THE CIRCULATION. 251 



When the ejection of blood from the ventricle is retarded by the 

 aortic valves, permitting regurgitation or by a high arterial tension, 

 then there is a strain upon the cardiac muscle which is met by increased 

 contraction. In the muscle of the heart the greater the resistance, up 

 to a certain point, to contraction, the stronger the force of contraction. 

 .If the heart has continuously to meet these resistances, then its muscle 

 hypertrophies and compensates for the defective valve in order to 

 properly carry on the circulation. 



INNERVATION OF THE HEART. 



If the heart be removed from the chest or all of its nerves be 

 severed, it will still continue to beat for a variable time, dependent 

 upon the class of animal operated upon. In the case of the frog and 



Fig. 75. Heart of the Frog. (LivoN.) 



I. Anterior view. II. Posterior view. A, A, Aortas. Vc, Superior vena cava. 

 Or, Auricle. V, Ventricle. Ba, Aortic bulb. 8V, Sinus venosus. Vci, Inferior 

 vena cava. VI), Hepatic vein. Vh, Pulmonary vein. 



other cold-blooded animals the beating of the heart will continue for 

 hours under favorable conditions. From this it would seem that 

 there must reside within the heart itself some mechanism whereby 

 the rhythmical movements of the heart are maintained. 



Like every other organ of the body, the heart receives its pro- 

 per quota of nerve-supply, through whose medium are conducted cer- 

 tain impulses from without and by whose influence its rhythm may 

 be altered. 



Cardiac Ganglia. This internal mechanism has been chiefly 

 studied in the frog, where there exists in the heart three distinct 

 ganglia: Eemak's, Bidder's, and von Bezold's. From the cells of these 

 ganglia there are discerned numerous small fibers which form a plexus 

 over the surface of the auricles and upper portion of the ventricles. 



Remaps ganglion is seen at the orifice of the superior vena cava 

 or sinus venosus. Bidder's is located at the junction of the auricles 



