182 PHYSIOLOGY FOR DENTAL STUDENTS. 



first elevation is very rapid and abrupt (Fig. 24, a). This is 

 caused by the sudden increase in pressure of the blood, due to 

 cardiac systole, resulting in the sudden expansion of the artery. 

 Following the abrupt rise, the curve gradually descends till the 

 next heart beat occurs. During this period the arterial blood 

 pressure is maintained by the elastic recoil of the stretched 

 arteries. On the descending curve there are as a rule several small 

 waves and depressions. Of these waves the large one (Fig. 

 24, b) is always present and is known as the dicrotic wave, and 

 the dicrotic notch is the depression immediately preceding the 

 wave. The presence of this wave is explained as follows: At 

 the end of cardiac systole the blood, under the influence of the 

 pressure exerted by the stretched walls of the arteries, is forced 

 both towards the peripheral vessels and back towards the heart. 



Fig. 24. Pulse tracing made by sphygmograph. A, systolic wave; B, 

 dicrotic wave. 



The cardiac semilunar valves, being tightly closed at the end 

 of systole, arrest the back flowing blood, and it rebounds, as it 

 were, producing the depression with the wave (a) which is re- 

 flected over the entire circulation. When the blood pressure is 

 high, the secondary waves make very little depression, because of 

 their relatively low pressure, but in conditions where the blood 

 pressure is low, as in typhoid fever, surgical shock, a faint, and 

 in deep anesthesia, the dicrotic wave is easily felt by the finger. 



Other qualities of the pulse which may assist the physician in 

 judging of the condition of the circulatory system are its rate, 

 and its compressibility. Its rate tells us how fast the heart is 

 beating, and its compressibility gives a rough idea of the blood 

 pressure. 



The Circulation Through the Lungs. In general the same 

 conditions are present in the circulation of the blood through 



