234 



PHYSIOLOGY 



CHAP. 



the columns of fluid in all the piezometers can be joined by a 

 straight line. This straight line, which represents the gradual 

 tall of lateral pressure along the tube, is more or less steep in 

 proportion to the velocity of outflow. 



As shown in the diagram, the total force represented by the 

 height H of the column of fluid contained in the vessel falls into 

 two parts : that employed in overcoming the resistance offered to 

 the free passage of the fluid through the tube li ', and that employed 

 in driving the fluid through the tube h. The first is termed lateral 

 pressure or resistance-head, the second velocity -head. 



(c) When the tube connected with the vessel varies in diameter 



FIG. 88. Schema to demonstrate the laws which regulate the flow of a liquid, at constant pressure, 

 through a conducting tube with rigid walls and uniform diameter. 



in its different parts, the same fundamental law applies as has been 

 laid down for tubes of constant bore. Since fluid is incompres- 

 sible, an equal amount must flow through every section of the tube, 

 independent of its diameter in the time-unit. 



In consequence of this law, the velocity of the current in 

 sections of tubes that vary in diameter stands in inverse pro- 

 portion to the sectional area. 



Since the resistance in the wider parts of the tube is less than 

 in the narrower sections, it follows that the fluid requires less force 

 to propel it through the former than it does to pass through the 

 latter. The lateral pressure accordingly sinks more slowly in the 

 former and more rapidly in the latter, as shown in Fig. 89. At 

 points where a wider section of the tube passes into a narrower, 

 velocity rises, and there is considerable diminution of pressure, 

 owing to the greater resistance which the fluid encounters ; where, 

 on the contrary, a wider section follows a narrower, velocity falls, 

 while pressure on the contrary increases or remains unchanged, or 



