MOTION OF THE HEART AND BLOOD 



We should not let the thickness of the arterial 

 walls mislead us into believing that the pulsating 

 power moves along them from the he^rt.^ In some 

 animals the arteries do not differ from the veins, 

 and in the distant parts of the body where the ar- 

 teries are finely divided, as in the brain and hand, 

 no-one can tell arteries from veins by their walls, 

 for the tunics are the same in both. In an aneurysm 

 arising from an injured or eroded artery, the pulse 

 is just the same as in the other arteries, but it has no 

 arterial tunic. The renowned Riolan supports me 

 in this in his 7th Book. 



It is not to be supposed that the function of the 

 pulse is the same as that of respiration because the 

 respiration is made more frequent and powerful, 

 as Galen says, by the same causes as running, bath- 

 ing or any other heating agent. Not only is ex- 

 perience opposed to this (though Galen strives to 

 get around it), when by immoderate gorging the 

 pulse becomes great and the respiration less, but in 

 children the pulse is rapid when respiration is slow. 

 Likewise in fear, trouble, or worry, in many fevers, 

 of course, the pulse is very fast, the respiration 

 slower than usual. 



^ As in a peristaltic wave. The elasticity of blood-vessel walls, 

 depending on their elastic fibers and smooth muscle tunics, is now 

 recognized as a factor in maintaining blood pressure. Harvey demon- 

 strated that the pulsation in the arteries is due to their sudden disten- 

 tion by blood forced out of the heart during its contraction. Andreas 

 Caesalpinus (i 519-1603) argued this point in his ^uaestiones Peri- 

 pateticae (Lib. V,Quaest. 4) in 1571, but without apparently impressing 

 any of the anatomical or medical investigators of his or the next century 



[IS] 



