South African Clawed Frog. 



45 



It must be borne in view that the decerebrate preparation suspended as in 

 the experiments is not very quiescent. Alternating with periods of quiescence 

 are periods of violent " struggling " movements. Presumably afferent 

 impulses initiated by the stretching are being received by the centres from 

 the sensory receptors of the limbs which are hanging extended under the 

 influence of gravitation, and these impulses cause the motor cells to discharge 

 at intervals. There will thus be periods of gradual increase in the excitability 

 of the motor centres ending in discharge. Perhaps the shortest times 

 recorded are due to mechanical stimuli which chance to be imparted at a 

 time when the motor centres are nearing their maximal excitability, and are 

 to be regarded as examples of what may be termed " hair-trigger " action. 



The Rejiex Nexus 'between Forelinib and Hindlivib. 



Some experiments were performed in which the reflex activity of the hind- 

 limb was recorded in response to a mechanical stimulus applied in the one 

 case to the hand of the same side, and in the other to the hand of the 

 opposite side. The electrodes were applied at knee and ankle and the hand 

 was subjected to the strong squeeze. On stimulation of the hand of the 

 same side the following delays were obtained from three decerebrate frogs 

 at room temperature of 22° C. : — (1) 16"4o-, sixty-eight days after operation ; 

 (2) 16"9 a, seventy-six days after operation ; (3) 16'7 a, thirty-eight days 

 after operation ; (4)- 23'9 a, thirty-eight days after operation ; and (5) 11-8 a, 

 forty-eight days after operation, giving an average delay of 17"1 a. On 

 stimulating the hand of the opposite side the following delays were recorded 

 from two frogs at room temperature of 22° C. : — (1) 17'o cr, forty-eight days 

 after operation and (2) 15 cr, thirty-eight days after operation. 



From these figures one cannot assume a difference in time between the 

 cases where impulse descends on the same side of the cord from the hand 

 stimulate, and where it has to cross in descending to the opposite side. 

 Further, the times recorded agree well with those obtained from a hind-limb 

 on stimulating the opposite foot. Tlie length of path to be ascribed to nerve 

 conduction from hand to spinal centre is found on dissection to be shorter 

 than that from foot to spinal centre by about 5 cm., so that we deduct 1'4 cr 

 from the average time of the latter reflex in order to compare with the former, 

 and obtain the same figure. 



We conclude that the reflex path from fore-limb to hind-limb, either 

 homonymous or heteronymous, contains the same number of synapses as the 

 path from foot to opposite hind-limb, viz. : three synapses with a delay at 

 each of about 3'7 cr. 



We may compare tliis path with that for the scratch reflex in the spinal 



