542 



OF THE CIRCULATION OF BLOOD. 



a. 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 

 geometrical 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 com- 

 pare 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 re- 

 sult 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. 



SQ.UAHE. 



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 S4UARES. 



81.25 



52 



13 



50 

 290.25 



102 

 21.25 



65 



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

 stated that the unit, in the above measurements, is no more than one-fortieth of an inch ; 

 and when it is remembered that any error in the measurement is greatly increased in the 

 calculation. 



b. From Mr. Paget's observations, however, it appears that there is seldom an exact 

 equality between the area of the trunk and that of its branches, but the area sometimes in- 

 creases, and sometimes diminishes; the former being the general rule for the subdivision 

 of the aorta and its principal branches in the upper extremities; the latter in the lower. 

 The following Table shows the relative areas of several arterial trunks, and of the branches 

 proceeding from them. 



TRUNK. BRANCHES. 



Arch of Aorta 1 1-055 



Innominate. ....... 1 1-147 



Common carotid ...... 1 1-013 



External carotid 1 1-190 



Subclavian ....... 1 1-055 



Abdominal Aorta, to last lumbar art. . . 1 1-183 



, just before dividing . . 1 -893 



Common Iliac 1 -982 



External Iliac 1 1-150 



711. That the movement of the Blood through the Arterial trunks and the 

 Capillary tubes is, in Man, and in other warm-blooded animals, chiefly de- 

 pendent upon the action of the Heart there can be no doubt whatever. It can 

 be easily shown by experiment, that, if the Arterial current be checked, the 

 Capillaries will immediately cease almost entirely to deliver the blood into 

 the veins, and the Venous circulation will be instantaneously arrested. And 

 it has also been proved, that the usual force of the Heart is sufficient to propel 

 the blood, not only through the Arterial tubes, but through the Capillaries, 

 into the Veins; since even a less force will serve to propel warm water through 

 the vessels of an animal recently dead.t But there are certain "residual phe- 

 nomena" even in Man, which clearly indicate that this is not the whole truth : 

 and that forces existing in the Blood-vessels have a considerable influence, in 

 producing both local and general modifications of the effects of the Heart's 

 action. There are also indications of the nature of an influence, in which the 

 blood-vessels do not partake, arising from those changes occurring between 

 the Blood and the Tissues, that constitue the processes of Nutrition, Secretion, 



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



I See Dr. Williams' Principles of Medicine, p. 143, note. 



