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TEXT-BOOK OF PHYSIOLOGY 



area of individual tubes in the section B c, while in the section c D, there must 

 follow a progressive decrease in the total sectional area of the collective tubes coin- 

 cident with a progressive increase in the sectional area of individual tubes, conse- 

 quently there will be a combination of the two conditions alluded to in the two 

 preceding paragraphs, namely, an enlargement of the stream bed coincident with 

 a diminution in size of the individual tubes composing it, in the middle section. 

 Moreover, for the purpose here intended it may be assumed that the tubes com- 

 posing the middle section c are microscopic in size and that their total sectional 

 area bears to the sectional area of tube A the ratio of 600 to i . 



If the system is connected with a pressure vessel, as in the preceding instance 

 and the stopcock is suddenly opened, the column of water will exert a downward 



FIG. 149. PRESSURE VESSEL WITH A SERIES OF PROGRESSIVELY BRANCHING AND AGAIN UNITING 



TUBES. 



a, x, a series of stopcocks by which the peripheral resistance can be increased or decreased. 



pressure, and in consequence the water will be driven into and through the system 

 with a definite velocity and pressure. 



The velocity of the fluid will gradually decrease from B to c in a ratio inversely 

 proportional to the total area of each cross-section until at c, it will attain its mini- 

 mal value; the velocity will again increase from c to D in a ratio inversely pro- 

 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 



