124 



PBACTICAL PHYSIOLOGY 



and travels down the elastic arteries at the rate of about 5-8 metres 

 a second. The secondary waves are produced by the elastic vibra- 

 tions of the wall of the large arteries which result from their sudden 

 distension. The first secondary, or predicrotic wave is probably pro- 

 duced by the final contraction of the base of the heart, which wrings 

 the blood out of the ventricle. The second or dicrotic wave follows 

 the dicrotic notch. The dicrotic notch is synchronous with the tension 

 of the closed semilunar valves and the second sound of the heart. 



The dicrotic notch is caused by the back swing of the blood towards 

 the closed aorta valves, and the wave by the rebound. The size of the 



dicrotic wave depends on its relation 

 to that phase which the primary wave 

 happens to be in, as well as on the con- 

 ditions which increase the back swing 

 and rebound. A forcible beat and an 

 arterial system which can quickly empty 

 itself favour dicrotism. 



The sphygmograph fixed by a band 

 round the wrist may act like a plethys- 

 mograph, and be affected by changes 

 of volume in the limb. To avoid this, 

 the instrument is suspended, and thus 

 applied to the artery without use of the 

 band. 



Take another pulse tracing, using this device, and forcibly inspire and 

 expire during the record. 



A deep intercostal respiration, if not prolonged, yields a fall of 

 pressure, and, conversely, a deep diaphragmatic inspiration yields a 

 rise. But the effect of respiration is very complex, and it is difficult to 

 say what the effect of normal respiration in any individual case will be. 

 The ordinary statement that inspiration raises and expiration lowers 

 blood pressure is not borne out by the records. (T. Lewis.) The 

 pressure falls during forced breathing. The fall occurs with inspiration, 

 and is caused by the violent contraction of the diaphragm obstructing 

 the vena cava inferior. 



In Valsalva's experiment, a deep expiration with mouth and nose 

 shut, the abdominal and arterial pressure rise. A stiff walled rubber 

 tube inserted into the rectum and connected with a manometer 

 indicates the big changes of abdominal pressure thus produced, e.g. 

 + 20- + 90 mm. Hg. 



Blood Pressure in Man. The pressure may be measured by the 

 Leonard Hill sphyginometer. This consists of a graduated glass tube, 



FGI. 123. Arrangement of levers in 

 Dudgeon's sphygmograph. 



