HISTORY OF PHYSIOLOGY. 7 



later times have shown the accuracy of his observations on 

 points which at first seemed mere fancies of his mind. He 

 wrote several works on physiological matters. His works 

 entitled, " Natural History of Animals," " On the Parts of 

 Animals," on "Animal Locomotion," "On Respiration," 

 are in part preserved, while a treatise on anatomy seems to 

 have been lost. From these writings the views of his 

 time are accessible. The main purpose with Aristotle 

 was to explain the function of every part and organ 

 described. Some of the main conceptions of Aristotle are 

 here briefly given: The heart is the central organ, and 

 the motive power of the whole body. It is the seat of 

 vitality, because it is the source of the animal heat of the 

 body, all the warmth of the body being produced by the 

 heart. The heart makes the blood, gives to it its vital stimu- 

 lus, and then sends it all over the body through the blood 

 vessels. The blood does not circulate, it never returns to 

 the heart, but as fast as it is used up at the periphery of 

 the body it is replaced in the heart. The blood is stored 

 in the blood vessels as in a jar. Whence the name blood 

 vessel used to this day. (The name "aorta" dates from 

 Aristotle.) Aristotle describes the stomach and intestines, 

 and hints at the proper conception of absorption. He 

 describes the trachea by the name artery, and says that it 

 carries air into the lungs. In the lungs the spiritus or 

 pneuma is extracted from the air and by channels (pul- 

 monary artery and veins, no doubt referred to) this 

 "spirit" substance is carried to the heart, where it is 

 mixed by the heart with the blood, and serves to give it 

 its vital stimulus. The ancient notion that the air or 

 ' ' spiritus ' ' is the vital principle still lingers in our every- 

 day language when we refer to vital things as spiritual, or 

 call the active element of wine spirits, although now hav- 

 ing an entirely different meaning. Aristotle explains all 

 vital processes as caused by lieat. Heat was the "causa 

 operandi " for everything. "Heat changes the metals, 

 disintegrates ores, liquids, and then vaporizes ice, causes 



