144 ACTION OF THE HEART. 



The majority of the circulations we examine in organic forms are ac- 

 complished without any heart. Plants have none ; fishes have no sys- 

 temic heart ; even in man, at the first period of embryonic existence, there 

 is no such central organ ; in his adult condition the portal circulation has 

 none. The current of blood in the capillaries, seen under the microscope, 

 exhibits no jetting movements, but, on the contrary, a steadiness of 

 flow, sometimes for long in one channel, then a cessation, then perhaps 

 a retrogradation, and then a new path. It looks as though the blood 

 was flowing spontaneously, and not by any force acting behind. The 

 heart of an animal may be suddenly cut out, and yet the capillary motion 

 may go on in the same direction as before. After death the arterial 

 tubes are most commonly found empty : a result which is a mechanical 

 impossibility on the supposition that the heart alone drives the blood, 

 but which ensues as a necessary consequence if the capillaries draw it. 

 In acardiac monsters the blood circulates without difficulty, and, though 

 it was at one time supposed that in these twins the hearted foetus drove 

 the blood through the heartless one, this is now demonstrated not to be 

 the case. The circulation, moreover, varies locally, and at special epochs, 

 as in the development of the generative organs, the mammary glands, the 

 flow to the erectile tissues. Ubi irritatio ibi fluxus is an old medical 

 aphorism, and these local variations are incompatible with the action of 

 one central unvarying force. In cases of spontaneous gangrene, it some- 

 times occurs that the circulation through the part has declined, while the 

 capillaries are all open, as subsequent examination proves. The applica- 

 tion of cold to a part checks the circulation through it, and this not 

 through any contraction of the vessels ; so, likewise, does a jet of carbon- 

 ic acid gas directed upon them. Moreover, any retardation in the supply 

 of air to the lungs restrains the circulation, and this not alone in the 

 pulmonary vessels, but also in the systemic capillaries, producing an in- 

 creased pressure in the arterial tubes, a diminished one simultaneously 

 occurring in the veins ; and if, in the various cases now mentioned, the 

 propulsive action of the ventricles can not be relied on to explain the dif- 

 ficulties, neither can any supposed suction or exhausting action of the 

 auricles. When a ligature is tied round a vein, the action of the auricle 

 is cut off, but the vein distends beyond the obstruction, showing that 

 there is a force acting from the capillaries. Flexible tubes, such as are 

 those vessels, would at once collapse under the exertion of a very moder- 

 ate suction power, far less in intensity than would be necessary to draw 

 the blood in the veins. 



In spasmodic asthma, and in all pulmonary congestions, the right side 

 of the heart circulates, the blood with difficulty through the lungs, show- 

 ing the existence of a great obstruction to its motion through the pulmo- 

 aary capillaries. An examination of the condition of the various por- 



