chap. XXL] MECHANICS OF THE CIRCULATION. 223 



The blood-vessels form a system of branching 

 tubes of varying diameter. Beginning at one ex- 

 tremity in a large artery, the aorta, which gives off 

 branches at various angles, and these again other 

 branches, and so on, of constantly diminishing calibre, 

 the system passes into a series of remarkably small 

 vessels (the capillaries), which, in their turn, pass into 

 vessels now increasing in size, and uniting at various 

 angles to form the larger veins, which ultimately 

 end in two large vessels. Thus, to speak generally, 

 you have two series of wide vessels in communi- 

 cation through the medium of very small vessels. 

 The total calibre of the vessels increases from the 

 aorta to the capillaries, and again diminishes from 

 the capillaries to the great veins which open on the 

 right side of the heart. The force that circulates the 

 blood through this complex system of tubes is that 

 of the heart. To apply what has been noted of the 

 flow of fluid through such an arrangement of tubes, 

 the force exerted by the heart will be expended in 

 two directions, (1) to overcome resistance due to the 

 friction of the blood against the walls of the tubes (see 

 page 212), and (2) to produce a certain rate of flow. 

 Experiment proves that the laws applicable to fluid 

 flowing in tubes are equally applicable to the blood 

 flowing in the vessels. One of these laws is that the 

 pressure diminishes regularly from the source of force 

 onwards, and, in accordance with this law, it is found 

 that the pressure of the blood against the walls of the 

 vessels diminishes with the distance from the heart. 

 Since, however, we have here not tubes of uniform 

 diameter, but tubes of varying diameter, the pressure 

 will not diminish uniformly but irregularly. Thus, 

 owing to the resistance offered by the capillaries, the 

 pressure in the arteries diminishes slowly, but in the 

 capillaries themselves very fast, and again slowly in 

 the veins, which offer little resistance to the passage 



