12 



THE BLOOD 



These experiments do not give us the natural 

 pathway of the blood." 



But, in his second letter, he describes how he 

 has examined, with a microscope of two lenses, 

 the lung and the mesentery of a frog, and has 

 seen the capillaries, and the blood in them :— 



"Such is the divarication of these little vessels, 

 coming off from the vein and the artery, that the 

 order in which the vessel ramifies is no longer 

 preserved, but it looks like a network woven from 

 the offshoots of both vessels." 



He was able, in a dead frog, to see the capil- 

 laries ; and then, in a living frog, to see the blood 

 moving in them. But, in spite of this work, it 

 took nearly half a century before Harvey's teaching 

 was believed by all men — Tantum consuetudo apud 

 omnes valet. 



2. The Blood-pressure. 



Harvey had seen the facts of blood-pressure — 

 the great quantity of blood passing through, and the 

 swiftness of its passage — but he had not measured 

 it. Keill's experiments on the blood-pressure ( 1 7 1 8) 

 were inexact, and of no value ; and the first exact 

 measurements were made by Stephen Hales, who 

 was rector of Farringdon, Hampshire, and minister 

 of Teddington, Middlesex ; a Doctor of Divinity, 

 and a Fellow of the Royal Society. His experi- 

 ments, in their width and diversity, were not 

 surpassed even by those of John Hunter, and were 

 extended far over physiology, vegetable physiology, 



