THE CIRCULATION 225 



be seen just before the great upstroke. This corresponds 

 to the contraction of the auricles. 



In various diseases of the heart the cardiogram is mate- 

 rially modified. Hence it is important to have a clear 

 conception of the various parts of the trace. 



The elucidation of the various parts of the cardiogram is 

 only possible after careful study of the other changes in the 

 heart during the cycle. 



5. Changes in the Intracardiac Pressure. 



These can be studied only in the lower animals. 



The most common way of determining the pressure in a 

 cavity is to connect it to a vertical tube and to see to what 

 height the fluid in the cavity is raised. If such a method is 

 applied to the heart, the blood in the tube undergoes such 

 sudden and enormous changes in level that it is impossible 

 to get accurate results. 



The same objection applies to the method of connecting 

 the heart with a U tube filled with mercury. When this is 

 done the changes in pressure are so sudden and so extensive 

 that the mercury cannot respond to them on account of its 

 inertia. 



Various means of obviating these difficulties have been 

 devised. One of the best is to allow the changes of pressure 

 to act upon a small elastic membrane tested against known 

 pressures. A tube is thrust through the wall of the heart 

 and connected with a tambour covered by a membrane to 

 which a lever is attached. 



A. Pressure in the Great Veins (small dotted line in 

 Fig. 114). When the auricles contract the flow of blood from 

 the great veins into these chambers is arrested, and, as a 

 result, the pressure in the veins rises. As the auricles relax 

 the blood is sucked from the veins and the pressure falls, but, 

 as the auricles fill up, it again rises. When the ventricles 

 relax and suck blood from the auricles, blood again flows in 

 from the great veins and the pressure falls, again to rise as 

 the auricles and veins are both filled up, towards the end of 

 the pause. 



B. Pressure in the Auricles (dash line in Fig. 114). 



At the moment of auricular contraction there is a marked 



15 



