288 



CIRCULATION 



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 =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 LI. 

 THICKNESS OF WALLS AND DIAMETER OF LUMEN OF ARTERIES IN MM. 





(MacWilliain 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 R/R^/l and p/p l =3/2. 

 Now tlt 1 =RIR l and as p/p 1 =6/2=3/l. 



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 witli 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 approxima- 

 tion to the ratio 3 to 1 . 



The valves of the heart and veins are interesting mechanical 

 structures. During the two 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 

 vessels at the root of the neck. These sluice-gates are very 

 simple contrivances just little pockets set in pairs opposite each 

 other in the vein. 'Fabricius noted that the openings of the pockets 

 were always directed toward the central part of the body. He 

 interpreted this as indicating a mechanism to prevent the blood 



