CENTRAL NERVOUS SYSTEM. 99 



ANIMAL offers a most suitable preparation for the study of reflex 

 activities. Immediately following its isolation from the brain the 

 cord enters into a depressed condition called SPINAL SHOCK. This 

 is quickly recovered from in the lower animals, but may take 

 months to disappear in the higher. 



Experiment 40. Pith a frog, destroying the brain only and sus- 

 pend the animal by fixing the lower jaw to a clamp (Fig. 29). 

 Apply mechanical stimuli (by pinching) to the skin of various 

 regions and note the location and character of the response. 

 Commence the application of the stimuli immediately after 

 decerebration and observe by repeating at intervals whether 

 they improve with time. If they do so, what is your conclusion ? 

 Proceed now to demonstrate the duration of the latent period or 

 reflex time using as the stimulus a 0.1 per cent, solution of HoSO 4 . 

 Measure accurately the time which elapses between placing the 

 foot in the acid and the movement of the leg. Remove all 

 traces of acid from the skin by means of water. Make three 

 observations with the same strength of acid using the two feet 

 alternately and allowing some time (several minutes) to elapse 

 between the stimulations. Repeat the observations using 0.3 

 per cent. acid. Are any differences in latent period and in- 

 tensity of response observed, depending on the strength of 

 stimulus? 



It is likely, with the strong stimuli employed in the above 

 experiments, that some spread (or irradiation) will have occurred 

 in the cord, so that the opposite hind limb or the fore limbs 

 show movements. How is this explained? 



Experiment 41. To investigate the "march" of the irradia- 

 tion more precisely and to demonstrate that the isolated cord 

 is capable of synthesising a complicated and apparently pur- 

 poseful group of movements saturate a piece of filter paper in 

 30 per cent, acetic acid, remove the excess of acid and place a 

 small square of this paper on one of the flanks of the frog. 

 After noting the character of the movements, wash away the 

 acid and repeat, holding the leg on the irritated side. 

 The Spreading of Reflexes in the cord is greatly facilitated by 

 strychnine, but the movements are not of the same "purposeful" 

 character as those just studied. 



