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TEXT-BOOK OF PHYSIOLOGY. 



portional to the total area of each cross-section until at E, when it will attain the 

 value it had in A if the entrance and exit tubes have the same area. 



The lateral pressure will gradually fall from the beginning to the end of the sys- 

 tem, though the fall must be more rapid in B-C than in A-B as will be clear from 

 the following considerations. 



In the section B-C the two factors viz., the widening of the stream bed which 

 decreases the resistance, and the narrowing of the individual tubes which increases 

 the resistance exert an opposing influence on the pressure; hence the fall of 

 pressure will be proportional to the ratio between these two factors. As the 

 increase in the resistance due to the progressive decrease in the size of the individual 

 tubes preponderates considerably over the decrease in the resistance due to the 

 widening of the stream bed, there must be an increase in resistance in the area 

 B-C and therefore a more rapid fall of pressure than in A-B. This fall, however, 

 will not be as steep as it might be for the reason that the decrease in the velocity 

 is attended by a decrease in the resistance and hence a lessened consumption of 

 the propelling power. In the section C-D the two factors, viz., the narrowing of 

 the stream bed which increases the resistance, and the enlarging of the individual 



D E 



FIG. 151.- 



-PRESSURE VESSEL WITH A SERIES OF PROGRESSIVELY BRANCHING AND REUNITING 



TUBES. 



tubes which decreases the resistance, exert an opposing influence on the pressure, 

 hence the fall of pressure will be proportional to the ratio between these two factors. 

 As the decrease in the resistance due to the progressive enlargement of the individual 

 tubes preponderates considerably over the increase in the resistance due to the 

 narrowing of the stream bed, there should theoretically be a rapid fall of pressure 

 from c to E. This rapid fall, however, will be to some extent prevented for the 

 reason that the increase in velocity due to the narrowing of the stream bed in- 

 creases the resistance to a high value and hence the pressure falls less rapidly than 

 it otherwise would. 



