16 CIRCULATION OF THE BLOOD 



blood, and sent out to the body as a sort of nervous 

 fluid. The distribution of this imaginary fluid took 

 place through the nerves, which were regarded as hollow. 

 The animal spirits were held responsible for all the kinds 

 of activity that we call nervous. 



To return to the blood itself we may emphasise that 

 Galen's view of the action of air on the blood was not 

 wholly imaginary. It was grounded on observation in 

 so far as it took account, firstly, of the necessity of air 

 for life ; secondly, of a real difference between venous 

 and arterial blood ; thirdly, of the fact that the systems 

 in which both kinds of blood, arterial and venous, are 

 contained have direct connection with the heart ; and 

 fourthly, of some relation to the warmth of the body, 

 the supposed nature of which we must now consider. 



Long before Galen's time physicians had come to 

 associate heat with life. They had observed that animals 

 grew cold when dead, while living animals remained 

 warm without the aid of any apparent fire. They thus 

 thought that the animal heat produced without fire was 

 a special property of living matter and was different from 

 the heat that was derived from fire. It was therefore 

 given a special name and called the innate heat. The 

 innate heat was held to be developed specially by the 

 heart, an organ on which, as we have seen, great stress 

 had been laid by Aristotle, who lived five hundred years 

 before Galen. The name of the great naturalist, Aristotle, 

 thus became connected, in the minds of later writers, 

 with the idea of this mysterious innate heat. The con- 

 nection of so important a name as that of Aristotle with 

 this idea had very important effects in a later age. 



Galen, however, was dissatisfied with the current 

 view of the innate heat. He refused to accept its peculiar 

 and inexplicable nature, and he sought to explain it 

 and to bring it within his physiological system. He 



