144 THE HUMAN MECHANISM 



10. The driving force for the flow of blood from the aorta ; 

 pressure in arteries and veins. The hydraulic conditions in 

 the aorta may be illustrated by means of the following sim- 

 ple piece of apparatus : To an ordinary rubber syringe attach 

 a piece of elastic rubber tubing, the other end of which is 

 closed by a detachable nozzle. If now the nozzle be removed 

 and water pumped into the tube, it will be found that the 

 flow from the open end consists of squirts or spouts and 

 continues only during the stroke of the pump; if, however, 

 we attach the nozzle and again pump water into the .tube, 

 the resistance caused by the small orifice of the nozzle pre- 

 vents the water from flowing out of the tube as fast as the 

 syringe pumps it in. The tubing becomes distended with water. 

 Since, however, the tube is elastic, 1 and so tends to return 

 to its original size, it forces the liquid out through the 

 nozzle even between the strokes of the pump. The imme- 

 diate cause of the steady flow from the nozzle is therefore 

 the elastic squeeze of the rubber tube. The intermittent 

 stroke of the pump produces distention of the tube, and the 

 elasticity of the distended tube constantly forces the water 

 out of the nozzle. 



Closely similar conditions obtain in the arterial reservoir. 

 Here the outlet is also through very small tubes, the small 

 arteries, whose bore is not greater than -^ or yl ^ of an inch ; 

 which fact introduces the same condition as does the nozzle 

 of our apparatus, that is, a resistance to the outward flow of 

 the blood. Consequently the blood cannot flow out of the 

 aorta as rapidly as it is driven in, and the extensible and 

 elastic walls are necessarily stretched. The immediate effect 



1 An elastic body is one which returns to its original shape when it has 

 been stretched, compressed, or otherwise deformed. Elasticity must not be 

 confounded with " extensibility," or the property of allowing stretching. 

 Thus when we "pull" taffy we deal with a body which is very extensible 

 but which is practically inelastic. A body, indeed, may be extensible only 

 with difficulty, but possess a very high degree of elasticity ; ivory is a good 

 example of this kind. 



