THE CIRCULATION OF THE BLOOD. 295 



clamp, until only every second or third auricular contraction is con- 

 ducted to the ventricle, and then stimulating the sympathetic. At 

 once the auricular contraction forces the block, and passes to the 

 ventricle, calling forth a normal contraction. In the mammal the 

 same effects follow stimulation of the vagus and the sympathetic as 

 in the frog. If the thorax of a dog is opened and artificial respira- 

 tion maintained, the heart will continue to beat in a practically 

 normal manner for a long time. Stimulation of the vagus with 

 induced currents of moderate strength will be followed by a com- 

 plete standstill of the heart in diastole, during which the walls are 

 relaxed and the heart-cavities filled with blood. If the currents are 

 of feeble strength, the heart will come to rest gradually through a 

 gradual diminution in the rate and force of the contraction. 

 Stimulation of the sympathetic may be followed by only a slight 

 increase in the rate, especially if the heart action is normally 



FIG. 133. INCREASE IN THE FORCE OF THE VENTRICULAR CONTRACTION (CURVE OF 

 PRESSURE IN RIGHT VENTRICLE) FROM STIMULATION OF THE SYMPATHETIC 

 FIBERS. THERE is LITTLE OR NO CHANGE IN FREQUENCY. (Franck.) 



very rapid. There is, however, an augmentation of the force and 

 an increase in conductivity (Fig. 133). 



The Cardio-inhibitor Center. In the dog, and probably in 

 many other mammals also,f*the cardio-inhibitory center, in the 

 medullajlexerts a more or less constant inhibitory or restraining in- 

 fluence on the heart's activity, ft This is indicated by the fact that the 

 rate of the heart-beat is very much increased by simultaneous divi- 

 sion of both vagi. For this and other reasons it is believed that this 

 center is in a state of tonic activity, discharging nerve impulses which 

 exert a regulative influence on the cardiac mechanism in accordance 

 with its needs, and especially in reference to the variable resistances 

 offered to the flow of blood which the heart must overcome. The 

 question has been raised as to whether the tonic activity of the center 

 is maintained by causes within itself, the result of an interaction of 

 cell substance and the surrounding lymph, or by nerve impulses 



