LABORATORY MANUAL OF PHYSIOLOGY. 



or bone scissors specially made for the purpose, cut through the 

 sternum, keeping the lower blade of the scissors closely applied to 

 the under side of the sternum. With care, the internal mammary 

 artery will probably be avoided. If it is cut, it must be secured 

 with artery forceps and tied. 



The artificial respiration should be stopped during the operation 

 of opening the thorax, in order to avoid injuring the lungs. Start 

 the artificial respiration again. Pull the divided thorax apart and 

 expose the pulsating heart in the pericardium. Note its relations 

 to the lungs during inspiration and expiration. Stop the respira- 

 tion for a moment and open the pericardium. Note the character 

 and sequence of contraction of the various chambers. Note the 

 change of position of the heart with each beat. Compare ventric- 

 ular systole with diastole. Note the coronary arteries and veins on 

 the surface of the heart. How does the method of nutrition of the 

 mammalian heart compare with that of the frog ? Feel the heart 

 between the thumb and finger. Note the hardening of the ven- 

 tricle with each systole and its softening with each diastole. 



With artificial respiration stopped, note the change in the rate 

 of the heart-beat as asphyxia continues, also the distention of the 

 right ventricle. Also note the change in color of the blood. Begin 

 respirations again and note the recovery of the heart and the change 

 in the color of the blood. 



XVII. STIMULATION OF EXTRINSIC CARDIAC NERVES. 



i. In the same rabbit used in the previous experiment, expose 

 and isolate the vagus and depressor nerves. Tie both nerves be- 

 tween ligatures, Cut between the ligatures. Stimulate the distal 

 end of the vagus with a strong tetanizing current and observe the 

 heart's action. Stimulate the central end of the divided depressor 

 and observe the effect upon the heart-beat. Stimulate the distal 

 end of the depressor. Is there any change in the beat of the heart ? 



Trace the cervical sympathetic down into the thorax to the stel- 

 late ganglion. Place this upon the electrodes and stimulate 

 with a medium strong tetanizing current. What is the effect 



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