214 THE STRUGGLE FOR LIFE. 
began to speculate on the nature of life, and used his flint 
knives for other purposes than to cut the juicy steak from 
the bones of his prey. But, although there were dim 
streaks shooting up above the horizon, heralding the coming 
dawn of knowledge, it was reserved for our countryman, 
the immortal Harvey, first to understand the whole truth 
concerning the circulation of the blood, and so lay, broad 
and deep, the indestructible foundations of Medicine and 
the allied sciences. 
Harvey inferred from his dissections and experiments, 
that small channels, now called capillaries, must exist, and 
connect the smallest branches of the arteries with those of 
the veins, and that the arteries during life contained blood, 
instead of Vital Spirits, as had been supposed. He stated 
that during life there is a ceaseless circulation of the blood, 
from the right side of the heart, through the lungs, the left 
side of the heart, the arteries, capillaries and veins, back 
again to the starting point. The blood therefore is con- 
stantly circulating in a closed channel as long as life lasts. 
Harvey himself never saw a capillary except with the eye 
of his mind, but now, thanks to the invention of: the 
microscope, the veriest tyro can demonstrate their existence 
and use, and:show the actual passage of the blood through 
them. The central organ. of the circulation is, of course, 
the heart, and, Alas for the poets! the heart is nothing more 
or less than a pump! and has no more to do with the 
affections and emotions than has the great toe. Like other 
pumps it is provided with valves, so that the blood’ can 
only be propelled in one direction. 
Briefly, the scheme of the circulation is as follows. The 
arterial, or purified blood, received into the left side ofthe 
heart from the lungs, is propelled through the arteries at 
the average rate of from 70 to 80 strokes a minute; these 
vessels keep on dividing until they end in fine hair-like or 
capillary channels, where the blood gives up-its oxygen, 
