CIRCULATION OF THE BLOOD. 



the rate and strength of the heart-beat ? Stimulate, again, with a 

 somewhat stronger current and note the effect. 



Increase the strength of the current, a little each time, until the 

 full inhibitory vagus effect is obtained. What is the result on blood 

 pressure ? What is the after-effect of vagus stimulation ? 



(b) Stimulate the central end of the divided nerve with medium 

 strong tetanizing current. What is the effect on blood pressure? 

 Note the respiratory movements before and during the stimula- 

 tion of the central end of the nerve. 



4. Depressor Stimulation. Tie the two ligatures which were 

 passed around this nerve and cut the nerve between the ligatures. 



(a) Stimulate the peripheral end of the cut nerve; (the end 

 toward the heart) with medium strong tetanizing currents. Is 

 there any effect on the rate or strength of the heart-beat or upon 

 the blood pressure? 



(b) Stimulate the central end of the divided nerve in the same 

 way. Is there any effect on the rate and strength of the heart-beat 

 or upon the blood pressure, or both ? How does this tracing com- 

 pare with that obtained through stimulation of the peripheral end 

 of the cut vagus, using the same strength of stimulus ? 



Compare the result obtained through stimulating the central 

 end of the depressor with that obtained through stimulating the 

 peripheral end. Is the depressor effect a direct or an indirect 

 effect? Is the nerve an efferent or an afferent pathway? If it 

 forms part of a reflex arc, what nerve pathways form the other 

 limb or limbs of the arc ? 



(c) Now cut the intact vagus of the opposite side. Note any 

 further change in the blood-pressure tracing. 



Again stimulate the central end of the depressor nerve. Is 

 there any fall in blood pressure ? Is there any slowing of the heart- 

 beat ? What change has vagus section made in the depressor re- 

 flexes ? Explain. In the light of the above observations, what are 

 the functions of the depressor nerve ? 



5. Section of the Cervical Sympathetic. Using the same 

 rabbit, observe the appearance of the blood-vessels in the two ears. 



