64 THE HISTORY OF BIOLOGY 



effect than that on any other subject. As one of his services to science it may 

 be mentioned that he finally succeeded in overcoming the old preconception 

 that the arteries and the left heart-chamber contain air. In his opinion they 

 contain blood, with an admixture of "pneuma" — that half-airlike, half- 

 firelike life-principle which gave rise to so much discussion in ancient times 

 and on which the existence of living creatures depends. In one place Galen 

 expresses the hope that the time may come when someone will discover the 

 component in the air which forms pneuma, the substance which is the com- 

 mon precondition of life and combustion — a curious idea, which in fact 

 the discovery of oxygen was eventually to bring to realization. He gives a 

 detailed description of the heart, both its structure and its functions; on the 

 other hand, like his predecessors, he lets both veins and arteries convey the 

 blood from the heart to the rest of the body, in which it is consumed. He 

 is not aware of any blood flowing from the body to the heart, while his idea 

 of the movement of the blood is still further confused by his belief that the 

 liver is to a certain extent the centre of the venous system, since the blood 

 flows from it not only to the heart, but also to the rest of the body. The 

 left ventricle receives through the vena pulmonalis "pneuma" from the lungs; 

 from the right ventricle the excremental products proceed to the lungs, these 

 products being "soot" from the combustion process in the heart, which is 

 got rid of by exhalation. The wall between the right and the left ventricles 

 of the heart is porous, permitting the blood to pass through. The walls of 

 the blood-vessels are carefully described, and, generally speaking, Galen's 

 detailed study of the construction of the individual organs is one of his strong 

 points. He is aware of the connexions between the arteries and the veins, 

 but, as is seen from the above, he has not realized the idea of circulation, and 

 this fact, combined with the vagueness with which he explains his ideas on 

 these organs, proved an obstacle to the development of biology for the next 

 fifteen hundred years. 



Galen carefully studied the respiratory process and on the whole de- 

 scribed it correctly. With regard to the sense organs, in spite of his thorough 

 investigations into the subject he made very little advance on his predeces- 

 sors, and the same may be said of his description of the genital system and 

 the embryonic process, in which he remains, on the whole, where Aristotle 

 stood. 



Splendid as Galen's scientific work was, it does not appear to have been 

 highly appreciated by his contemporaries. He himself complains that but few 

 understand him, but consoles himself with the thought that the Creator, in 

 spite of man's ingratitude, never wearies of doing good. Here posterity has to 

 an unusually generous extent made up for what his contemporaries failed to 

 give him. The fact that Galen did not impress his contemporaries may have 

 been due to the peculiar transitional attitude he adopted; to the survivors of 



