PRACTICAL EXERCISES 885 



roots, which are smaller than the posterior. Stimulate the central 

 end of one : there is no effect. Stimulation of the peripheral end 

 of the other causes contractions of the corresponding muscles. 



(b) Stimulation of the roots may be repeated on the mammal, 

 using the dog employed for the experiment on the motor areas 

 (p. 889). 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 corresponding 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 move- 

 ments 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 contractions 

 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 stimulated nerves or below that level. 



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

 determine whether reflex movements are elicited by a single induced 

 shock applied 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 leg into a beaker of 

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

 beakers with water at 10 C., 20 C., 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 temperature. 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 

 blotting-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 in- 

 creasing the area of the field stimulated, and observe the extent 

 and order of spread of the reflex movements. 



