330 CIRCULATION OF NUTRITIVE FLUID. 



it cannot be vis a tergo of the Heart, that occasions the enlargement of 

 certain arterial trunks, and of no others ; since any increase in its pro- 

 pulsive power would affect all alike. It must be, therefore, through 

 a power inherent in themselves, that the dilatation takes place ; and 

 there seems much reason for attributing to the Sympathetic system of 

 nerves a control over this power, and consequently the office of regu- 

 lating the local distribution of blood, in accordance with the wants of 

 the different parts. It is well known that the nerves of this system are 

 copiously distributed upon the arterial walls ; and it has been experi- 

 mentally shown, that they have the power of producing contractions in 

 the larger arteries. Moreover, there is every reason to believe that 

 the diameter of the Capillary blood-vessels, and the rate of the move- 

 ment of the blood through them is much influenced by these nerves 

 ( 603) ; and it seems highly probable, therefore, that they should have 

 a corresponding influence upon the size of the trunks, from which these 

 capillaries are derived. 



587. The Arterial system possesses nearly the same relative capacity 

 in every part ; that is, if a section could be made through all the sys- 

 temic arteries at a certain distance from the heart, the united areas 

 would be found equal to that of the aorta ; and those of the branches 

 of the pulmonary arteries would equal those of their trunk. This re- 

 sults from the fact, that, at every subdivision, the united areas of the 

 branches are almost precisely equal to that of the trunks from which 

 they proceed ; although the united diameters of the former far exceed 

 that of the latter. According to a well-known mathematical law, the 

 areas of circles are as the squares of the diameters ; consequently, in 

 making such comparisons, it is necessary to square the diameters of the 

 trunk and those of the branches, and to contrast the former with the 

 sum of the latter. Thus a trunk whose diameter is 7, may subdivide 

 into two branches, each having a diameter of nearly 5 ; for the square 

 of 7 is 49, and twice the square of 5 is 50. Or a trunk whose diameter 

 is 17 may subdivide into three branches whose diameters are 10, 10, 

 and 9 J (making 29 J as the sum of the diameters] ; for the square of the 

 diameter of the trunk is 289, whilst the sum of the squares of those of 

 the branches is 290J. It appears, however, from Mr. Paget's recent 

 admeasurements, that there is seldom an exact equality between the 

 area of the trunk and that of its branches ; the area sometimes increas- 

 ing, and sometimes diminishing. The former seems the general rule 

 in the upper extremities ; the latter in the lower. Thus the area of the 

 trunk of the external carotid is to that of its branches, as 100 to 119 ; 

 whilst the area of the abdominal aorta, just before its final division, 

 is to that of its branches as 100 to 89. 



588. In almost every part of their course, the ramifications of the 

 arteries communicate freely with each other, by anastomosis ; and this 

 communication is most important, as affording the means by which the 

 circulation is sustained, when the current through the main trunk is ob- 

 structed. There is scarcely an artery in the body, except the aorta, 

 which may not be tied, with the certainty that the blood will still be 

 conducted to its destination, by the collateral circulation. At first, 

 the quantity which thus passes is very insignificant, and is by no means 



