504 Time taken in passing Synapse in Spinal Cord of Frog. 



tion of the motor fibres. The difference of time between the two responses 

 was hardly altered by the administration of strychnine so long as the dose 

 was not strong enough to produce spasms or to affect the general circulation. 

 It was, however, slightly shortened, the cord-delay varying in normal cords 

 between 0*012 and 0*022 second, and in strychnine cords between 0*009 and 

 0*020 second. With stronger doses of strychnine the cord-delay was con- 

 siderably lengthened, probably as the result of impairment of the circulation. 



When the sciatic of the opposite side was stimulated and the excitability 

 of the cord raised just sufficiently for a reflex effect at all to be produced 

 by the stimulus, the cord-delay was always about double what it was in the 

 same-side reflex. In the strychnine cord of preparations the circulation of 

 which was still unimpaired, the extra-cord-delay in the case of the crossed 

 reflex became very rapidly shorter as the action of the drug continued and 

 might even be reduced to one-fifth of its original value in a preparation in 

 which the total cord-delay in the same-side reflex was hardly altered during 

 the experiment. It was, however, never reduced to less than 0*004 second. 

 In preparations in which the reflex responses were spasms, or in which the 

 circulation was impaired, it also was always long, although not usually 

 quite so long as the total cord-delay in the same-side reflex. In such 

 preparations in both kinds of reflexes the central stimulus often failed to 

 produce its maximum effect on the muscle at the beginning of the response. 



The effects of altering the temperature of the cord, and of altering the 

 strength of the artificial stimulus applied to the nerve, and of fatigue, on 

 cord-delay and on extra delay respectively in the two kinds of reflexes are 

 dealt with in the full paper. 



It is suggested that the method of measuring cord-delay employed gives 

 the clue required for ascertaining the number of synapses involved in 

 succession in any particular reflex, and that there is normally one synapse' 

 interposed in the conductive path of each individual fibre concerned in the 

 same-side reflex investigated, two in that of each fibre concerned in the 

 crossed reflex. It is pointed out that the numerical results obtained refer, 

 as a rule, rather to the modal time than to the shortest time in which a 

 synapse may be passed, and reasons are given for believing that the set of 

 synapses alone involved in the same-limb reflex, the primary synapses, are 

 wont to act much more synchronously than the set of secondary synapses 

 involved, in addition, in the crossed reflex, when the cord is normal. The 

 nature of the action of strychnine on the cord is also discussed. 



The experiments were made in Oxford, in the autumn of 1906, with the 

 aid of the Government Grant administered by the Eoyal Society. 



