172 THE HUMAN HEART. 



extent ; and the result is, the propagation of the wave from the cen- 

 tres to the circumferences of the system. The heart in successive 

 moments forces life upon the arteries ; the arteries, by an even pres- 

 sure, tend to contraction or death, and thereby diffuse the life, or 

 make it universal. Organic beings are expanded ab extra, but con- 

 tract of themselves. Were it not for pressure from without, the 

 contractility of all things would result in general death. But the 

 influx of life opens the narrow into the wide, and ultimately effects 

 a compromise, whereby contraction or individuality rules conjointly 

 with expansion or receptiveness, and the two together perpetuate 

 the commonwealth. This is the indispensable strife between pro- 

 gress and conservation, here represented by the heart and arteries. 



The arteries, forming a tree whose stem is the aorta, terminate by 

 their capillary twigs in the veins, which form another tree corres- 

 ponding to the former, but the reverse of it in motion or function ; 

 for the blood that runs from the largest to the lesser and least arte- 

 ries, returns to the heart through first the least, then the lesser, and 

 then the largest veins. The arterial pulse is quite lost in the mi- 

 nute branches and twigs of the arteries, and the blood passing into 

 the veins presents one continuous flow not manifestly influenced by 

 the beating of the heart. Nevertheless it receives the force of the 

 heart, which is the grand cause of the venous circulation, there being 

 many secondary causes promotive of the same effect, and particularly 

 the respiratory movements. The blood in the veins is prevented 

 from running back, both by the vis a tergo, and during muscular 

 efforts by a set of valves, something like those in the aorta, and 

 which are placed at short intervals in many of the veins, and deter- 

 mine the wavering current onwards. Arrived in the venae cavse, 

 the heads of the veins, the blood receives a new stroke from the 

 muscular strength added to the cava?, and presses with all its forces 

 into the right auricle of the heart, which thus receives the last of 

 the blood, and in the words of Harvey, " is the first part of the 

 heart to live, and the last to die." Thus intruded, it distends the 

 auricle; which, when it has borne the distention to the utmost, 

 begins to resist, to react, to contract ; it does contract and expels the 

 intruded blood. Whither does it expel it ? Not of course into the 

 venae cavae, excepting in the slight proportion between the whole 



