CARDIAC VAGUS OF THE FROG. 215 



the ligatured spot and the pericardium. Gently raise the 

 whole heart upwards to expose the somewhat whitish 

 V-shaped "crescent" between the sinus venosus and the 

 right auricle. 



(b.) Prepare previously an induction machine arranged to 

 give an interrupted current. Place the electrodes which 

 must be fine, and their points not too far apart (2 milli- 

 metres) upon the crescent, and faradise it for a second ; 

 if the current be sufficiently strong, the auricles and ventricle 

 cease to beat for a time, but they begin to beat even in 

 spite of continued stimulation. 



(c.) Stimulate the auricles, there is no inhibition or arrest. 



(d.) If a drop of solution of sulphate of atropia (Lesson 

 XL VIII. , 1) be applied to the heart, stimulation of the 

 crescent no longer arrests the action of the heart, for the 

 atropine paralyses the inhibitory fibres of the vagus. 



3. Seat of the Motor Centres. 



(a.) Expose a pithed frog's heart, cut out the ventricle 

 with the auricles attached to it, and observe that the heart 

 continues to beat. Divide the ventricle vertically by two 

 parallel cuts into three portions. The middle portion con- 

 tains the auricular septum, in which lie ganglioiiic cells. It 

 continues to beat while the right and left lateral parts do 

 not beat spontaneously, but respond by means of a single 

 contraction if they are stimulated. 



LESSON XLVII. 



CARDIAC VAGUS AND SYMPATHETIC OF 

 THE FROG AND THEIR STIMULATION. 



1. Cardiac Vagus of the Frog How to Expose it. In this 

 case a preliminary dissection must be made before the student 

 attempts to stimulate the vagus. 



(a.) Pith a frog, or destroy its brain and curarise it. Lay 

 it on its back on a frog-plate. Expose the heart, remove 



