956 ON THE FORM AND BRANCHING [ch. 



assumption of a peculiar power acting on the blood, of whose 

 existence in the animal economy we have as yet no other evidence *," 



Sir Charles Bell, whose anatomical skill was great but his mathe- 

 matical insight small, drew the conclusion, of no small historic 

 interest, that "the laws of hydraulics, though illustrative, are not 

 strictly applicable to the explanation of the circulation of the blood, 

 nor to the actions of the living frame." He goes on to say: 

 *' Although \^e perceive admirable mechanism in the heart, and in 

 the adjustment of the tubes on hydraulic principles: and although 

 the arteries and veins have form, calibre and curves suited^ to the 

 conveyance of fluid, according to our knowledge of hydraulic engines: 

 yet the laws of life, or of physiology, are. essential to the explanation 

 of the circulation of the blood. And this conclusion we draw, not 

 only from the extent and minuteness of the vessels, but also from 

 the peculiar nature of the blood itself. Life is in both, and a mutual 

 influence prevails f." This peculiar form of vitalism savours more 

 of Bichat and the French school than of the teaching of John 

 Hunter or Thomas Young. It is precisely that idea of "organic 

 control" or "organic coordination," which the physiologists are 

 always reluctant to accept, always unwilling to abandon: which 

 is said to be inherent in every process or operation of the body, and 

 to differentiate biology from all the physical sciences : and of which 

 in our own day Haldane has been the chief and great protagonist. 

 But it is a subject with which this book is not concerned. , 



To conclude, we may now approach the question of economical 

 size of the blood-vessels in a broader way. They must not be too 

 small, or the work of driving blood through them will be too great; 



* See Sharpey's article on Cilia, in Todd's Cyclopaedia, i, p. 637, 1836; also 

 Allen Thomson's admirable article on the Circulation, ibid. p. 672. Alison's views 

 were based not only on Haller, but largely on Dr James Black's Essay on the 

 Capillary Circulation, London, 1825. 



t Practical Essays, 1842, p. 88. When Sir Charles Bell declared that hydraulic 

 principles were not enough, but that "the laws of life" were needed to explain the 

 circulation of the blood, he was right from his point of view. He was slow to see, 

 and unwilling to admit, that hydrodynamical principles suffice to explain a large, 

 essential part of the problem ; but as a physiologist he had every reason to know 

 that that part was not the whole. He may have had many things in mind: the 

 arrest of the circulation in inflammation, as we see it in a frog's web ; that a cut 

 artery bleeds to death while a torn one does not bleed at all; that blood does not 

 coagulate when stagnant within its own vessels — a fact which, as John Hunter said, 

 "has ever appeared to me the most interesting fact in physiology." 



