174 THE PULSE. [BOOK i. 



normal conditions, as we have already seen, the maximum expansion 

 is soon reached, but in cases where the arterial walls are unusually 

 rigid and the heart at the same time not abnormally weak, the 

 ventricle may continue to empty itself against a resistance which 

 increases rapidly with the amount of blood passing into the aorta, 

 so that in spite of the diminishing rapidity with which the blood 

 is leaving the ventricle the insufficient distensibility of the vessels 

 causes the pressure in their interior to continue to rise until nearly 

 the end of the outflow from the heart. An anacrotic pulse also 

 frequently accompanies hypertrophy and dilation of the left 

 ventricle. 



The pulse then is the expression of two sets of conditions : one 

 pertaining to the heart, and the other to the arterial system. The 

 arterial conditions remaining the same, the characters of the pulse 

 may be modified by changes taking place in the beat of the heart ; 

 and again, the beat of the heart remaining the same, the pulse may 

 be modified by changes taking place in the arterial walls. Hence 

 the diagnostic use of the pulse-characters. It must however be 

 remembered that arterial changes may be accompanied by com- 

 pensating cardiac changes, to such an extent, that the same 

 features of the pulse may obtain under totally diverse conditions, 

 provided that these conditions affect both factors in compensating 

 directions. 



Venous Pulse. Under certain circumstances the pulse may be 

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

 Thus 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 dilation of the small arteries of the gland. 

 This dilation of the small arteries diminishes the peripheral resist- 

 ance by allowing more blood to pass through them with less friction; 

 in consequence the elasticity of the arterial walls is brought into play 

 to a less extent than before, and this may in certain cases go so 

 far, that as in the case of the artificial apparatus, where the elastic 

 tubing has an open end (see p. 129), not enough elasticity is brought 

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

 tinuous one. A similar venous pulse is also sometimes 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 intracardiac pressure, and which may perhaps be 

 spoken of as constituting a "venous pulse"; but at present they 

 need further elucidation. In cases of insufficiency of the tricuspid 

 valves, the systole of the ventricle makes itself felt in the great 

 veins; and a distension travelling backwards from the heart be- 



