FATHERS OF BIOLOGY. 



heat, Aristotle for its ventilation, Erasistratus for the 

 filling of the arteries with spirits. All these opinions are 

 discussed and commented upon by Galen, who deter- 

 mines the purposes of respiration to be (i) to preserve 

 the animal heat ; (2) to evacuate from the blood the 

 products of combustion. 



He conjectured that there was in atmospheric air not 

 only a quality friendly to the vital spirit, but also a 

 quality inimical to it, which conjecture he drew from 

 observation of the various phenomena accompanying 

 the support and the extinction of flame ; and he says that 

 if we could find out why flame is extinguished by absence 

 of the air, we might then know the nature of that sub- 

 stance which imparts warmth to the blood during the 

 process of respiration. 



On another occasion he says that it is evidently the 

 quality and not the quantity of the air which is necessary 

 to life. He further shows that he recognized the analogy 

 between respiration and combustion, by comparing the 

 lungs to a lamp, the heart to its wick, the blood to the 

 oil, and the animal heat to the flame. 



From certain observations in various parts of his 

 works, it appears that, although ignorant of the doctrine 

 of atmospheric pressure, he was acquainted with some 

 of its practical effects. Thus, he says, if you put one 

 end of an open tube under water and suck out the air 

 with the other end, you will draw up water into the 



