198 



CIRCULATION OF THE BLOOD 



difference is much less than when male and female individuals of the same 

 age, without regard to size, are compared. 



Again we meet with considerable variation of pulse rate in different indi- 

 viduals, a very low (26-20 per minute) and a very high rate (120 per min- 

 ute) having been observed in men of perfectly sound health. 



THIED SECTION 



THE BLOOD FLOW 



1. THE FLOW OF A LIQUID IN RIGID TUBES 



If a reservoir be so arranged as to deliver a liquid through a tube of uni- 

 form diameter as represented in Fig. 72, the mean velocity of the flow will be 

 the same in every cross section of the tube. This follows from the fact that 

 fluids are not compressible. Moreover, the internal friction of the fluid creates 

 a resistance which causes it to flow more slowly than it would if it were pouring 

 directly from an opening in the reservoir; and in consequence of this friction 

 the liquid in the tube is subjected to a tension, which, however, is smaller than 

 it would be if the flow from the end of the tube were hindered iir some way. 

 This tension manifests itself as a lateral pressure which can be measured by 

 means of vertical tubes leading off at intervals along the delivery tube. If the 

 highest points of the columns of liquid be connected with each other, a straight 

 line is obtained i. e., the lateral pressure decreases uniformly in the direction 

 of the current, to the end, where it is nil. The lateral pressure at any given 

 point along the delivery tube is equal to that part of the whole pressure which 





1 <t 3 * J 



FIG. 72. The flow of a liquid through a rigid tube of uniform diameter. 



is necessary to overcome the resistance of the current from that point onward. 

 If the system through which the liquid flows consist of a number of tubes 

 of different diameter fastened together, as in Fig. 73, the same fundamental 

 laws hold good. Because the liquid is not compressible the same quantity must 

 flow through every cross section of the tube, whatever its size, in unit time. 

 Consequently the velocity in the different sections of the tube stands in inverse 

 relation to their cross section. The lateral pressure within the different sections 

 falls at different rates most rapidly in the narrowest, most slowly in the widest. 

 In sections of the same diameter, whether they are separated by narrow or wide 



