently be uſeleſs, or per⯑haps prejudicial, was it not prevented by the different Structure both of the Strainer itſelf, and the Preparatory Veſſels. For howſoever we may imagine the Juice to be ſeparated from the circulating Maſs, we cannot with any Colour of Reaſon ſuppoſe ſuch a various and exquiſite Contri⯑vance, as is diſcovered not only in the Formation of the Gland itſelf, but alſo in that of the Veſſels leading to [37]and from it, ſo widely differing among themſelves in their Structure, Texture, and Capacity ^(a), ſhould ever be deſigned only to perform what any one Species of them was capable of.

Hence then it muſt follow, that the principal Differences in Animal Bodies are owing to the different Structure of the original Stamina, with regard not only to their exterior Shape and Actions, but that even the Fluids themſelves, and that ſolid Part called the Fleſh, derive their chief and principal Difference from this Source; and conſequently, that there is a real Identity of Animal [38]Bodies incommunicable to any other, depending on this very Cauſe.

Nor is this relating to the Fluids a mere Conjecture, but a real Fact, as appears from thoſe morbid Caſes, where either the Glands, Preparatory Veſſels, or even the fleſhy Parts are vitiated and depraved; which never fail to ſeparate unnatural Juices, tho' derived from the moſt healthful Blood.

Analogous to this we ſee in the Vegetable World, that it is not the Root, or even Trunk of the Tree, tho' all the Juices are thereby ſup⯑plied, but the Bud or Scion only, which governs the whole, and pro⯑duces the Species of Bloſſoms and [39]Fruit peculiar to itſelf. In like manner the Veſſels of the Animalcule, of what Species ſoever it be, convert the Juices deſigned for its Nouriſh⯑ment, each, into ſuch as are ſuitable to their different Structures; and thence keep up an infinite Variety in Animals, tho' ſupplied by the ſame kind of Food, according to the dif⯑ferent Veſſels by whoſe means it is elaborated and perfected, being in ſome highly volatiliſed and attenuat⯑ed, which gives the high Taſte; in others more viſcous and inactive, which produces the oppoſite, and are therefore leſs pleaſing to a volup⯑tuous Appetite.

I am not here ſuppoſing, that the Qualities of the circulating Juices of [40]Animals depend ſo intirely upon the Structure of the Solids, that the Di⯑verſity of Food is herein of no Con⯑ſequence, daily Experience ſufficiently confuting ſo abſurd an Opinion. But only that in thoſe of a different Spe⯑cies, the Taſte, Odour, and other Qualities of their Juices, are princi⯑pally owing to this different Struc⯑ture; as is manifeſt from hence, viz. that the Qualities of the Juices of differing Animals, though ſuſtained with the ſame Kind of Food, are more diſſonant to each other, than are thoſe of the ſame Species, though ſuſtained with Food of a very differ⯑ent Nature; and conſequently, this Identity in the various Species of Animals, muſt principally conſiſt in the Structure of the Solids, or, in [41]other Words, in the original Stamina of each individual Species.

Thus has our Reaſoning at length led us to the Diſcovery and Demon⯑ſtration of a Truth, which at firſt Sight muſt appear beyond all Proba⯑bility, and we can now with Cer⯑tainty affirm, that from ſo ſimple a Cauſe as the different Modification of the ſame Matter, the wiſe Author of Nature has produced not only one kind of Fleſh of Men, another of Beaſts, another of Fiſhes, and another of Birds; but theſe alſo infinitely dif⯑ferent in the different Species of each.

Hence we may ſee, why ſome Ani⯑mals ſhall neceſſarily require ſuch a [42]Degree of Warmth, to preſerve their Juices ſufficiently exalted and attenu⯑ated for their comfortable Subſiſtence, as others cannot ſuſtain without Pre⯑judice, even ſcarcely without a Diſ⯑ſolution; and vice verſa, why ſuch a Degree of Cold, as is only agree⯑able, or perhaps neceſſary to the Health of theſe, ſhould prove ſo far deſtructive of others, as on the leaſt Condenſation of the Fluids from that Cauſe, to render them torpid, mo⯑tionleſs, and ſeemingly dead, till re⯑ſtored by the kindly Influence of the approaching Summer.

Hence alſo it is obvious, why the Blood and Juices of ſome particular Species of Animals ſhall be affected with Diſeaſes of a Peſtilential Nature [43]with regard to them, from ſuch Cauſes, as ſhall produce very few In⯑conveniences to many other Kinds.

Hence alſo we may ſee the Reaſon, why each individual Perſon has ſome Particularities in his Conſtitution pe⯑culiar to himſelf, not to be found in the Generality of the Species, nor de⯑pending on his Manner of Life, by which he is as it were diſtinguiſhed from the reſt: Which alſo probably may ſometimes be the Caſe in Brutes, tho' more rarely than in Men, could we equally arrive at the Knowledge of it, as is manifeſt from the different Sprightlineſs, Vigour, and Activity of ſome compared with others of the ſame Species.

Laſtly, hence we may ſee, why an intire Change of the Conſtitution ſhould be attended with almoſt inſu⯑perable Difficulties: Why great Al⯑terations require a long and gradual Proceſs, nor are to be attempted by haſty Methods, leſt the Patient, in⯑ſtead of Relief, meet with certain Ruin and Deſtruction.