322 TEXT-BOOK OF PHYSIOLOGY 



tricular activity, but after a variable period of time, fifty seconds or more, 

 the ventricle develops an independent rhythm which gradually increases in 

 frequency, but seldom, if ever, attains that of the auricles. Under such 

 circumstances tetanic stimulation of the auriculo-ventricular tissues by 

 means of the clamp now transformed into stimulating electrodes, failed to 

 bring about a stoppage of the ventricles. Moreover, if during the time the 

 clamp is applied and after the ventricle has developed a rhythm of its own, 

 the vagus is stimulated, the auricles will cease to beat as usual, but the ven- 

 tricles will continue to beat at their usual rate. These and similar facts 

 lead to the conclusion that vagal inhibitor action is limited to the auricles. 



From foregoing facts it is apparent that the accelerator and augmentor 

 effects of the sympathetic nerve impulses, and the inhibitor effects of the 

 vagus nerve impulses, closely resemble on the one hand, the accelerator and 

 augmentor effects of increasing amounts of diffusible calcium salts, and on 

 the other hand, the inhibitor effects of increasing amounts of diffusible 

 potassium salts in the blood or other circulating fluid; and so closely do 

 these two sets of phenomena resemble each other, that they are by some 

 observers regarded as identical. 



Some additional facts in this connection have been presented by Ho well, 

 viz., that an increase (within limits) and a decrease in the percentage of 

 diffusible calcium salts in a circulating fluid passing through the cavities of 

 the mammalian (cat) heart, increases on the one hand, and decreases on 

 the other hand, the sensitiveness of the heart to sympathetic acceleration 

 and augmentation. From this the inference is deduced that the acceleration 

 and augmentation of the heart-beat which follow stimulation of the sympa- 

 thetic nerves are due to the presence in the heart tissue of a certain percentage 

 of diffusible calcium salts, which have been freed from combination with 

 organic matter by the action of the sympathetic nerve impulses. Again, 

 that an increase (within limits) and a gradual decrease in the percentage 

 of diffusible potassium salts in a circulating fluid passing through the cavities 

 of the frog and the cat heart, increases on the one hand and decreases and 

 finally abolishes on the other hand the sensitiveness of the heart to vagus 

 inhibition. From this the inference is deduced that the inhibition of the 

 heart-beat which follows stimulation of the vagus nerve is due to the presence 

 in the heart tissue of a certain percentage of diffusible potassium salts, 

 which have been freed from combination with organic matter by the action 

 of the vagus nerve impulses. 



The foregoing effects of the sympathetic and vagus nerves on the heart 

 muscle, viz.: changes in its irritability, conductivity, rapidity, and the 

 energy of the beat, have been termed by Engelmann bathmo tropic, dromo- 

 tropic, chronotropic, and inotropic. Any one of these effects, e.g., the chrono- 

 tropic, may be modified in a positive direction by the sympathetic, or in a 

 negative direction by the vagus. 



The Cardio-accelerator Center. The collection of nerve-cells from 

 which the pre-ganglionic fibers of the sympathetic system arise is known as 

 the cardio-accelerator or augmentor center. The exact location of this 

 center in the central nerve system has not been as yet accurately determined. 

 It is probably located in the medulla oblongata. 



From experiments which have been made on the sympathetic nerve 

 apparatus in its entirety, it is believed that the function of this center is the 



