958 THE CENTRAL NERVOUS SYSTEM 



the dog employed for the experiment on the motor areas (p. 962). 

 Place the animal, belly down, and insert a good-sized block of wood 

 between it and the board at the level of the lumbar vertebrae of the 

 spine. Divide the skin and muscles on either side of this region till the 

 laminae of the vertebrae are exposed. Snip through them with strong 

 forceps, and open the spinal canal, exposing a length of cord correspond- 

 ing to three or four vertebrae. Ligate and stimulate the roots as in (a). 



2. Reflex Action in the ' Spinal ' Frog. Pith the brain of a frog, 

 destroying it down to the posterior third' of the medulla oblongata. 

 (i) Note the position of the limbs immediately after the operation, and 

 again thirty to forty minutes later. Its hind-legs possess tone, and are 

 drawn up against the flanks. The animal can still execute certain 

 co-ordinated movements e.g., pulling away its leg if a toe is pinched. 

 The power of maintaining equilibrium is lost. If placed on its back, it 

 lies there. When thrown into water it sinks usually without any 

 attempt at swimming. Verify the following facts, using mechanical 

 stimulation (pinching the toes or skin of the leg) : (a) If the stimulus 

 provokes muscular movements only on one side of the body, this is 

 usually on the same side as the stimulated point, (b) When the stimulus 

 causes reflex movements on both sides of the body, the stronger con- 

 tractions are on the side to which the stimulus was applied. 



Determine whether it is easier to obtain movement of a portion of the 

 body innervated from a region of the cord above the level of the stimu- 

 lated nerves or below that level. 



(2) With electrical stimuli (using a coil arranged for single shocks de- 

 termine if reflex movements are elicited by a single induced shock ap- 

 plied to the skin. Verify the fact that a series of shocks is more efficient, 

 the effects of the separate stimuli being summated in the reflex centres. 



(3) To test the effect of thermal stimuli, dip the leganto a beaker of 

 warm water. Vary the temperature of the water, using a series of 

 beakers with water at ioC., 15 C., 2OC., etc., above the temperature of 

 the room. Place the leg for a moment in each, and determine which is the 

 most efficient stimulus. Immediately on withdrawing the leg from each 

 of the hot-water beakers immerse it in a beaker of water at room tem- 

 perature. Finally, dip the leg into a beaker of cold water, and heat it 

 gradually to a temperature at which a reflex was previously obtained. 

 Probably it will not be elicited by the gradual warming. 



(4) ' Purposive ' Movements. Touch the skin of one thigh with blot- 

 ting-paper soaked in strong acetic acid. The leg is drawn up, and the 

 foot moved as if to get rid of the irritant. If the leg is held, the other 

 is brought into action. Immerse the frog in water to wash away the acid . 



(5) Spread (Irradiation) of Reflexes. Gently stimulate a toe or a small 

 spot on the flank with weak induction shocks or weak mechanical 

 stimuli, and note the reflex effect obtained. Then go on gradually 

 increasing the strength of stimulation without increasing the area of 

 the field stimulated, and observe the extent and order of spread of the 

 reflex movements. 



3. Reflex Time. Pass a hook through the jaws. Holding the frog by 

 the hook, dip one leg into a dilute solution of sulphuric acid (0*2 to 

 0-5 per cent.), and note with the stop-watch the interval which elapses 

 before the frog draws up its leg (Tiirck's method of determining the 

 reflex time). Wash the acid off with water. 



Determine how the reflex time varies with the strength of the stimulus. 

 This can be done by using various strengths of acid. The reflex time 

 will be shorter the stronger the stimulus up to a certain point. Compare 

 the reflex time of movements on the same side of the body as the point 

 of application of the stimulus and on the opposite side. 



