CIRCULATION OF THE BLOOD. 



521 



Fig. 216.— Diagram of Variations in 

 Pressure in Branching Tubes. 

 ( Wunclt.) 



The changes in pressure in the tube A, B, C, D are 

 represented by the broken line A, b, c, d, e. 



ment of the circulatory system of animals, where a vessel divides into 

 several branches of greater total calibre than the parent stem, and where 

 after repeated subdivision, the branches again unite to form a single tube 

 whose calibre is about the same as that of the original tube. 



A simple representation of such a series of branching tubes is given 

 in Fig. 216. 



In such a series the pressures are seen in the broken line a,b,c, d, e. 

 At B, taking into consideration only the increase in calibre, a sudden 

 increase in pressure occurs. On the 

 other hand, considering only the occur- 

 rence of bends and angles in the tube, 

 the pressure would suddenly sink. 

 These two causes, however, oppose 

 each other, and the most ordinary rep- 

 resentation of the case would be 

 indicated by a slower sinking in the 

 line of pressure than in the single 

 parent stem. The condition is, how- 

 ever, different where the branches again unite to form a single trunk; 

 here the pressure must fall, because the bed of the stream becomes 

 contracted, while at the same time an angle is also met with. Both 

 these facts, therefore, work in the same direction, and the pressure 

 undergoes a sudden fall which would be greater than that produced 

 by mere contraction of the stem. 



It follows from the above that in a symmetrical system of tubes the 

 pressure does not symmetrically increase and decrease, but will be greater 

 in any portion in the centre of the system of the tubes (at M, Fig. 216) 

 than the mean of pressure at any 

 two points equally distant in front 

 or behind this point. It may, there- 

 fore, happen that in a complicated 

 system of branching tubes the re- 

 sistance is not greater than in a single 

 tube, or may even be smaller, since 

 the increase in the diameter may 



diminish the resistance more than the branching increases it. If the 

 resistance is the same, it is evident, also, that the rapidity is the same 

 in both cases, and as a consequence more fluid will flow out of such a 

 branching system of tubes when the resistance is smaller than would 

 escape from a single tube (Fig- 211). 



The angle formed by the branches with the original stem seems to 



produce no marked influence on the resistance and velocity of movement. 



The above relationship between resistance and velocity of movement 



Fig. 217.— Diagram of System of Branch- 

 ing Tubes. (Wundt.) 



