vin BLOOD-STREAM: MOVEMENT IN VESSELS 237 



The validity of this law would be al isolate if the blood, which like 

 all fluids is incompressible, were circulating in a system of rigid 

 tubes. But since the walls of the vessels are extensible and elastic, 

 it is evident that there may be a temporary infraction of it without 

 disturbing the fundamental conditions of the circulation. The 

 mass of blood driven through the pulmonary arteries, e.g., may for a 

 few seconds be greater than that which simultaneously enters the 

 left auricle through the four pulmonary veins : this must produce 

 a certain degree of pulmonary congestion, compatible with life. 

 If, however, this condition be maintained too long, and if the 

 converse phenomenon does not immediately succeed it, so that 

 normal circulatory conditions are restored, the congestion in the 

 pulmonary vessels will obviously increase to such an extent in a 

 short time that it presents an invincible obstacle to the flow of 

 blood. Obviously, therefore, the above law has only a relative 

 value, when the time-unit is taken as an interval of a few seconds ; 

 it has an absolute value where a longer period, i.e. of one or more 

 minutes, is taken. 



(&) Law of Velocity. It follows as the necessary result of the 

 law of outflow that the velocity is inversely proportional to the 

 cross-section, in the different parts of the vascular system. In 

 order to determine the proportion in which velocity alters in the 

 different parts of the circulatory system it is enough to measure 

 the total sectional area of the vessels in that part. This is only 

 possible with the large arteries and veins nearest the heart, which 

 is the centre of the system : it is necessary, however, to measure 

 them not on the dead body, but on the living subject, under the 

 most normal circulatory conditions possible, so as to obtain the 

 sectional area under physiological tension and filling of the vessels. 

 Since the total sectional area or current bed increases slowly on the 

 one hand, from the large to the small arteries, and rapidly from 

 these to the capillaries ; and on the other hand, falls rapidly from the 

 capillaries to the small veins, and slowly from these to the large 

 veins, it may be stated in general terms that velocity alters in the 

 inverse sense and same proportion. Since, further, the sectional 

 area of the aorta is less than the sum of the sectional area of the 

 two venae cavae, while, 011 the other hand, the sectional area of the 

 pulmonary artery is larger than the sum of the sectional area of 

 the four pulmonary veins, the velocity of the blood-stream in the 

 systemic circulation will be maximal in the aorta, minimal in the 

 aortic capillaries, medium in the venae cavae ; while in the 

 pulmonary system it will be maximal in the pulmonary veins, 

 minimal in the capillaries, medium in the arteries. 



The difference in mean velocity of the flow in the vessels of the 

 pulmonary and aortic circulations can also be arrived at a priori, 

 starting from the fact that the capacity of the first system is to 

 that of the second as 2 : 11. It follows that the pulmonary circula- 



