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serum Obstrucis the capillary tubes and refusos to floW; of another When it speeds iis course With life and alacrity, and tries toforestati the nisus of the compressing artery. It is of One LindWhen the arteries turgesce b0yond their duo limit with a largo abundatice of their enclosed suid; of another Wheu they are compressed Within their duo bound s by reason Os iis Scantiness. In sine, this generat equilibrium os pressure, as confined Withintho limits of iis own maximum and minimum, is subject tochanges every monili, Weeli, day, hour, Πny, CVery moment,
according to the changing states of the brain and of the body. Conformably to this state os pressure it is, that ali the chemicat, physical, and mechanical operations of the blood are carried on ;so that this generat equilibrium is as it Woro tho basis os the Whole animal economy. The knowledge of iis laWs and natureis tho cardinal potui os ali the sciences of angi ol Ogy, ad enology, and myology, comprised in the living animal; sor tho lise of thobody consisis in the existence and regulation Os the generalpressure and circulation.
182. In order to promote this generat pressure and circulationis the veraeis, there is required a muscular tunio, provided witha multi id series of motive rinys, and continued Dom the muscleos the heurt to the capillary vesseis. The third tunic, . . 'says Verheyen, is muscular Or fibrous, consisting os annularfibres set thichly together. These seshy and motive fibres Sur- round ing the arteriat iube, are nos disposed in a thin and single Series, as in the Venous coat, but aggregated and superimposed
ablo thichnessV n. 116). And Heister observes, that this cont is a dense and thicli tunic, made up of a vast number of plane muscular fibres, surround ing the arteries like a circle Or ring, and whicli iunio, by reason os iis thichness, is also readilydivisibio into severat layers n. 120). BOerhaave states that the muscular coat is made up of highly elastic annular fibres, many roWs thich, and divisibie into a number of lamellae;
and that the second is a thin and cellular costi, very dilatablewhen iis celis are inflated; and Whicli, by pouring sorti, an Oily, salty and lubricating suid iapon the muscular fibres, admirably sis them to persorm their incessant contractions and
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circumstances to be tinen into account in the arteries, and as is ample reason sor their elevation and dilatation Were not to be
this muscular coat lie ali the strength and force of the artery. For iis motive circles en compass the artery in a denSe SerieS. They are continued Dom the heari even to the minutest Vesset S. To these circlos bolongs the muscular sorce. They are condenSed
and Strong in proportion to the magnitude of the volume theypropel. For this reason the heari, Whicli propeis the largest Volume, is One entire muscle, proportioned in iis dimensions tothe volume it has to urge; nexi in dimension after the heartsollows the norta, Whicli is evidently possessed of a sphincter muscle and fleshy girilis. Aster this there is in the artery a Successive decrease in the number of forces, in proportion as theenclosed volume of blood successively decreases in magnitude and enereases in aptitude sor fluency. ΜOreover, the arterycompresses the finger When inserted into it, repeis the entrance of air, carries sorWard an injected liquid, and after death, is an artery be tied so as to intercepi iis communication Withthe heari, it Will, even below the ligature, convey sorWardS injected liquor, and urge ouWard as iis OWn proper suid, entruStedio itS OWn management, the Wave it encloses, and Whicli regardsevery potui in the artery, Dom the place Where it Was injected, RS iis oWn proper hestri. The compressive force of the artery is the greater in proportion to iis grenter degree of expanSion, larWhen expanded it has a tende cy to return to iis least diameter. Iis fibro can receivo inspiration Dom the bratus; cau also by
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tough, Was more adapted to the expansion or contraction thanto the elongation of the arteryV n. 125). Τhrough this smali cylindrical caual some litile drop or particle is frequently transmitted, Whicli is not in contigui ty With iis antecedent, and whicli is thus independent of the motion of the heart. Never-theless, ad these bucome duly put in motion, inasmuch as the
principes and proximate cause of the generat pressure, aS Weli RSthe concomitant cause of the circulation, has iis resideuce in the muscular tunic. Assuredly, then, even the least reSi Stanee, cither of the current paris of the blood, or of the vesseis, Would Superinduce a generat resistance of the whole and a state os quiescenoe. The sphere os activity there re extending betWeenthe heari and arteriai extremities, dous not belong to the heartalone; sor Uithout the aid of the arteries, the fluid would Dothin limine, and be reluctant to move on, While iis impetus Would continue to languisti tili noue rumuined to urge forWard theblood. All the forco exercised by the heari may be accounted sor in iis injecting the Wave, Whicli arrives at the end of iis course Only in the extremities; and When it has arrived there, the heari may bo considered as having discliarged the whole os
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That the pressure of the tunic is continuous, and of the Same Lind With that whicli is exorcised by tho huist at distinet inter-
ing ratio, it does nos cense at the instant of the nexi force censing to be superadded, but remaius equabie in the ratio Whicli it liud acquired, untii it reaches the nexi potnt of progreSSion, according to the laW of motion belonging to sutus, and particularly Where that motion is undulatOry. 185. From these observations the reason is evident sorWhicli the arteries are sost, flexile, Fielding, and not hard oruncompliant, OtherWise they could not be acted upon by any undulating suid, Dor could themselves react upon the suid. In order for anything to be, action and passion are both requisite, and there must be a concert betWeen the tWO, in Orderthat the effect may be perpetuat and may respond to iis One and Only universat sirst principie. 186. Τhus likeWiso the reason is evident lar Whicli the ringsare nos spirat but circular; for in consequence of their buingcircular, When the series is divaricaten they only untWist, and donot stretch out longitudinalty, Deither is more Of their contexture opened than is proportioned to the disseretice betWeen thoprevi Ous diameter of the artery and the existing diameter of the Wave; a disseretice to Whicli the arteriai sWelling or dilatation is
exactly eques: it Would be otherWise is the muscular contexture Were longitudinal-spirat, instead Os transverse and annular; forin this ense the artery Would nos urge the enclosed volume in aforWard direction toWard the extremittes, but in a backWard direction toWard the heart. Heiace it sollOWs, that aS many RS arethe circles in Series, so many are the forces in contiguity, and
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srom these sorcos thus multiplied, and subtended by one common
tinuous, exisis in the heari as contiguous; for Whicli reason the arteries have circular fibres, and the beari oblique and spiralfibres; sor the latter propeis the sanguineous volume DOm thenarroWer part of iis ventricle or Dom iis cone, into the bronderpari or toWards iis base, and thus that is essected in an antecedent direction Whicli is immediately after essected in a conSequent direction, Or as Soon as the volume is received by the artery, Whether it be the pulmonary artery or the aOrta. 187. There is also required an interior membrane, wh08e inceit shali be to collect these rinys, and to determine them to the production of this generat esset in the manner which we find obtainino in the arteries. The laurili and internat tunic,V saysVerheyen, is the thinnest, and is almost entirely membranOUS, or is you prefer the term, nervous. Iis fibrillae extending longitudinalty, cui the annular fibres of the preceding tunic atright anglos n. II 6 . And Boerhaave says: Tho fifth and last coat, Whicli lines the internat sursace of the artern is thinand membranous, and appears to consist Of longitudinal fibres, Mitch are contractile, like those of the laurili coat' n. 126). Morgagni has the solioWing Words : For my pari, ... I do notacknowledge that there are any sufficiently manifest longitudines
fibros in the internat coat of the arteries. . . . In fine, I ObserVein the arteries no flesti v si es at ali, excepi Such as are annular
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thus subtends, also adjusis' and direcis them. 2. That it ru-Stores tension, adapis the compaginated series to the Wave asosten as it passes by, restores to the fibres their natural siluation and modo, and proscribes to them their laWs os extensionand compression. 3. That this it does more especialty in the arteries, in Whicli the common cause is carried On, at the Same time that the particular cause of Hl things is carried On; and that it is both upon the equilibrium os pressure exerciSed by the arteries, and upon the circulation of tho blood, that the WhOle natural economy depends; While nothing is permitted tobe transacted in the system, to the execution of Whicli thebraitis did nos conspire, Whose fibres therelare floW by continui ty Dom this most universat membrane. See n. 137. 189. From a careses consideration os these circumStanees, we are noW at liberty to form a conception Of the manner in Whicli the wave onoe injected into the arteriai cones, aftem d8
propeis itself and is urged sorWard through tho Wholo longilios the arteries to iis ultimate limiis, With the greatest facili ty,
yet with the greatest force; sor there cannot be a more efficient, but at the fame time a more easy, mode of exerciSing larce, than the one presenten to us in the case of undulation. The propulsion of the Wave then We may conceive to talie place in the solioWing manner. Is at the moment it is omitted Dom thelieari, it assumes the inverse figure of the heari, but the direct figure of the arteries, it becomes parabolico-conoid, beingbroader at iis upper pari, Rud narrOWer and more acuminaledat iis loWer, just as in the case of Water throWn into a similarstate os fluctuation. Henee the blood, When it distends the artery beyond iis previ ous Width, compeis it also the more Stronglyto reaci in a direction toWard the ramiscations. Τhis is thecase not Only With the larger volume, but also With the small-eSt, or everi With a parti cle, percolating the capillary iubes, Where being in the close embrace of the vesset, it is conSequently urged sorWard by this means Mone to the beginning of the vetus. For LeeuWenhoeli observed that in a tadpole the particles of blood mero stat and ovat, and that sometimes, byreason Os the tenuity of an artery, they Were made to aSSume R
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as the larger globulesJ Were stretched out or elongated, thesmaller assumed the fame tengthenen figure, tili they becamo liku thronds ;V and again, that the globules, . . . in pnSSingthrough the smali capillary arteries and vetus, changed to an Oblong figure, . . . three times as long as broad n. 29 . Hen coit follows, that the larce of the tunic, multiplied into the yield-ance of iis fluid, produces a corioint essori, Whicli is alWays Ontho increase Dom the trutilis to the branches, and Dom thebranches to the capillary vesseis. But dismissing the consideration os these particulars, Whicli Obscure the series of universaland generat principies, let us proceed to the investigation os
190. The case, however, is disserent in reuard to the veins. In these there is no circulation, but a bare impletion and depletion, Or a pressure et on the blood they contain equalty in every direction, et w d' downwarsis, and laterat , as emen Esed in liquidssilino conical venets. Τhat the successive circulation Or undulation of the sanguineous Volume ceases at the termination Osthe arteries, and that another circulation commetices at the be-
ginning of the vetus, Whicli is rather to be denominated adimpletion Or equation, is attested by a comparison of the arteries With the volns; by the cause of the circulation and of the pulse, Whicli in tho vetus censes; by the definition of undulation, Which does not agrest With the phe Omena Os Venous circulation ;by the essentials of the blood, Whicli are changed immediatelyon iis transition Dom the extremities of the arteries tuto thesrst beginnings of the VeiΠS. 191. Τhis position is also confirmed by facis. For, I. The vetus are destitute os ali pulsation, consequently of nil transferrence Of undulation Dom One place to another. 2. Τhey are provided With valves, Whicli the arteries stro not. In the cavlty of the vetus,'' says Verheyen, at disserent intervais, thereare certain very thin, light pellicles, called valves, Whicli at oneextremity, namely, toWard the Smaller tWigs, are continuous With the interior venous coat, While at the other, namely, toWard the heari, their edges are Dee. . . . They RrO DO-quently found near the divisions of the vetus, or the junction Osthe branches; and somelimes are Single, Sometimes in piars,
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Veius as having, not Only single valves, but somelimes as havingtWo, three, and even sive at a time, some of them conoid and pyrisOrm, SOme Semilianar, Others semiorbitat. Furthermore hestates, that these valves lio at very disserent distances Dom eachother Hong the vetus' n. 117). These circumstances clearly indicate, that the vetiis exercise no perpetuat joint nisus tending Dom a smaller to a largor branch, but that theyeven exercise a j Oint nisus tending Dom a Jarger to a smaller
placed besore the entrance of the Vena cava as it ascends Domitio livor, occupies iis half and anterior portion, and then sorma a bag, Whicli is concave upWard and conVex doWnWard. Respecting this valve see Eustachius, Lancisi, Winsto' and
cular areas Of this Vein a very copious reflux of blood talios place, Which When the auricle is compressed, cannot be sentinio the right ventricle of the heari. 4. When the vetus aretiod, there is an equabie intumescetice belWeen the ligature and the extremities, While there is an equabie detumescetice belWeenthe ligature and the heart. The case is disserent in the arteries. 5. In the blood enclosed in the vetus there is a flux and reflux, as is iis course Were indeterminate, since the blood does notflos into them Dom the arteries at any stated intervals; as
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Where the circulation is interrupted, a disserent sori of effluxprevalling. For While the arteries are pressing the blood in asorWard direction, the blood is pressing upon the arteries in alateria direction; in the fame manner as Streams, Whicli do notiose the property of lateria pressure because they may happento be tending downWard. Were the case OtherWiSe, the arteryWould Deither become expanded ΠOr Would it pulsate.
ticularly is We compare the vetus With the arteries. For Wemay not unsasely conclude Dom the character of the tunics fothe nature of the blood, and to the manner in Whicli it acts n. 135). Thus, 1. The muscular tu te of the vetus is invested vitii a simple serius os motive rings ; this tunic is the inmost, and is not subtended by any other that collecis the single forces of the fibres and combines them into One common force, as me find to be the case in the arteries. The laurili and in-most tunic of the vetusJ,V says Verheyen, like tho third tunic of the artery, consists of annular fibres, but thinner, and arranged in a very simple series' n. II 6). And according to
and a muscular coat, but these are much thinner than in thoarteries n. 120). BOerhaave observes of the veitis, that alltheir membranes are much thinner aud more inert than illoso
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the sanguineous fluid indeterminately, Whicli is thus compelledio reaci by pressure in overy direction. 2. Τho blood floWs
into the vena cava, and the sanguineous stream is received Dominnumerable venous Origitis. The case is OtherWise With the arteries, into ali os Whicli there is only one influx DOm One fouΠ-tain, namely, the norta; While on the Other haud, the vena cava is like a longiliened conduit pipe, Whicli admits of as many littie Waves as there are inosculating branches; lience there cannot possibiy result one particular, but Only a common Orgenerat pulse, or impletion. The fame lam obtains in thebranches, Whicli receive the sanguineous iide Dom their tWigs in like manner as the common recipient trunti receives it Domthe branches. It follows, therofore, that in the case of a multiplici ty of currenis discliarging themselves into One commori trunk, and One common current disclinrging itself into a multiplicity of vesself, the laW of progression is not similar. 3. This flow of blood in the vetus does not fine place at regularstated intervais, but at severat disserent moments and almost continuOusly. It comes, sor instance, ut disserent momenis, Om various members of the body siluated at a distance Domoach other in the system, as Dom the wrist, the piam of
and abdominat viscera: consequently, the Wave nearest thetruuli has precedence of the Others more remote. It comes at
different moments Dom the muscies; for the muscies Iose theirblood, groW pale and constricted, according to the motions, Wilis, and natural instincts of the mitid, the blood of the muscies being at that time Wholly expressed into the litile vetus; a circumstance Which takes place Whenever the Will acts, whether the heart be in a state of systole or diastole. At different moments it comes Dom the gland 8, Vesicles, and pores, Whicli arenti muscularly acted upon, and excited to aut according to their Severat appetencies Whicli are continuat; and When excited, they present the Dod to the hungry veins, ns the Stomaeh, me- Sentery, and thoracie duci present their chyle to the subclavianVein. It comes ut disserent moments DOm the sinuses of thebrain into the jugular vetus, and at disserent moments againfroin the agygos into the heari; lar being annexed to the