a discourse on method-第11章

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ving　been　the　first　to　teach　that　there　are　many　small　passages　at　the　extremities　of　the　arteries；　through　which　the　blood　received　by　them　from　the　heart　passes　into　the　small　branches　of　the　veins；　whence　it　again　returns　to　the　heart；　so　that　its　course　amounts　precisely　to　a　perpetual　circulation。　　Of　this　we　have　abundant　proof　in　the　ordinary　experience　of　surgeons；　who；　by　binding　the　arm　with　a　tie　of　moderate　straitness　above　the　part　where　they　open　the　vein；　cause　the　blood　to　flow　more　copiously　than　it　would　have　done　without　any　ligature；　whereas　quite　the　contrary　would　happen　were　they　to　bind　it　below；　that　is；　between　the　hand　and　the　opening；　or　were　to　make　the　ligature　above　the　opening　very　tight。　　For　it　is　manifest　that　the　tie；　moderately　straightened；　while　adequate　to　hinder　the　blood　already　in　the　arm　from　returning　towards　the　heart　by　the　veins；　cannot　on　that　account　prevent　new　blood　from　coming　forward　through　the　arteries；　because　these　are　situated　below　the　veins；　and　their　coverings；　from　their　greater　consistency；　are　more　difficult　to　compress；　and　also　that　the　blood　which　comes　from　the　heart　tends　to　pass　through　them　to　the　hand　with　greater　force　than　it　does　to　return　from　the　hand　to　the　heart　through　the　veins。　And　since　the　latter　current　escapes　from　the　arm　by　the　opening　made　in　one　of　the　veins；　there　must　of　necessity　be　certain　passages　below　the　ligature；　that　is；　towards　the　extremities　of　the　arm　through　which　it　can　come　thither　from　the　arteries。　　This　physician　likewise　abundantly　establishes　what　he　has　advanced　respecting　the　motion　of　the　blood；　from　the　existence　of　certain　pellicles；　so　disposed　in　various　places　along　the　course　of　the　veins；　in　the　manner　of　small　valves；　as　not　to　permit　the　blood　to　pass　from　the　middle　of　the　body　towards　the　extremities；　but　only　to　return　from　the　extremities　to　the　heart；　and　farther；　from　experience　which　shows　that　all　the　blood　which　is　in　the　body　may　flow　out　of　it　in　a　very　short　time　through　a　single　artery　that　has　been　cut；　even　although　this　had　been　closely　tied　in　the　immediate　neighborhood　of　the　heart　and　cut　between　the　heart　and　the　ligature；　so　as　to　prevent　the　supposition　that　the　blood　flowing　out　of　it　could　come　from　any　other　quarter　than　the　heart。

But　there　are　many　other　circumstances　which　evince　that　what　I　have　alleged　is　the　true　cause　of　the　motion　of　the　blood：　　thus；　in　the　first　place；　the　difference　that　　is　observed　between　the　blood　which　flows　from　the　veins；　and　that　from　the　arteries；　can　only　arise　from　this；　that　being　rarefied；　and；　as　it　were；　distilled　by　passing　through　the　heart；　it　is　thinner；　and　more　vivid；　and　warmer　immediately　after　leaving　the　heart；　in　other　words；　when　in　the　arteries；　than　it　was　a　short　time　before　passing　into　either；　in　other　words；　when　it　was　in　the　veins；　and　if　attention　be　given；　it　will　be　found　that　this　difference　is　very　marked　only　in　the　neighborhood　of　the　heart；　and　is　not　so　evident　in　parts　more　remote　from　it。　　In　the　next　place；　the　consistency　of　the　coats　of　which　the　arterial　vein　and　the　great　artery　are　　composed；　sufficiently　shows　that　the　blood　is　impelled　　against　them　with　more　force　than　against　the　veins。　　And　why　should　the　left　cavity　of　the　heart　and　the　　great　artery　be　wider　and　larger　than　the　right　cavity　and　the　arterial　vein；　were　it　not　that　the　blood　of　the　　venous　artery；　having　only　been　in　the　lungs　after　it　has　passed　through　the　heart；　is　thinner；　and　rarefies　more　readily；　and　in　a　higher　degree；　than　the　blood　which　proceeds　immediately　from　the　hollow　vein？　　And　what　can　physicians　conjecture　from　feeling　the　pulse　unless　they　know　that　according　as　the　blood　changes　its　nature　it　can　be　rarefied　by　the　warmth　of　the　heart；　in　a　higher　or　lower　degree；　and　more　or　less　quickly　than　before？　　And　if　it　be　inquired　how　this　heat　is　communicated　to　the　other　members；　must　it　not　be　admitted　that　this　is　effected　by　means　of　the　blood；　which；　passing　through　the　heart；　is　there　heated　anew；　and　thence　diffused　over　all　the　body？　　Whence　it　happens；　that　if　the　blood　be　withdrawn　from　any　part；　the　heat　is　likewise　withdrawn　by　the　same　means；　and　although　the　heart　were　as…hot　as　glowing　iron；　it　would　not　be　capable　of　warming　the　feet　and　hands　as　at　present；　unless　it　continually　sent　thither　new　blood。　　We　likewise　perceive　from　this；　that　the　true　use　of　respiration　is　to　bring　sufficient　fresh　air　into　the　lungs；　to　cause　the　blood　which　flows　into　them　from　the　right　ventricle　of　the　heart；　where　it　has　been　rarefied　and；　as　it　were；　changed　into　vapors；　to　become　thick；　and　to　convert　it　anew　into　blood；　before　it　flows　into　the　left　cavity；　without　which　process　it　would　be　unfit　for　the　nourishment　of　the　fire　that　is　there。　　This　receives　confirmation　from　the　circumstance；　that　it　is　observed　of　animals　destitute　of　lungs　that　they　have　also　but　one　cavity　in　the　heart；　and　that　in　children　who　cannot　use　them　while　in　the　womb；　there　is　a　hole　through　which　the　blood　flows　from　the　hollow　vein　into　the　left　cavity　of　the　heart；　and　a　tube　through　which　it　passes　from　the　arterial　vein　into　the　grand　artery　without　passing　through　the　lung。　　In　the　next　place；　how　could　digestion　be　carried　on　in　the　stomach　unless　the　heart　communicated　heat　to　it　through　the　arteries；　and　along　with　this　certain　of　the　more　fluid　parts　of　the　blood；　which　assist　in　the　dissolution　of　the　food　that　has　been　taken　in？　　Is　not　also　the　operation　which　converts　the　juice　of　food　into　blood　easily　comprehended；　when　it　is　considered　that　it　is　distilled　by　passing　and　repassing　through　the　heart　perhaps　more　than　one　or　two　hundred　times　in　a　day？　　And　what　more　need　be　adduced　to　explain　nutrition；　and　the　production　of　the　different　humors　of　the　body；　beyond　saying；　that　the　force　with　which　the　blood；　in　being　rarefied；　passes　from　the　heart　towards　the　extremities　of　the　arteries；　causes　certain　of　its　parts　to　remain　in　the　members　at　which　they　arrive；　and　there　occupy　the　place　of　some　others　expelled　by　them；　and　that　according　to　the　situation；　shape；　or　smallness　of　the　pores　with　which　they　meet；　some　rather　　than　others　flow　into　certain　parts；　in　the　same　way　that　some　sieves　are　observed　to　act；　which；　by　being　variously　perforated；　serve　to　separate　different　species　of　grain？　　And；　in　the　last　place；　what　above　all　is　here　worthy　of　observation；　is　the　generation　of　the　animal　spirits；　which　are　like　a　very　subtle　wind；　or　rather　a　very　pure　and　vivid　flame　which；　continually　ascending　in　great　abundance　from　the　heart　to　the　brain；　thence　penetrates　through　the　nerves　into　the　muscles；　and　gives　motion　to　all　the　members；　so　that　to　account　for　other　parts　of　the　blood　which；　as　most　agitated　and　penetrating；　are　the　fittest　to　compose　these　spirits；　proceeding　towards　the　brain；　it　is　not　necessary　to　suppose　any　other　cause；　than　simply；　that　the　arteries　which　carry　them　thither　proceed　from　the　heart　in　the　most　direct　lines；　and　that；　according　to　the　rules　of　mechanics　which　are　the　same　with　those　of　nature；　when　many　objects　tend　at　once　to　the　same　point　where　there　is　not　sufficient　room　for　all　（as　is　the　case　with　the　parts　of　the　blood　which　flow　forth　from　the　left　cavity　of　the　heart　and　tend　towards　the　brain）；　the　weaker　and　less　agitated　parts　must　necessarily　be　driven　aside　from　that　point　by　the　stronger　which　alone　in　this　way　reach　it　I　had　expounded　all　these　matters　with　sufficient　minuteness　in　the　treatise　which　I　formerly　thought　of　publishing。　　And　after　these；　I　had　shown　what　must　be　the　fabric　of　the　nerves　and　muscles　of　the　human　body　to　give　the　animal　spirits　contained　in　it　the　power　to　move　the　members；　as　when　we　see　heads　shortly　after　they　have　been　struck　off　still　move　and　bite　the　earth；　although　no　longer　animated；　what　changes　must　take　place　in　the　brain　to　produce　waking；　sleep；　and　dreams；　how　light；　sounds；　odors；　tastes；　heat；　and　all　the　other　qualities　of　external　objects　impress　it　with　different　ideas　by　means　of　the　senses；　how　hunger；　thirst；　and　the　other　internal　affections　can　likewise　impress　upon　it　divers　ideas；　what　must　be　understood