THE VASCULAR MECHANISM. 



217 



wave, P, corresponds with the systole of the auricles, which, when they 



contract, become galvanometrically negative to the resting apex. The 



second wave, R, occurs at the beginning of the ventricular systole, and 



is due to the systole commencing at the base of the ventricle, which 



becomes negative to the apex. As the wave of contraction travels to 



the apex, the thread returns 



to its resting position and 



remains steady for a short 



time, during which the whole 



ventricle is in contraction. 



The final rise at T is due to 



the systole lasting longer at FIG. 80. Electro-cardiogram from human heart. 



the base than at the apex, (Hume.) Explanation of letters in text. 



particularly round the root of 



the aorta and pulmonary artery. The records obtained in the lower 

 animals show the same general form; in the frog and tortoise the 

 prolonged contraction of the ventricle is accompanied by a long period 

 between P and T during which the heart is isoelectric. 



The Nutrition of the Heart Muscle. The nutrition of the heart 

 muscle is dependent on the amount and composition of the nutrient 

 fluid supplied to it, that is, in normal conditions, of the blood. The 



influence of alterations in 

 the composition of the nu- 

 trient fluid are very easily 

 studied in the isolated frog's 

 heart, the muscular wall of 

 which is nourished entirely 

 by interchanges between 

 the muscular fibres and the 

 blood flowing through the 

 heart. For this purpose a 

 cannula, such as that shown 

 in fig. 81, is tied into an 

 auricle, and the heart is 

 excised. The cannula is 



- attached to a bottle con- 

 FIG. 81. Diagram of apparatus for perfusion or 



frog's heart. taming a suitable perfusion 



fluid, which enters the 



heart at a pressure of 1 to 2 cm. of water. The fluid expelled through 

 the bulbus arteriosus from the ventricle flows over the surface of the 

 heart, keeping it moist. The apex of the ventricle is connected by 

 a thread with a recording lever. 



