CIRCULATION, RESPIRATION AND EXCRETION 129 



corresponding to the strokes of the pump. By the addition of 

 a nozzle of smaller diameter than the hose this intermittent 

 outflow is converted into a steady stream. The resistance of 

 the nozzle to the passage of the water gives rise to a pressure 

 which stretches the walls of the hose, and their elastic force 

 maintains the flow between the strokes of the pump. 



Substantially the same conditions exist in the body. The 

 walls of the arteries are elastic while the capillaries in which 

 the arteries terminate may be compared to the nozzle of the hose. 

 The resistance caused to the flow of the blood by these minute 

 channels tends to hold it back and produces a pressure in the 

 arteries which, like the pressure in the hose, causes a steady 

 movement of blood through the capillaries. In other words, 

 the immediate cause of the motion of the blood through the 

 capillaries is the elasticity of the arterial walls. If the latter 

 become weakened and lose their tone or become hardened as in 

 old age (arteriosclerosis), the driving force is lessened and the 

 circulation slows down, since the veins can return blood to the 

 heart only as fast as it is forced through the capillaries by the 

 arterial pressure. The blood pressure in the arteries, therefore, 

 is an important indicator of the activity of the circulatory sys- 

 tem. The veins serve substantially as a return system, the 

 blood being pushed along them by the residual pressure from the 

 capillaries, perhaps aided somewhat by the expansion of the 

 auricle of the heart, while valves prevent any backward flow. 

 As compared with the arterial pressure, therefore, the blood 

 pressure in the veins is low. 



185. The lymph. The body cells are not closely packed to- 

 gether but are imbedded more or less loosely in connective tis- 

 sue (83) leaving spaces between them (intercellular spaces). 

 These spaces contain a colorless transparent fluid called the 

 lymph which is the real nutritive medium in which the cells 

 live. From it, by means of osmosis through their outer mem-- 

 branes and perhaps in other ways, the cells derive the substances 

 required for their vital activities and into it they discharge 

 the waste products of their action. 



The lymph in its turn stands in relation to the blood, from 

 which it is separated by the thin walls of the capillaries. While 

 the minute capillaries penetrate all the tissues and convey blood 

 to all parts of the body, it should be understood that the cir- 



