EXTENSOR EEFLEXES 117 



u delay in impulse transmission in the reflex cf. C Sherrington and S. C. M. Sowton, 



process situate in the spinal cord. Jul. of Physiol, vol. xlix, p. 331, 1915 ; N. B. 



Although its latency is longer and its Dreyer and C. Sherrington, Proc. Roy. Sac. 



threshold of stimulation higher than with JB. vol. xc, p. 272, 1918. For threshold cf. 



the break-shock applied to motor nerve, the E. L. Porter Am. Jnl. of Physiol. voL xxxi 



muscle contraction in reflex response is often p. 141, 1912. 

 much greater both in height and duration : 



EXERCISE XIX 



KEFLEX ACTION IN AN EXTENSOR MUSCLE (VASTOCEUREUS) ; POSTURAL 

 REFLEXES, AND REFLEX INHIBITION OF THE CROSSED EXTENSION 

 REFLEX 



[Parts of the exercise which may be omitted at its first performance are indicated in 



square brackets.] 



I. See that the decerebrate preparation provided is in good condition. 

 It should exhibit extensor rigidity, reflex ' standing '; this reflex posture in the 

 decerebrate condition is maintained even when the preparation lies on its 

 back or side ; the limbs ofi'er resistance to passive reflexion of their joints. 

 Examine this in the fore-limb. With the preparation lying on its side, place 

 one hand on the dorsal edge of the upper-lying scapula and, with the other 

 hand under the pads of the corresponding fore-foot, exert pressure such as 

 were the extensors of shoulder and elbow not acting would fold the limb. 

 A considerable resistance is felt, but this, when the pressure exceeds a certain 

 degree, almost suddenly yields. This is the ' lengthening reaction ', so called 

 because in it the extensor muscles yield, undergoing an inhibitory relaxation 

 which allows of their lengthening. In the hind-limb, test the resistance to 

 passive flexion of the knee. 



II. Get ready on the experiment table the myograph, two induction 

 apparatus for faradic stimulation and two pairs of stimulating electrodes. 

 The myograph-drum should be arranged for moving (right to left) at a rate 

 of about 1 cm. a second. In addition to your usual operating instruments 

 a large cutting bone-forceps is provided. The automatic break-key and the 

 striker attached to the drum-spindle for the previous exercise are not wanted 

 for this. But the drill and one drill-pin and a brass rod and heavy 

 standard for holding it are wanted : also the myograph and a light pulley 

 on a separate stand. See that the lever's writing-point is set at right angles 

 to the radius of the drum, as the extensions of the lever will be rather ample ; 

 if not at right angles to the radius the point tends to leave the drum-surface 

 when the arc is long. z 



