374 TEXT-BOOK OF PHYSIOLOGY 



The first positive wave, a, is due to an expansion of the vein, the result 

 of a sudden rise of pressure. As it occurs before the ventricular systole, it 

 is pre-systolic in time and caused by the contraction of the auricle, the 

 effect of which is to cause a temporary retardation of the blood-stream 

 flowing toward the auricle and hence a backward wave of pressure. 



The^r.^ negative wave is due to a recoil of the veins following a diminu- 

 tion of the pressure as the blood again moves forward in consequence of the 

 relaxation of the auricular walls. 



The second positive wave, c or s, is also caused by a wave of positive pres- 

 sure in the vein, reflected from the auricle, though it is not of auricular origin. 

 As it begins with the ventricular contraction and develops during the closed 

 period, the protosystolic period, i.e., between the closure of the tricuspid 

 valve and the opening of the semilunar valves (see page 287), it is believed 

 to be due to the bulging of the auriculo-ventricular valve into the auricular 

 cavity, by the still higher intra-ventricular pressure thus diminishing its size 

 and raising its pressure. 



The second negative wave, Af, is due to a marked fall of pressure, a col- 

 lapse of the walls of the vein and a rapid flow of blood to the auricular cavity. 

 These phenomena begin with the opening of the semilunar valves and are 

 due in part to the relaxation of the auricular walls, but more especially to a 

 descent of the more central portions of the auriculo-ventricular valve or 

 septum, into the ventricular cavity in consequence of the contraction of the 

 papillary muscles. The hollow cone thus formed, enlarges the auricular 

 cavity, withdraws some of its contained blood, and hence lowers the pressure, 

 which leads to the inflow of blood from the veins and hastens the auricular 

 filling. 



The third positive wave, v, is caused by a third wave of pressure reflected 

 from the auricle. It occurs toward the end of the ventricular systole and 

 is probably due to a slight retardation of the blood flow in consequence of 

 the return of the auriculo-ventricular septum to its normal position, the 

 result of a relaxation of the papillary muscles, when the intra-ventricular 

 pressure is still higher than the intra -auricular pressure. 



The third negative wave, Vf, is caused by a third fall of pressure in the 

 vein and appears very shortly after the beginning of the ventricular relaxa- 

 tion, and the closure of the semilunar valves. It develops during the common 

 pause of auricles and ventricles. The fall of the venous pressure follows 

 the passage of the blood from the auricle into the ventricle. It continues 

 during the ventricular filling but disappears on the return of the auricular 

 contraction. 



The Volume Pulse. If an individual artery expands with each systole 

 and recoils with each diastole of the heart, the same is true of all arteries, 

 and as a result the volume of any organ or part of the body must undergo 

 similar changes. To such alternate changes in volume the term volume 

 pulse is given. The extent to which an organ will increase in volume will 

 depend to some extent on its elasticity. The reason for the increase in 

 volume is the resistance offered to the flow of blood into and through the 

 capillaries; the decrease in volume to the overcoming of the resistance through 

 the arterial recoil. 



The variations in volume may be recorded by enclosing the organ in a 

 rigid glass or metal vessel, which at one point is in communication with a 



