3 2o TEXT-BOOK OF PHYSIOLOGY. 



The origin of this second expansion has been the subject of much 

 investigation, and at present it may be said that the question is not 

 fully decided. It is asserted by some investigators that it is central 

 in origin, beginning at the base of the aorta and passing to the pe- 

 riphery; by others, that it is peripheral in origin, beginning near the 

 capillary region and reflected to the heart. The former view is the 

 one more generally accepted. According to it, the expansion is the 

 result of the sudden closure of the aortic valves, a backward surge of 

 the blood-column against them. The sudden arrest of the blood 

 and its accumulations again expands the aorta. 



/ The dicrotic notch is therefore taken as the moment at which the 

 ventricular systole ceases and the aortic valves close. --- From this fact 

 it is evident that immediately after the first expansion the pressure 

 begins to fall, even though the ventricular systole continues, owing 

 to the discharge of blood from the arterial into the capillary and 



FlG ' J53 Mosso's PLETHYSMOGRAPH. G. Glass vessel for holding a limb. F 

 Plask for varying the water-pressure in G. T. Recording apparatus. (Landois 

 and Stirling.) 



venous systems. The height of the dicrotic wave or the depth of 

 the dicrotic notch is favored by low arterial tension and highly elastic 

 arteries. Both features are diminished by the reverse conditions. 

 The apex is sometimes rounded and even flat, indicative of a great 

 diminution in arterial elasticity. The sphygmogram not infrequently 

 varies ^ considerably from the normal type in different pathologic 

 conditions of the circulatory apparatus. A consideration of these 

 vanations does not fall within the scope of this work 



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 



