88 ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



A 1.0 per cent, solution of pilocarpin acts in the same way as muscarin. 

 Note the gradual inhibition of the ventricle. 



Annotation. It will be remembered that this poison produces its characteristic 

 effect either by paralyzing the contractile elements of the muscle-fibers or by stim- 

 ulating the inhibitor nerve mechanism. The latter view seems the more plausible, 

 because if a few drops of a 0.5 per cent, solution of atropin are applied to a heart 

 which has been arrested with muscarin, this organ resumes its beat. Atropin acts 

 antagonistically to the muscarin by counteracting the effect of the latter. 



5. Reflex Cardiac Inhibition. Etherize a frog and lay it upon its 

 back. Make a rounded orifice in the ventral surface of the thorax in the 

 region of the heart so as to expose this organ fully to the view. With 

 the flat handle of a scalpel continue to tap lightly upon the ventral 

 aspect of the abdomen until the heart shows a material reduction in its 

 frequency. Explain this result, comparing it with the symptoms follow- 

 ing strokes upon the solar plexus in man. 



Divide both vagi nerves and repeat this experiment. Note that the 

 aforesaid procedure now fails to inhibit the heart, because the paths by 

 means of which these impulses reach the heart have been cut. 



Annotation. In the frog the vagi nerves are not easily found. Insert a glass 

 rod in the esophagus to distend it. Remove the tissues over the petrohyoid muscle, 

 extending from the angle of the jaw to the thyroid process of the hyoid bone. Two 

 nerves will be seen pursuing a course across this muscle, namely, the hypoglossal 

 and the glossopharyngeal. The first is easily recognized by tracing its course to 

 the tongue. It lies closer to the midline. Next to the lower border of the petrohyoid 

 muscle and close to a blood-vessel lies the vagus (Fig. 53). 



6. Acceleration of the Heart. Arrange the apparatus for registering 

 the contractions of the frog's heart upon the smoked paper of a moder- 

 ately rapid kymograph. Pith a frog and expose the heart by making a 

 median incision through the wall of the thorax. Expose the spinal 

 column at the base of the skull by pushing the esophagus and trachea 

 to one side. Identify the vagus ganglion. It lies under the upper part 

 of the levator scapula muscle. After its formation the sympathetic 

 nerve turns outward along the base of the jaw to become united with 

 the vagal fibers. Black pigment marks the course of this nerve. Isolate 

 this portion and insulate it with a narrow strip of rubber membrane. 

 Allow the heart to register its beats while the drum revolves once around 

 its axis in about one minute. Add the record of a chronograph. Di- 

 rectly underneath register a second line of heart-beats, but stimulate 

 the sympathetic nerve during this entire period with a tetanic current 

 of very moderate intensity. Count the beats in each line, and com- 

 pare. The stimulation of this nerve in the frog does not produce a very 

 decisive acceleration; still, records of this length should show a difference 

 of from ten to fifteen beats. 



