m 



distribution of pressure there is a constant flow of blood 

 from arteries to veins. 



Many points in connection with the circulation may be 

 conveniently studied on a model made of indiarubber tubes 

 and a Higginson's syringe. (Practical Exercise.) 



A. Blood Pressure. 



The distribution of pressure is the cause of the flow of 

 blood, and must first be considered. 



1. General Distribution of Pressure. 



(See Fig. 125, p. 258.) 



The pressure in any part of a system of tubes depends 

 upon two factors 



1st. The force propelling fluid into that part of the 

 tubes. 



2nd. The resistance to the outflow of fluid from that part 

 of the tubes. 



The pressure in the arteries is high, because with each 

 beat of the heart about 80 grms. of blood are thrown with 

 the whole contractile force of each ventricle into the cor- 

 responding artery; and because the resistance offered to 

 the outflow of blood from the arteries into the capillaries 

 and veins is enormous. For, as the blood passes into 

 innumerable small vessels, it is subjected to greater and 

 greater friction just as a river in flowing from a deep 

 narrow channel on to a broad shallow bed is subjected to 

 greater friction. 



Thus in the arteries the powerful propulsive force of the 

 heart and the great resistance to outflow keep the pressure 

 high. 



When the capillaries are reached much of the force of 

 the heart has been lost in dilating the elastic coats of the 

 arteries, and thus the inflow into the capillaries is much 

 weaker than the inflow into the arteries. At the same 

 time the resistance to outflow is small, for in passing from 

 capillaries to veins the channel of the blood is becoming 



