268 CIRCULATION OF THE BLOOD. 



the tricuspid valve in the dead heart, it is important to notice 

 what are the conditions which render the valve incompetent, 

 i. e., prevent it from closing completely. The most important 

 of these conditions is over-distension of the ventricle, by which 

 the ostium becomes too large to be covered by the valve. 

 When this occurs during life, the phenomenon known as the 

 venous pulse presents itself. The right ventricle being still in 

 communication with the venous system at the moment that it 

 hardens, blood is injected by it backwards. When, in the 

 human subject, this condition is permanent, it leads first to 

 dilatation of the great veins, and, secondly, to similar incom- 

 petence of the vein-valves nearest the heart. In such persons 

 two large swellings are seen on either side of the neck the 

 distended jugular veins which pulsate nearly synchronously 

 with the heart. 



SECTION VI. ENDOCABDIAL PRESSURE. 



By this term is understood the pressure exercised by the 

 blood contained in the heart, against its internal surface. It 

 can be measured in the frog and in mammalia. 



63. Investigation of the Endocardial Pressure in 

 the Heart of the Frog under various Conditions. In 

 the frog the action of the heart is maintained unimpaired after 

 the separation of the organ from the cerebro-spinal nervous 

 centres. It is not even necessary that "it should be supplied 

 with blood. Serum (if perfectly fresh) of another animal may 

 be substituted for it, without apparent^ affecting either the 

 vigor or regularity of the cardiac contractions. These two 

 facts render it possible to use the heart of the frog for the solu- 

 tion of a number of problems, in reference to which it is desira- 

 ble to investigate the mechanical functions of the heart inde- 

 pendently of the influence of the nervous system. 



The method of preparing the heart for such experiments is 

 that first employed by Dr. Coats, of Glasgow, in an investiga- 

 tion relating to the mechanical work done b}' the heart in a 

 given time, in Lud wig's laboratory. It has been since used 

 with various modifications byBowditch, Brunton,Blasius, and 

 others. The brain and spinal cord having been destroyed by 

 the introduction of a needle, the body of the frog is cut across 

 below the liver. The sternum with the anterior extremities 

 are removed, great care being taken to reserve on one side a 

 large flap of skin which may be used as a cover for the nerves, 

 and the heart. The heart is then freed of its pericardium, and 

 the little serous ligament by which it is connected with the 

 posterior surface of that membrane is ligatured and divided. 

 The next step is to tie one branch of the aorta, and then to pass 

 a canula through the other and the bulb into the ventricle. The 



