14 CIRCULATION OF THE BLOOD 



to the other. Although a seeker, or even a fine bristle, 

 cannot be made to penetrate from one ventricle to the 

 other through any of these pits, yet for ages, and in spite 

 of the evidence of their senses, men continued to believe 

 that these channels really existed. The great Galen had 

 believed in their existence, and that was enough ! If they 

 could not be found, it was thought that it was because 

 they were so small that they were beyond the reach of 

 vision. Their existence was necessary to explain the 

 nature of the heart's action. It must be remembered 

 that there were then no microscopes or lenses to increase 

 the power of vision, so that it was the easier to believe in 

 the existence of these channels. Furthermore, no better 

 explanation of the physiology of the heart than that 

 provided by Galen was forthcoming for very many 

 centuries. 



We have now to consider what Galen and his followers 

 believed to be the fate of the small amount of blood that 

 made its way, as they thought, drop by drop, through 

 this septum by means of the imaginary passages. Galen 

 knew well of the difference in appearance between 

 arterial and venous blood, he fully recognised the light 

 colour of the blood in the arteries as compared with the 

 dark colour of the venous fluid. Though he had no clear 

 idea of the nature of respiration, though he knew nothing 

 about the composition of the atmosphere, and though 

 oxygen had not yet been discovered, yet he attributed 

 this difference between the arterial and venous blood to 

 a cause not very different from that assigned to it to-day. 

 He considered it was the action of the air that produced 

 the lighter colour of the arterial blood. 



Galen knew too of the existence of the important 

 vessel that empties itself into the left auricle which we 

 call the pulmonary vein, but which he called the venal 

 artery. We know now that this vessel brings oxygenated 



