THE PH YSIOL OGY OF THE NER VO US S YSTEM 215 



(a) How do contraction of the legs in response to direct stimuli 

 compare ? 



(6) Has severing the sacral plexus altered the duration of the con- 

 traction of the muscles supplied? 



(c) If veratrin still produces its typical effects, to what elements 

 in the reflex arc have you limited its action? 



(d) Compare the effect of severing the sacral plexus in a frog 

 poisoned with veratrin with that of a frog poisoned with strychnine. 



(4) Ligate the thigh of a pithed frog at the junction with the body, 

 not including in the ligature the sciatic nerve. Sever all tissues just 

 below the ligature except the nerve and the femur. Carefully separate 

 the cut surfaces with rubber tissues so as to prevent diffusion of the 

 drug. Inject 0.003 grm. veratrin into the dorsal lymph space. 



(a) By means of a diagram show the distribution of the poison. 



(6) Compare the contraction of the legs, noting particularly the 

 difference in the duration rather than the difference in the force of 

 contraction. 



(c) If the protected limb reacts normally, to what elements in the 

 reflex arc have you limited the possible action of veratrin. 



(d) Compare results with similar experiment with strychnine. 



(5) From the frog used in experiment (4) make two gastrocnemii 

 preparations. Fasten in myograph by means of femurs and stimulate 

 them directly, making tracings of contractions. 



(a) Compare tracings. 



(6) To what elements have we limited the action of veratrin? 



(c) Suggest an experiment which would limit the action to one 

 element. 



(6) Very cautiously sniff veratrin. Describe the sensation. 



(7) General observations and comparisons. 



(a) Review your notes on the action of curare, strychnine, and 

 veratrin upon the reflex arc. 



(6) How would you prove that a drug paralyzed by its action on 

 the spinal cord? 



(c) How would you prove that a drug destroyed reflex activity by 

 its action on some part of the sensorium? 



VI. SENSATION. 



The phenomena of reflex action and the function of the several 

 elements of the reflex arc have been studied. It will be remembered 

 by the subject on whom were observed the phenomena of human 

 reflexes that he was conscious of all the stimuli, though he was un- 

 conscious of the response until it had already been effected. 



The sensory element of the reflex arc the dendritic element of 

 the afferent spinal neuron carries from the periphery to the spinal 



