CARDIAC MUSCLE. 63 



Normally in the frog the vagus carries very few impulses to 

 the heart, therefore cutting the vagi has little influence upon the 

 rate. But in mammals the vagi continually hold the heart in 

 check so that cutting of these nerves permits an increase in rate. 

 The vagus is reflexly called into action by sensory stimulation. 

 A good example of this is obtained by sharply striking the abdomen 

 of a frog with a blunt instrument. Frequently the heart may be 

 stopped or slowed in this way. 



An increased blood pressure will also reflexly act along the vagus 

 to slow the heart. This is due to the stimulation of afferent fibres 

 ending in the arch of the aorta. High pressures may also directly 

 stimulate the vagus centre. 



The vagus seems to exercise a beneficial influence on the heart, 

 tending to oppose acceleration and overwork of the heart. The 

 vagus is most effective in animals which are accustomed to strenuous 

 muscular exercise and therefore great activity of the heart. 



Acceleration. Certain fibres, which are called accelerator 

 fibres, oppose the action of the vagus. They are found in the 

 cervical sympathetic beloAV the inferior cervical ganglion. Stimu- 

 lation of these fibres increases the rate of the heart and sometimes 

 the amplitude as well. On the other hand cutting of these fibres 

 permits the vagus to have full sway (in the mammal) and thus 

 the heart beat becomes slower. The accelerators are called into 

 action reflexly, although it is difficult to say just what stimuli will 

 affect the accelerators and what will call the vagi into action. 



The arrangement of opposed fibres, i.e., accelerator and in- 

 hibitor, renders quick adjustment to changing requirements 

 easier. 



Experiment 28. Frog's Heart. Pith a frog, destroying the 

 brain only. Expose the heart by slitting the upper part of the 

 abdomen lengthwise. Avoid injury to the anterior abdominal 

 vein by cutting a little to one side of the mid-line. Observe 

 the beating of the heart, especially the rate. Then open the 

 pericardium and pass a thread under the fraenum and tie. 

 The fraenum is a slender band of tissue which connects the 

 posterior surface of the heart with the pericardium. Has this 

 operation influenced the rate? By means of the thread raise 

 the heart and notice the large veins, the sinus, the white cres- 



