CENTRAL NERVOUS SYSTEM. 



101 



Experiment 42. Inject a few drops of a 0.5 per cent, solution of 

 strychnine sulphate into the dorsal lymph sac of the frog. Apply 

 weak stimuli at intervals to the skin of different parts of the 

 body and note the exact nature of the responses. Explain the 

 action of the strychnine. 

 Inhibition of Reflexes. Reflex activities may be inhibited 



either through afferent nerves or from the higher centres. 



FIG. 30. Exposed brain of frog: (c) cerebrum; (o) optic lobes; 

 (d) cerebellum; (e) medulla. 



Experiment 43. Expose the sciatic nerve in the left thigh. Tie 

 a ligature about the nerve near the knee and cut the nerve 

 distal to the ligature. Immerse the right leg of the frog in 

 0.5% sulphuric acid and note the latent period for reflex action. 

 Immediately wash off the acid. After three minutes place 

 electrodes on the exposed sciatic nerve and as the right foot is 

 again immersed in the acid, stimulate the nerve with a weak 

 tetanizing current. Prove that the sensory endings in the skin 

 are still irritable by the acid. Explain the inhibition. 



