190 THE VASCULAR MECHANISM. 



is still possible, however, even on this view, as on the other, to suppose that 

 an antecedent reflux, due to the negative pressure succeeding the cessation 

 of flow from the ventricle, closes the valves and starts the pre-dicrotic wave. 

 But the matter is one not yet beyond the stage of controversy. 



137. In an anacrotic pulse the first rise is not the highest, but a 

 second rise (B, Fig. 69) which follows and is separated from it by a notch 

 is higher than, or at least as high as, itself. Such an anacrotic wave, though 

 it may sometimes be produced temporarily in healthy persons, is generally 

 associated with diseased conditions, usually such in which the arteries are 

 abnormally rigid. In describing the ventricular systole, we spoke of the 

 pressure within the ventricle as reaching its maximum just before the 

 opening of the semilunar valves ; and this is apparently the normal event ; 

 but there are curves which seem to show that after the first sudden rise of 

 pressure which opens the valves, followed by a brief lessening of pressure, 

 which appears on the curve as a notch, the pressure may again rise, and 

 that to a point higher than before. And a similar curve is sometimes de- 

 scribed by the front-to-back diameter of the ventricle. The systole opens 

 the valve as it were with a burst ; this is followed by a slight relapse, 

 and then the systole, strengthening again, discharges the whole of the ven- 

 tricular contents into the aorta and so brings about a tardy maximum ex- 

 pansion. And what is thus started in the aorta travels onward over the 

 arterial system. It is difficult to see how these anacrotic events can be pro- 

 duced, except by a certain irregularity in the ventricular systole ; and, 

 indeed, the anacrotic pulse is frequently associated with some disease or 

 defect of the ventricle. 



138. Venous pulse. Under certain circumstances the pulse may be 

 carried on from the arteries through the capillaries into the veins. Thus, 

 as we shall see later on, when the salivary gland is actively secreting, the 

 blood may issue from the gland through the veins in a rapid pulsating 

 stream. The nervous events which give rise to the secretion of saliva, lead 

 at the same time, by the agency of vasomotor nerves, of which we will pre- 

 sently speak, to a dilatation of the small arteries of the gland. When 

 the gland is at rest the minute arteries are, as we shall see, somewhat con- 

 stricted and narrowed, and thus contribute largely to the peripheral resist- 

 ance in the part ; this peripheral resistance throws into action the elastic 

 properties of the small arteries leading to the gland, and the remnant of 

 the pulse reaching these arteries is, as we before explained, finally de- 

 stroyed. When the minute arteries are dilated, their widened channels 

 allow the blood to flow more easily through them and with less friction ; the 

 peripheral resistance which they normally offer is thus lessened. In conse- 

 quence of this the elasticity of the walls of the small arteries is brought 

 into play to a less extent than before, and these small arteries cease to do 

 their share in destroying the pulse which comes down to them from the 

 larger arteries. As in the case of the artificial model, where the " periph- 

 eral " tubing is kept open, not enough elasticity is brought into play to 

 convert the intermittent arterial flow into a continuous one, and the pulse 

 which reaches the arteries of the gland passes on through them and through 

 the capillaries, arid is continued on into the veins. A similar venous pulse 

 is also sometimes seen in other organs. 



Careful tracings of the great veins in the neighborhood of the heart 

 show elevations and depressions, which appear due to the variations of intra- 

 cardiac (auricular) pressure, and which may, perhaps, be spoken of as 

 constituting a " venous pulse," though they have a quite different origin 

 from the venous pulse just described in the salivary gland ; but at present 

 they need further elucidation. In cases, however', of insufficiency of the 



