a history of science-2-第25章

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〃　Having　convinced　himself　that　no　intra…ventricular　opening　existed；　he　proceeded　to　study　the　action　of　the　heart　itself；　untrammelled　by　too　much　faith　in　established　theories；　and；　as　yet；　with　no　theory　of　his　own。　He　soon　discovered　that　the　commonly　accepted　theory　of　the　heart　striking　against　the　chest…wall　during　the　period　of　relaxation　was　entirely　wrong；　and　that　its　action　was　exactly　the　reverse　of　this；　the　heart　striking　the　chest…wall　during　contraction。　Having　thus　disproved　the　accepted　theory　concerning　the　heart's　action；　he　took　up　the　subject　of　the　action　of　arteries；　and　soon　was　able　to　demonstrate　by　vivisection　that　the　contraction　of　the　arteries　was　not　simultaneous　with　contractions　of　the　heart。　His　experiments　demonstrated　that　these　vessels　were　simply　elastic　tubes　whose　pulsations　were　〃nothing　else　than　the　impulse　of　the　blood　within　them。〃　The　reason　that　the　arterial　pulsation　was　not　simultaneous　with　the　heart…beat　he　found　to　be　because　of　the　time　required　to　carry　the　impulse　along　the　tube；　By　a　series　of　further　careful　examinations　and　experiments；　which　are　too　extended　to　be　given　here；　he　was　soon　able　further　to　demonstrate　the　action　and　course　of　the　blood　during　the　contractions　of　the　heart。　His　explanations　were　practically　the　same　as　those　given　to…dayfirst　the　contraction　of　the　auricle；　sending　blood　into　the　ventricle；　then　ventricular　contraction；　making　the　pulse；　and　sending　the　blood　into　the　arteries。　He　had　thus　demonstrated　what　had　not　been　generally　accepted　before；　that　the　heart　was　an　organ　for　the　propulsion　of　blood。　To　make　such　a　statement　to…day　seems　not　unlike　the　sober　announcement　that　the　earth　is　round　or　that　the　sun　does　not　revolve　about　it。　Before　Harvey's　time；　however；　it　was　considered　as　an　organ　that　was　〃in　some　mysterious　way　the　source　of　vitality　and　warmth；　as　an　animated　crucible　for　the　concoction　of　blood　and　the　generation　of　vital　spirits。〃＇3＇　In　watching　the　rapid　and　ceaseless　contractions　of　the　heart；　Harvey　was　impressed　with　the　fact　that；　even　if　a　very　small　amount　of　blood　was　sent　out　at　each　pulsation；　an　enormous　quantity　must　pass　through　the　organ　in　a　day；　or　even　in　an　hour。　Estimating　the　size　of　the　cavities　of　the　heart；　and　noting　that　at　least　a　drachm　must　be　sent　out　with　each　pulsation；　it　was　evident　that　the　two　thousand　beats　given　by　a　very　slow　human　heart　in　an　hour　must　send　out　some　forty　pounds　of　bloodmore　than　twice　the　amount　in　the　entire　body。　The　question　was；　what　became　of　it　all？　For　it　should　be　remembered　that　the　return　of　the　blood　by　the　veins　was　unknown；　and　nothing　like　a　〃circulation〃　more　than　vaguely　conceived　even　by　Harvey　himself。　Once　it　could　be　shown　that　the　veins　were　constantly　returning　blood　to　the　heart；　the　discovery　that　the　blood　in　some　way　passes　from　the　arteries　to　the　veins　was　only　a　short　step。　Harvey；　by　resorting　to　vivisections　of　lower　animals　and　reptiles；　soon　demonstrated　beyond　question　the　fact　that　the　veins　do　carry　the　return　blood。　〃But　this；　in　particular；　can　be　shown　clearer　than　daylight；〃　says　Harvey。　〃The　vena　cava　enters　the　heart　at　an　inferior　portion；　while　the　artery　passes　out　above。　Now　if　the　vena　cava　be　taken　up　with　forceps　or　the　thumb　and　finger；　and　the　course　of　the　blood　intercepted　for　some　distance　below　the　heart；　you　will　at　once　see　it　almost　emptied　between　the　fingers　and　the　heart；　the　blood　being　exhausted　by　the　heart's　pulsation；　the　heart　at　the　same　time　becoming　much　paler　even　in　its　dilatation；　smaller　in　size；　owing　to　the　deficiency　of　blood；　and　at　length　languid　in　pulsation；　as　if　about　to　die。　On　the　other　hand；　when　you　release　the　vein　the　heart　immediately　regains　its　color　and　dimensions。　After　that；　if　you　leave　the　vein　free　and　tie　and　compress　the　arteries　at　some　distance　from　the　heart；　you　will　see；　on　the　contrary；　their　included　portion　grow　excessively　turgid；　the　heart　becoming　so　beyond　measure；　assuming　a　dark…red　color；　even　to　lividity；　and　at　length　so　overloaded　with　blood　as　to　seem　in　danger　of　suffocation；　but　when　the　obstruction　is　removed　it　returns　to　its　normal　condition；　in　size；　color；　and　movement。〃＇4＇　This　conclusive　demonstration　that　the　veins　return　the　blood　to　the　heart　must　have　been　most　impressive　to　Harvey；　who　had　been　taught　to　believe　that　the　blood　current　in　the　veins　pursued　an　opposite　course；　and　must　have　tended　to　shake　his　faith　in　all　existing　doctrines　of　the　day。　His　next　step　was　the　natural　one　of　demonstrating　that　the　blood　passes　from　the　arteries　to　the　veins。　He　demonstrated　conclusively　that　this　did　occur；　but　for　once　his　rejection　of　the　ancient　writers　and　one　modern　one　was　a　mistake。　For　Galen　had　taught；　and　had　attempted　to　demonstrate；　that　there　are　sets　of　minute　vessels　connecting　the　arteries　and　the　veins；　and　Servetus　had　shown　that　there　must　be　such　vessels；　at　least　in　the　lungs。　However；　the　little　flaw　in　the　otherwise　complete　demonstration　of　Harvey　detracts　nothing　from　the　main　issue　at　stake。　It　was　for　others　who　followed　to　show　just　how　these　small　vessels　acted　in　effecting　the　transfer　of　the　blood　from　artery　to　vein；　and　the　grand　general　statement　that　such　a　transfer　does　take　place　was；　after　all；　the　all…important　one；　and　the　exact　method　of　how　it　takes　place　a　detail。　Harvey's　experiments　to　demonstrate　that　the　blood　passes　from　the　arteries　to　the　veins　are　so　simply　and　concisely　stated　that　they　may　best　be　given　in　his　own　words。　〃I　have　here　to　cite　certain　experiments；〃　he　wrote；　〃from　which　it　seems　obvious　that　the　blood　enters　a　limb　by　the　arteries；　and　returns　from　it　by　the　veins；　that　the　arteries　are　the　vessels　carrying　the　blood　from　the　heart；　and　the　veins　the　returning　channels　of　the　blood　to　the　heart；　that　in　the　limbs　and　extreme　parts　of　the　body　the　blood　passes　either　by　anastomosis　from　the　arteries　into　the　veins；　or　immediately　by　the　pores　of　the　flesh；　or　in　both　ways；　as　has　already　been　said　in　speaking　of　the　passage　of　the　blood　through　the　lungs；　whence　it　appears　manifest　that　in　the　circuit　the　blood　moves　from　thence　hither；　and　hence　thither；　from　the　centre　to　the　extremities；　to　wit；　and　from　the　extreme　parts　back　again　to　the　centre。　Finally；　upon　grounds　of　circulation；　with　the　same　elements　as　before；　it　will　be　obvious　that　the　quantity　can　neither　be　accounted　for　by　the　ingesta；　nor　yet　be　held　necessary　to　nutrition。　〃Now　let　any　one　make　an　experiment　on　the　arm　of　a　man；　either　using　such　a　fillet　as　is　employed　in　blood…letting　or　grasping　the　limb　tightly　with　his　hand；　the　best　subject　for　it　being　one　who　is　lean；　and　who　has　large　veins；　and　the　best　time　after　exercise；　when　the　body　is　warm；　the　pulse　is　full；　and　the　blood　carried　in　large　quantities　to　the　extremities；　for　all　then　is　more　conspicuous；　under　such　circumstances　let　a　ligature　be　thrown　about　the　extremity　and　drawn　as　tightly　as　can　be　borne：　it　will　first　be　perceived　that　beyond　the　ligature　neither　in　the　wrist　nor　anywhere　else　do　the　arteries　pulsate；　that　at　the　same　time　immediately　above　the　ligature　the　artery　begins　to　rise　higher　at　each　diastole；　to　throb　more　violently；　and　to　swell　in　its　vicinity　with　a　kind　of　tide；　as　if　it　strove　to　break　through　and　overcome　the　obstacle　to　its　current；　the　artery　here；　in　short；　appears　as　if　it　were　permanently　full。　The　hand　under　such　circumstances　retains　its　natural　color　and　appearances；　in　the　course　of　time　it　begins　to　fall　somewhat　in　temperature；　indeed；　but　nothing　is　DRAWN　into　it。　〃After　the　bandage　has　been　kept　on　some　short　time　in　this　way；　let　it　be　slackened　a　little；　brought　to　the　state　or　term　of　middling　tightness　which　is　used　in　bleeding；　and　it　will　be　seen　that　the　whole　hand　and　arm　will　instantly　become　deeply　suffused　and　distended；　injected；　gorged　with　blood；　DRAWN；　as　it　is　said；　by　this　middling　ligature；　without　pain；　or　heat；　or　any　horror　of　a　vacuum；　or　any　other　cause　yet　indicated。　〃As　we　have　noted；　in　connection　with　the　tight　ligature；　that　the　artery　above　the　bandage　was　distended　and　pulsated；　not　below　it；　so；　in　the　case　of　the　moderately　tight　bandage；　on　the　contrary；　do　we　find　that　the　veins　below；　never　above；　the　fillet　swell　and　become　dilated；　while　the　arteries　shrink；　and　such　is　the　degree　of　distention　of　the　veins　here　that　it　is　only　very　strong　pressure　that　will　force　the　blood　beyond　the　fillet　and　cause　any　of　the　veins　in　the　upper　part　of　the　arm　to　rise。　〃From　these　facts　it　is　easy　for　any　careful　observer　to　learn　that　the　blood　enters　an　extremity　by　the　arteries；　for　when　they　are　effectively　compressed　nothing　is　DRAWN　to　the　member；　the　hand　preserves　its　color；　nothing　flows　into　it；　neither　is　it　distended；　but　when　the　pressure　is　diminished；　as　it　is　with　the　bleeding　fillet；　it　is　manifest　that　the　blood　is　instantly　thrown　in　with　force；　for　then　the　hand　begins　to　swell；　which　is　as　much　as　to　say　that　when　the　arteries　pulsate　the　blood　is　flowing　through　them；　as　it　is　when　the　moderately　tight　ligature　is　applied；　but　when　they　do　not　pulsate；　or　when　a　tight　ligature　is　used；　they　cease　from　transmitting　anything；　they　are　only　distended　above　the　part　where　the　ligature　is　applied。　The　veins　again　being　compressed；　nothing　can　flow　through　them；　the　certain　indication　of　which　is　that　below　the　ligature　they　are　much　more　tumid　than　above　it；　and　than　they　usually　appear　when　there　is　no　bandage　upon　the　arm。　〃It　therefore　plainly　appears　that　the　ligature　prevents　the　return　of　the　blood　through　the　veins　to　the　parts　above　it；　and　maintains　those　beneath　it　in　a　state　of　permanent　distention。　But　the　arteries；　in　spite　of　the　pressure；　and　under　the　force　and　impulse　of　the　heart；　send　on　the　blood　from　the　internal　parts　of　the　body　to　the　parts　beyond　the　bandage。〃＇5＇

This　use　of　ligatures　is　very　significant；　because；　as　shown；　a　very　tight　ligature　stops　circulation　in　both　arteries　and　veins；　while　a　loose　one；　while　checking　the　circulation　in　the　veins；　which　lie　nearer　the　surface　and　are　not　so　directly　influenced　by　the　force　of　the　heart；　does　not　stop　the　passage　of　blood　in　the　arteries；　which　are　usually　deeply　imbedded　in　the　tissues；　and　not　so　easily　influenced　by　pressure　from　without。　The　last　step　of　Harvey's　demonstration　was　to　prove　that　the　blood　does　flow　along　the　veins　to　the　heart；　aided　by　the　valves　that　had　been　the　cause　of　so　much　discussion　and　dispute　between　the　great　sixteenth…century　anatomists。　Harvey　not　only　demonstrated　the　presence　of　these　valves；　but　showed　conclusively；　by　simple　experiments；　what　their　function　was；　thus　completing　his　demonstration　of　the　phenomena　of　the　circulation。　The　final　ocular　demonstration　of　the　passage　of　the　blood　from　the　arteries　to　the　veins　was　not　to　be　made　until　four　years　after　Harvey's　death。　This　process；　which　can　be　observed　easily　in　the　web　of　a　frog's　foot　by　the　aid　of　a　low…power　lens；　was　first　demonstrated　by　Marcello　Malpighi　（1628…1694）　in　1661。　By　the　aid　of　a　lens　he　first　saw　the　small　〃capillary〃　vessels　connecting　the　veins　and　arteries　in　a　piece　of　dried　lung。　Taking　his　cue　from　this；　he　examined　the　lung　of　a　turtle；　and　was　able　to　see