314 RESPIRATION 



current. The stimulus should be graduated carefully, for there is often such 

 a great increase in respiratory rate and volume that the animal may become 

 overanesthetized. 



b. The effect of Stimulating the Vagus Nerve. Isolate and stimulate 

 the vagus nerve with a medium strength of stimulus. The effect is usually 

 complete inhibition of respiratory movements. By means of graduated 

 stimuli one may demonstrate the accelerator effects from the stimulation 

 of the vagus. Stimulate also the superior laryngeal, and compare with the 

 effects of stimulating the whole vagus. 



c. The Effect of Cutting the Vagus Nerves. Isolate both vagus nerves 

 and section them as nearly at the same moment as possible. Be sure to 

 mark on the tracing the exact moment at which the nerves are cut. This 

 experiment should be performed with every accessory condition as constant 

 as possible, and the animal should not be disturbed for one or two minutes 

 so that the effects of the section will be recorded, figure 240. The result is 

 always a marked deepening and slowing of the respiratory movements. 



d. The Effect of Stimulating the Central End of the Vagus. Upon stimu- 

 lating the central end of the vagus after section, the respiration rate will be 

 inhibited as in b, showing that the vagus nerves carry afferent respiratory 

 fibers, figure 241. 



e. The Effect of Stimulation of the Phrenic Nerves. Isolate the right 

 phrenic nerve at its origin from the brachial plexus and stimulate it with a 

 medium strength of stimulus. Upon stimulating the nerve the diaphragm 

 will remain in contraction and the record will show that the thorax is in the 

 inspiratory phase. 



Section this nerve and note the change in the character of respiratory 

 movements; make direct observations on the diaphragm, examining from 

 the abdominal side. 



12. Demonstration of Apnea, Dyspnea, and Asphyxia. Produce 

 deep anesthesia, then disconnect the ether bottle and connect the tracheal 

 tube with a hand bellows. Produce deep and forced artificial respiration 

 for twenty to thirty seconds. Stop the artificial respiration; the animal will 

 remain quiet without any effort at breathing. This is the condition of apnea. 

 Allow the animal to recover its normal respiration rate and again produce 

 deep anesthesia. Now clamp off the tracheal tube so that the animal can no 

 longer receive air and leave it so until death. As the blood becomes more 

 and more venous there will first be a marked increase in the respiration rate 

 and depth. This is known as hyperpnea. This stage is followed by one 

 of increasing respiratory amplitude in which the accessory respiratory mus- 

 cles not previously active are brought into forcible contractions, both inspira- 

 tory and expiratory phases are now forced, dyspnea. The movements con- 

 tinue to increase, and the muscles of the neck, larynx, mouth, and nostrils 

 now take part. There is a rather sudden decrease in the respiratory move- 



