PROOF OF CONTRACTION OF ARTERIOLES. 143 



column at each pulsation diminished in extent (fig. 100). This 

 rise could be due either to the heart propelling a greater quan- 

 tity of blood into the aorta at each pulsation, or to the arteries, 

 contracting so as to hinder it from escaping from the arterial 

 into the venous system. The diminished height of the pulse- 

 wave seems sufficient of itself to negative the former idea and 

 to show that the increased pressure can only be due to con- 

 traction of the arterioles, but we think that a still clearer proof 

 is afforded by the form of the wave. The time occupied in the 

 ascent of the pressure-wave (indicated by the horizontal dis- 

 tance between the lowest and highest parts of the ascending 

 limb) is nearly the same in figs. 98 and 100, but the descending- 

 limb of the latter sinks very gradually indeed, while in the 



Fig. 100. 



former it falls almost as quickly as it rises. What then is the 

 explanation of this phenomenon ? During the diastole of the 

 heart, the sigmoid valves when healthy, as they were in this 

 case, completely close the cardiac end of the aorta. The whole 

 arterial system may then be compared to an elongated elastic 

 vessel, from which fluid is issuing b}^ a narrow opening. The 

 greater the pressure of fluid in the vessel the more rapidly will 

 it escape by the opening, the more quickly will the pressure 

 consequently fall, and the more abrupt will be the descent of 

 the pressure-curve. I^ow, the mean blood-pressure in the 

 normal tracing is somewhat over 70 millimetres,* and the maxi- 



* Tlie true heights are of course nearly double these, but for conyenient 

 comparison with the tracings we have taken the numbers as they stand in the 



