THE CIRCULATION 



119 



Apply two ligatures to this nerve, a few millimeters apart, and cut 

 between them. Record the blood-pressure and stimulate its central 

 and distal (heart) ends successively at intervals. Since the excitation 

 of the distal end remains ineffective, what conclusions may be drawn 

 regarding the direction of conduction in this nerve? What two factors 

 should be held responsible for the fall in blood-pressure? 



Divide both vagi nerves and repeat the stimulation of the central 

 end of the depressor as soon as the changes ordinarily following the 

 section of the aforesaid nerves, have become thoroughly established. 

 Observe that henceforth the fall in blood-pressure is no longer asso- 

 ciated with a reduction in the frequency of the heart. Give a detailed 

 explanation of the function of the depressor nerve, and mention condi- 

 tions during which this mechanism is called into play. 



FIG. 78. DIAGRAM TO SHOW THE COURSE OF THE DEPRESSOR NERVE IN THE RABBIT. 

 L, Larynx; T, thyroid gland; j, int. jugular vein; C, carotid artery; S, sympathetic 

 nerve extending between the superior and inferior cervical ganglia; V, vagus nerve; 

 SL, sup. laryngeal nerve; D, depressor nerve, entering the vagus by two branches. 

 The vagus is pulled over, permitting the sympathetic to appear next to the carotid artery. 



3. The Sciatic Nerve. Test the artificial respiration apparatus and 

 connect the ether-bottle with the tracheal cannula. Prepare a solution 

 of curare. Inject 1 c.c. of this solution intravenously by means of a 

 syringe. Allow this agent to produce its characteristic effect, and 

 institute artificial respiration as soon as the diaphragm shows the first 

 indications of ceasing its -action (fifteen minutes). Expose the sciatic 

 nerve in the thigh; place a ligature upon it, and cut it distally to the 

 ligature. Record the carotid pressure and stimulate the central end 

 of this nerve with a quickly interrupted current of moderate strength. 

 As a rule, currents of medium strength and frequency give rise to a 

 reflex vasoconstriction, and hence to an increase in the carotid blood- 

 pressure. Since the motor plates of the skeletal musculature have been 

 paralyzed by the curare, this result cannot be referred to the mechanical 

 influence of contracting muscle tissue. 



