200 EXPERIMENTAL PHYSIOLOGY 



one or other of the tracings, the observations being repeated with 

 stronger stimuli. The respirations are invariably increased in 

 depth and rhythm (hyperpnoea) and the heart rate is often reduced 

 but the effect on blood pressure as a rule is not readily measurable, 

 because of the exaggerated respiratory oscillations which the 

 tracing shows. Afferent fibres to the vagus and respiratory centres 

 are demonstrated in this experiment, and those to the vasomotor 

 centre can be inferred in cases where the blood pressure rises or 

 remains constant, while the pulse becomes slower. Why is the 

 conclusion permissible? 



One vagus nerve is cut after moderately tightening the ligature 

 around it and the central end is stimulated. Are the results similar 

 to those of sciatic stimulation? 



The results of the foregoing experiments indicate that the 

 centres of respiration and of vagus and vasomotor control are 

 simultaneously stimulated through afferent nerves. In order to 

 determine the relative importance of each effect it is necessary 

 to eliminate one or both of the other afferent influences. With 

 this object in view, the remaining vagus is cut so as to remove the 

 heart from vagus control. (What drugs could have been employed 

 to effect a similar denervation of the heart? Why is there a rise 

 in blood pressure?) The sciatic and vagus nerves are again stimu- 

 lated. What difference is there in the results? Explain the cause. 



Frequently, by using induced shocks that are relatively slow 

 and feeble, the reflex effects on arterial blood pressure are the 

 opposite of those observed when shocks of ordinary frequency and 

 strength are employed. What conclusions do you draw? 



Even the elimination of reflex vagus effects may not suffice to 

 demonstrate the pressor or depressor influence because of the pre- 

 dominance of the respiratory effects. It may be necessary to 

 eliminate the latter, which can be done by injecting through the 

 femoral vein a saturated solution of curare in physiological saline.* 

 Under the influence of curare the respirations cease, and to keep the 

 animal alive it is necessary to connect the respiratory pump to the 

 tracheal cannula, with the anaesthetic bottle inserted in the tubing. 



*The curare solution should be prepared several days previously and should be 

 tested on a frog before using in the above experiment. The curare at present on 

 the market is very unreliable. 



