CHAP, iv.] THE VASCULAR MECHANISM. 287 



place in the ventricle, the systole for instance not being equally 

 sustained. 



We may further call once to mind the fact to which we have 

 already called attention that, while sometimes the curve of ven- 

 tricular pressure reaches its maximum at the very beginning of 

 the systole, declining more or less slowly afterwards, at other times 

 the maximum is reached at the end of the systole, the curve of 

 pressure being anacrotic; we may add that the maximum may 

 also occur at any intermediate point. Further, and this is the 

 matter to which we wish to call attention, the curve of aortic 

 pressure follows that of the ventricular pressure, both being kata- 

 crotic or anacrotic as the case may be. As we have urged, the 

 anacrotic curve is seen when the peripheral resistance is such that, 

 for some time during the systole, the flow from the aorta towards 

 the periphery is slower than the flow from the ventricle into the 

 aorta. Such a condition is apt to be met with when the arteries 

 are more rigid than normal, and under these circumstances the 

 anacrotic characters of the pulse may become prominent. 



149. 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 vaso-motor nerves, of which we shall presently speak, to a widen- 

 ing of the small arteries of the gland. When the gland is at rest, 

 the minute arteries are, as we shall see, somewhat constricted and 

 narrowed, and thus contribute largely to the peripheral resistance 

 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 destroyed. 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 consequence of this the elasti- 

 city 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 

 ' peripheral ' tubing is kept open, not enough elasticity is brought 

 into play to convert the intermittent arterial flow into a con- 

 tinuous one, and the pulse which reaches the arteries of the gland 

 passes on through them and through the capillaries, and is con- 

 tinued on into the veins. A similar venous pulse is also some- 

 times seen in other organs. 



Careful tracings of the great veins in the neighbourhood of the 

 heart shew elevations and depressions, which appear due to the 

 variations of endocardiac pressure, and which may perhaps be 



