RESPIRATORY WAVE IN BLOOD-PRESSURE CURVE. 



303 



The cause of the undulations in the blood-pressure curve cor- 

 responding to the respiratory movements is not quite so simple 

 as it might appear to be at first sight, and it has often been mis- 



FIG. 136. 



Pick's Spring Manometer. A hollow C-sha|K?d spring (A), made of extremely thin 

 metal, is fixed at (6 b), where its cavity communicates with the tube (K). The top of the 

 C is connected at (a) with the writing lever. Any increase of pressure in the tube (K) 

 causes the spring to expand and move the writing point (<;) up and down. 



understood. Though many causes have been given, no single one 

 appears to explain adequately all the changes that may occur in 

 this phenomenon under varying circumstances. At first sight 

 the respiratory movements and conse- 

 quent pressure changes within the thorax 

 would seem to give a simple mechanical 

 explanation. But if the change occur- 

 ring in the intrathoracic pressure be ex- 

 amined carefully, it will be found not 

 to correspond exactly with the so-called 



Tracing of blood pressure taken 



respiratory wave of the pressure curve with Pick's manometer. 



FIG. 137. 



