8 STUDIES IN ADVANCED PHYSIOLOGY. 



the embryo to grow in the egg; in short, heat causes all 

 the processes of life." "But this heat must not go too 

 high, and so we have respiration, which enables us to 

 breathe in cold air and so cool the blood. For this reason 

 we breathe the faster the hotter we are ! ' ' 



While much of Aristotle's information is remarkably 

 exact when we remember his times, and shows him to have 

 been a scientist such as the world has rarely seen since, yet 

 here and there we find ideas that seem somewhat puerile. 

 Thus he says with great earnestness "that in all his dissec- 

 tions, and he has made many, he has always found the 

 windpipe leading to the lungs and the oesophagus to the 

 stomach, nor has he ever found them interchanged." 



PRAXAGORAS. 



The views of Aristotle were extended by Praxagoras, 

 whose important addition to physiological knowledge was 

 the distinction he found between arteries and veins. He 

 also noticed the pulsations of the arteries and the absence 

 of a pulse from the veins. This was a material gain. But 

 with this discovery there crept in a misconception which 

 remained for centuries and formed the fundamental con- 

 ception of all the earlier physiologists. This was that the 

 veins are filled by, and carry the blood, but that the arteries 

 have no liquid of any kind in them, but carry the mysterious 

 "spiritus" or "pneuma," something akin but not exactly 

 the same as ordinary air. This misconception arose no 

 doubt from the observation that after death the arteries are 

 empty and all the blood practically is found in the veins. 

 The word artery, arteria, was used by Aristotle for the 

 trachea, as it was clearly an air- tube, but was by Praxagoras 

 applied to the pulsating blood-vessels under the idea that 

 they too contained air. We still use the name artery, 

 although since the time of Galen we know that they carry 

 blood and not air. None of the works of Praxagoras are 

 preserved, but his views are quoted by later writers. 



