114 ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



stimulate again. Note the compensatory variations occurring at this 

 time. 



2. Division of One Vagus Nerve. Apply another loose ligature to 

 each vagus nerve. Record the normal blood-pressure. Ligate one 

 vagus twice and cut between the ligatures, so that its central and distal 

 stumps may be stimulated separately. Indicate the moment of the 

 division upon the tracing. What changes do you observe in the blood- 

 pressure? Obtain the cardiac frequency from the tracing before and 

 after the section. To what factor do you attribute the rise in pressure? 



Stimulate the distal end of this nerve. After the pressure has re- 

 turned to normal stimulate the central end of this nerve. What deduc- 

 tions may be drawn regarding the inhibitor qualities of the vagi? 



Annotation. It has been stated above that the inhibitor power of the vagi 

 nerves differs in different animals. Thus, cardio-inhibition is easily obtained in 

 the dog, but only with difficulty in cats. Secondly, in certain animals the stimulation 

 of the central end of one vagus the other being intact produces a reflex slowing 

 of the heart. 



3. Division of Both Vagi Nerves. Record the blood-pressure and 

 divide the opposite vagus in the same manner. Determine the cardiac 

 rate before and after the section. Are the effects previously noted now 

 more pronounced? Stimulate the distal and central ends of this nerve. 

 Compare the inhibitory power of the left nerve with that of the right. 



Annotation. The inhibitor power of these nerves varies considerably. Some- 

 times the left and sometimes the right nerve is the stronger of the two, but most 

 generally the right. With both nerves divided, the stimulation of either central end 

 must necessarily fail to produce a reflex slowing of the heart. 



4. Inhibition of the Exposed Heart. Test the artificial respiration 

 apparatus and adjust the check- valve for an air current of medium 

 volume. Connect the tracheal cannula of the animal with the ether- 

 bottle. Incise the skin over the sternum. By means of a pair of 

 strong scissors cut through this bone directly in the median line, avoid- 

 ing the mammary arteries. Institute artificial respiration. Vary the 

 position of the stop-cock upon the inlet tube to give to the lungs a normal 

 degree of inflation. Stop the bleeding by applying a compress or by 

 torsion and ligation of the vessels. Pull the edges of the sternum apart 

 by means of a strong string drawn around the board. Open the peri- 

 cardial sac and inspect the heart, repeating the observations made 

 in the course of Lesson XVI. Stimulate the distal end of either vagus 

 nerve. Note that the heart stops in diastole, greatly distended by 

 blood. Observe the swollen condition of the central veins, indicating a 

 rise in venous pressure. 



Endeavor to regain the heart in the inhibited condition by prolong- 

 ing the duration of the stimulation. W^hat happens? When the heart 

 resumes its beat it is apparent that it contracts first in the region of the 

 pace-maker, giving a well-marked contraction wave. 



