68 Greek Biology 



branches. Of these branches, one, our common vena cava, 

 entered the right side of the heart. For the blood that it 

 conveyed to the heart there were two fates possible. The 

 greater part remained awhile in the ventricle, parting with 

 its impurities and vapours, exhalations of the organs, which 

 were carried off by the artery -like vein ($Ae\|/ aprrjptto^s, the 

 mediaeval vena pulmonalis, our pulmonary artery) to the lung 

 and then exhaled to the outer air. These impurities and 

 vapours gave its poisonous and suffocating character to the 

 breath. Having parted thus with its impurities, the venous 

 blood ebbed back again from the right ventricle into the 

 venous system. But for a small fraction of the venous blood 

 that entered the right ventricle another fate was reserved. 

 This small fraction of venous blood, charged still with the 

 natural spirits derived from the liver, passed through minute 

 channels in the septum between the ventricles and entered 

 the left chamber. Arrived there, it encountered the external 

 pneuma and became thereby elaborated into a higher form of 

 spirit, the vital spirits (-nvtv^a fcoriKoV, spiritus viialis), which 

 is distributed together with blood by the arterial system to 

 various parts of the body. In the arterial system it also 

 ebbed and flowed, and might be seen and felt to pulsate 

 there. 



But among the great arterial vessels that sent forth arterial 

 blood thus charged with vital spirits were certain vessels which 

 ascended to the brain. Before reaching that organ they divided 

 up into minute channels, the rete mirabile (i:\iy\La jmeyiorov 

 Oavfjia), and passing into the brain became converted by the 

 action of that organ into a yet higher type of spirits, the 

 animal spirits (-nv^v^a ^V^LKOV, spiritus animalis), an ethereal 

 substance distributed to the various parts of the body by the 

 structures known to-day as nerves, but believed then to be 

 hollow channels. The three fundamental faculties (Swa/xa?), 

 the natural, the vital, and the animal, which brought into 



