PHYSIOLOGY OF THE CIRCULATION. 79 



in a very rapid ratio on the application of any im- 

 pediment to the free discharge of fluid from a tube, 

 so it is rendered probable that the outward or dis- 

 tending pressure of the arterial blood is very con- 

 siderable. 



A question here arises as to the proportion which 

 this lateral pressure of the arterial blood bears to the 

 impelling power of the heart. The data which we 

 possess may perhaps be deemed insufficient to war- 

 rant a positive answer, but, for the following reasons, 

 I should imagine that these forces must generally 

 be very nearly equal. The onward impulse of the 

 arterial blood is supposed, from experiments per- 

 formed on animals with the hasmadynamometer, to 

 be equal, in man, to a column of the same fluid eight 

 feet high. Poiseuille has, moreover, found this 

 impulse to be nearly equal in the chief arterial 

 branches. Now it follows, from the arborescent ar- 

 rangement of this system of tubes, that the force of 

 the blood entering the carotid artery must be nearly 

 the measure of the lateral pressure of the aortic blood; 

 and as the impulse of the blood entering the femoral 

 artery, which, in direction, is continuous with the 

 aorta, is no greater than that of the blood driven 

 into the carotid, which is a lateral branch, it would 

 appear that the mass of aortic blood presses equally 

 in all directions, its lateral being equal to its onward 

 pressure.* And, from the facts and arguments 



* An experiment with the hasmadynarnometer, on the renal artery, 

 which arises from the aorta at a right angle, would remove all doubt 

 on this point. 



