THE BLOOD-CURRENT. 159 



valves is well marked in the three heart-beats at 20-20. This closure is 

 indicated in analogous points in both curves, after which there is a descent of the 

 foot-support, and this corresponds to the downward propagation of the pulse-wave 

 through the aorta to the vessels of the feet. 



In insufficiency of the aortic valves, as shown in Fig. 70, IV, the vibration com- 

 municated to the body is very considerable. 



81. The Blood- Current. 



The closed and much-branched vascular system, whose walls are 

 endowed with elasticity and contractility, is not only completely 

 filled with blood but it is over-filled. The total volume of the blood is 

 somewhat greater than the capacity of the entire vascular system. 

 Hence it follows that the mass of blood must exert pressure on the 

 walls of the entire system, thus causing a corresponding dilatation of 

 the elastic vascular walls (Brunner). This occurs only during life; 

 after death the muscles of the vessels relax, and fluid passes into the 

 tissues, so that the blood-vessels come to contain less fluid and some 

 of the vessels may be emptied. 



If the blood were uniformly distributed throughout the vascular 

 system and under the same pressure, it would remain in a position of 

 equilibrium (as after death). If, however, the pressure be raised in 

 one section of the tube the blood will move from the part where the 

 pressure is higher to where it is lower ; so that the blood-current is a 

 result of the difference of pressure within the vascular system. If either 

 the aorta or the venae cavse be suddenly ligatured in a living animal, 

 the blood continues to flow, gradually more slowly, until the difference 

 of pressure is equalised throughout the entire vascular system. 



The velocity of the current will be greater the greater the difference 

 of pressure, and the less the resistance opposed to the blood-stream. 



The difference of pressure which causes the current is produced by the 

 heart (E. H. Weber). Both in the systemic and pulmonary circulations 

 the point of highest pressure is in the root or beginning of the arterial 

 system, while the point of lowest pressure is in the terminal portion of 

 the venous orifices at the heart. Hence, the blood flows continually 

 from the arteries through the capillaries into the venous trunks. 



The heart keeps up the difference of pressure required to produce 

 this result ; with each systole of the ventricles a certain quantity of 

 blood is forced into the beginning of the arteries, while at the same 

 time an equal amount flows from the venous orifices into the auricles 

 during their diastole (E. H. Weber). 



Bonders added another important fact viz., that the action of the 

 heart not only causes the difference of pressure necessary to establish a 

 blood-current, but that it also raises the mean pressure within the vascular 



