676 THE NERVOUS SYSTEM 



a. Stimulate the posterior root with weak induction currents. Vary 

 the strength till muscular contractions occur in the leg on the same side. 



b. Section the root in the middle. Stimulate the peripheral end. 

 Stimulate the central end. Do not allow escape of the stimulating current 

 to other tissues nor to the anterior root. 



c. Stimulate the anterior root and note the contractions that result. 



d. Section the anterior root in its middle and stimulate the peripheral 

 end, using as weak a stimulus as you can for effective contractions. Now 

 stimulate the central end. 



e. Demonstrate the crossed reflex if possible by stimulating the 

 central end of a posterior root strongly. Stimulate the central end of 

 the brachial nerve and look for crossed reflexes. 



/. Expose the third spinal nerve, cut it and stimulate the central 

 end in an attempt to show both crossed and descending reflexes that shall 

 include movements of the hind legs. 



5. The Reflex Act, Spinal Frog. Prepare a spinal frog by transecting 

 the cord where it joins the medulla, the posterior border of the tympanum, 

 and pith the brain. This spinal animal will remain quiet in whatever 

 position. 



a. Place in the prone position with the hind legs stretched full length, 

 pinch the tip of the toe, after a short latent period the leg of that side will 

 be drawn up into the usual position by its side. If the stimulation is 

 continued and strong enough contractions of the opposite muscles will 

 occur. 



b. Suspend the frog by its jaws and stimulate different points of the 

 skin with 4 per cent, sulphuric acid applied by wet bits of blotting paper. 

 Complicated though orderly contractions occur in the muscles of now one, 

 now the other, or both legs according to which spot is stimulated by the 

 acid. 



c. Stimuli applied in the median line of the body lead to symmetrical 

 contractions. These contractions fail if the spinal cord is destroyed. 

 Hence they are reflex acts, simple, or coordinated; unilateral or crossed, 

 etc. Convulsive reactions occur when the stimulation is too violent. 



6. Decerebrate Frog. Prepare a series of frogs by destroying portions 

 of the brain as follows: 



a. The upper portion of the cerebrum, not injuring the olfactory or 

 optic tracts. 



b. All the cerebrum without injury to the optic lobes and tracts. 



c. Destroy the cerebrum and optic lobes, without injury to the base 

 of the brain or optic tracts. 



d. Destroy all parts of the brain in front of the medulla. 



e. Section at the point of union of the spinal cord and medulla and 

 destroy the brain completely. 



