THE CIRCULATION OF THE BLOOD 439 



important discovery, made a very capital and important 

 error in another direction, although it was a very natural 

 error. If, in any animal which is recently killed, you open 

 one of those pulsating trunks which I referred to a short 

 time ago, you will find, as a general rule, that it either 

 contains no blood at all or next to none ; but that, on the 

 contrary, it is full of air. Very naturally, therefore, Erasis- 

 tratus came to the conclusion that this was the normal 

 and natural state of the arteries, and that they contained 

 air We are apt to think this a very gross blunder ; but, 

 to anybody who is acquainted with the facts of the case, 

 it is, at first sight, an exceedingly natural conclusion. Not 

 only so, but Erasistratus might have very justly imagined 

 that he had seen his way to the meaning of the connection 

 of the left side of the heart with the lungs ; for we find that 

 what we now call the pulmonary vein is connected with the 

 lungs, and branches out in them (Fig. 1). Finding that the 

 greater part of this system of vessels was filled with air after 

 death, this ancient thinker very shrewdly concluded that 

 its real business was to receive air from the lungs, and to 

 distribute that air all through the body, so as to get rid of the 

 grosser humours and purify the blood. That was a very 

 natural and a very obvious suggestion, and a highly ingenious 

 one though it happened to be a great error. You will 

 observe that the only way of correcting it was to experiment 

 upon living animals, for there is no other way in which this 

 point could be settled. 



And hence we are indebted, for the correction of the error 

 of Erasistratus, to one of the greatest experimenters of 

 ancient or modern times, Claudius Galenus, who lived in 

 the second century after Christ. I say it was to this man 

 more than any one else, because he knew that the only way 

 of solving physiological problems was to examine into the 

 facts in the living animal. And because Galen was a skilful 

 anatomist, and a skilful experimenter, he was able to show 

 in what particulars Erasistratus had erred, and to build up 

 a system of thought upon this subject which was not im- 

 proved upon for fully 1,300 years. I have endeavoured, in 

 Fig. 2, to make clear to you exactly what it was he tried to 

 establish. You will observe that this diagram is practically 

 the same as that given in Fig. 1, only simplified. The same 

 facts may be looked upon by different people from different 

 points of view. Galen looked upon these facts from a very 

 different point of view from that which we ourselves occupy ; 



