VALVES OF THE VEINS 



375 



The wall of the apex of the heart has the largest mean curvature 

 {R is least and, therefore, t is least). 



Similar reasonino- may l)e a})|)lie(l to the consideration of the 

 thickness of the walls of the blood vessels. The pressure (P) 

 within the vessel is balanced by (1) the elastic tension of the wall 

 (7^) divided by the radius of curvature (R), and (2) by the pressure 

 brought to bear on the external surface of the wall by the resist- 

 ance to distortion of the surrounding tissues (p). 



Thus T = R{P - p), 



or putting t = thickness and C = a constant, we may write 



t = CR{P -p). 



That is, if (P — p) be kept constant the thickness of the walls will 

 vary as the radius of curvature. 



TABLE LX 

 Thickness of Walls and Diameter of Lumen or Arteries in mm. 



(Mac William and Kesson.) 



According to measurements made on excised vessels the carotid 

 artery of the ox has a lumen of 6 mm. while that of the sheep 

 is 3 mm. The maximal pressure developed in these vessels at 

 body temperature amounted to 60 and 40 mm. Hg respectively. 



That is RIR^ = 2/1 and p/p^ = 3/2. 



Now t/t^ = RjR^ by pjp^ == 6/2 = 3/1. 



The actual thickness of the carotids as measured by Mac- 

 William and Kesson are 1-74 and 0-61 mm. respectively. Of 

 course, the main elastic resistance to distortion is met with in the 

 muscular tunica media, which in the ox is 1-12 mm. and in the 

 sheep 0-42 wide. Either pair of figures gives a close approximation 

 to the ratio 3 to 1 (Table LX.). 



Valves. 



The valves of the heart and veins are interesting mechanical 

 structures. During the t^^'o years that Harvey studied at the 

 University of Padua, Fabricius, the renowned Professor of Anatomy 

 there, was investigating the valves of the veins. He demonstrated 

 their presence in the veins of the arms and legs and also in the 



