356 PRACTICAL PHYSIOLOGY 



CHAPTER XXIX. 



VASO-MOTOR SYSTEM CONTINUED. PERFUSION OF 

 BLOOD-VESSELS. 



Vaso-motor Changes following Excitation of the Sciatic. Pith the 

 cerebrum of a large frog and plug the opening. Curarise the frog very 

 lightly, but sufficiently to paralyse the muscles. Curare in large doses 

 paralyses the vaso-motor mechanism. Very carefully expose the sciatic 

 nerve in the thigh without damaging the blood-vessels ; tie, cut and 

 place it upon electrodes. Observe the capillaries in the web of the 

 foot under the microscope, and note the rate of flow. Tetanise with a 

 current strength just perceptible to the tongue. The blood-flow becomes 

 slower owing to constriction of the arteries. The vaso-con stricter effect 

 is slight and very easily exhausted. If the nerve be excited with single 

 shocks once every five seconds, vaso-dilatation and increased flow may 

 result. Now examine the circulation in the web of the other foot. 

 Note the rate of flow and then tetanise or pinch the skin of the frog. 

 Reflex constriction and consequent slowing of the current may result. 

 The afferent fibres excite the reflex discharge of the vaso motor centre 

 in the spinal bulb and cord. These results, difficult to obtain in the 

 frog, may be easily demonstrated in the cat by the plethysmographic 

 method. 



Perfusion of Frog's Blood-vessels. Destroy the brain and plug the 

 hole in the skull. Expose the heart. Tie one aorta. Place a ligature 

 under the other, snip it with sharp scissors, and allow the blood to 

 escape. Insert a fine-glass cannula into it pointing away from the 

 heart. Fill the cannula with normal saline by means of a capillary 

 pipette. Connect a rubber tube to a glass funnel and clip the tube. 

 Fill the funnel and tube with Ringer's fluid. Connect the tube with 

 the cannula. No air bubbles must be introduced. Snip the sinus 

 venosus and open the clip. Hang the frog in the vertical position. 

 The fluid circulates, runs out of the sinus, and drops from the toes of 

 the frog into a measure glass. Measure the outflow per minute. 

 Circulate Ringer's fluid plus 1 in 1000 sodium nitrate; the outflow is 

 increased owing to vaso-dilatation. Supra-renal extract produces the 

 contrary effect. 



