358 TEXT-BOOK OF PHYSIOLOGY. 



viz., a line of ascent from a to b, and a line of descent from b to d (Fig. 170). 

 In normal tracings the former is almost vertical and caused by the sudden 

 expansion of the artery immediately following the ventricular contraction; 

 the latter is in general oblique, due to the recoil of the arterial walls, occupies 

 a longer period of time, and is marked by several elevations and depressions, 

 both of which indicate that the restoration to equilibrium is neither immediate 

 nor uncomplicated. One of these elevations is quite constant and known as 

 the dicrotic wave, c; the depression or notch just preceding it is known as the 

 dicrotic notch. Pre- and post-dicrotic waves are not infrequently present. 

 The summit is generally sharp and pointed. 



The vertical direction of the line of ascent is taken as an indication that 

 the arterial walls expand readily, that the blood is discharged quickly, and 

 that the ventricular action is not impeded. An oblique direction of the 

 line of ascent is an indication that the reverse conditions obtain. The height 

 varies inversely as the arterial pressure, other things being equal; being 

 high with a low pressure, and low with a high pressure. 



The dicrotic elevation shows that a second expansion wave is developed 

 which interrupts temporarily the recoil of the arterial walls. 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 periphery; 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, and 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 ven- 

 tricular systole ceases and the aortic valves close. From this fact it is evi- 

 dent 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 venous systems. The height of the 

 dicrotic wave or the depth of the dicrotic notch is increased by low arterial 

 pressure and highly elastic arteries. Both features are diminished by the 

 reverse conditions. The apex is sometimes rounded and even flat, indica- 

 tive 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 varia- 

 tions does not fall within the scope of this work. 



The Carotid Pulse. The carotid pulse can be readily recorded by 

 applying over the carotid artery, anterior to the sternocleidomastoid muscle, 

 on a level with the thyroid cartilage, a funnel-shaped tambour in con- 

 nection with a suitable recording tambour and lever. The sphygmogram 

 thus obtained resembles in all essential respects that obtained from the 

 radial artery. It is often of advantage in the investigation of certain 

 problems of the heart, both physiologic and pathologic, to record the 

 carotid pulse and the cardiac impulse simultaneously. 



The Venous Pulse. By this term is meant a pulsation of the large veins 

 in the neighborhood of the heart but more especially in the jugular veins. 



