OF THE CIRCULATION IN GENERAL. 



359 



extremity by the Heart ; and at their peripheral extremity by the Capillary 

 vessels, which are nothing else than the minutest ramifications of the two sys- 

 tems, inosculating into a plexus. The systemic arteries all proceed from one 

 trunk, the Aorta; which first ascends, and gives off -branches to the head and 

 superior extremities ; then descends through the thorax and abdomen, giving 

 off branches to the parts near which it passes ; and terminates in the two large 

 trunks that proceed to the inferior extremities. Although the diameters of 

 the branches, at each subdivision, together exceed that of the trunk, yet there 

 is but little real difference in their size. For, according to a well-known geo- 

 metrical law, the areas of circles are as the squares of their diameters; and, 

 as the calibre of a tube is estimated by its area, not by its diameter, it follows 

 that, in comparing the size of a trunk with that of its branches, we are to square 

 the diameter of the former, and compare the result with the sum of the squares 

 of the diameters of the branches. When this is done, there is found to be a 

 very close correspondence. The following table gives the result of eight 

 measurements, taken with a view to determine the question. The first three 

 were taken from the mesenteric artery of a Sheep ; the next three from the 

 aorta and iliac arteries ; the last two from the Horse.* 



TRUNK. 



Diameter. 

 I. 9 

 II. 7.2 

 III. 

 IV. 

 V. 

 VI. 

 VII. 

 VIII. 



3.5 

 7.0 

 17 

 10 

 4.5 

 8 



Square. 



81 



51.64 



12.25 



49 



289 



100 



20.25 



64 



BRANCHES. 



Diameters. 



7.5+5 



6+4 



3+2 



5 + 5 



10+10+9.5 



7+7+2 



3.5 + 3 



4+7 



Sum of Squares. 



81.25 



52 



13 



50 



290.25 



102 



21.25 



65 



The discrepancy between the two results must be considered extremely small, 



Fig. 88. 



Web of Frog's foot, stretching between two toes, magnified 3 diameters showing the blood-vessels and 

 their anastomoses; 1, 1, veins ; 2, 2, 2, arteries. (After Wagner.) 



* Ferneley, in Medical Gazette, Dec. 7, 1839. 



