50 



Reflex Times in the South African Clawed Frog. 



experiments the crossed reflex time is the shorter, it is concluded that the 

 intraspinal paths of the two reflexes are similarly constituted as regards 

 number of synapses, that is to say, each path normally involves one synapse. 



In the second class, where the frog is not in the best condition, either on 

 account of a recent operation or from long survival since operation, the 

 short path is not open for the crossed reflex, and a longer path, containing 

 two additional synapses, or three synapses in all, is employed. The average 

 heteronymous time is here 23'7cr, and the average homonymous 15"7o-, 

 giving an excess delay for the crossed reflex of 8 cr. 



The delay at a synapse in the spinal cord is found to be about 3*7 a. 

 This is considered to be referable to a single synapse, but it is not intended 

 to exclude the possibility that it represents the delay at a set of synapses. 



The electrical variation recorded from the limb muscles is similar to that 

 of a twitch followed by a series of rapid oscillations. ^ 



In the decerebrate and in the intact frog, a group of delayed crossed 

 responses from the hind limb are described, having delays ranging from 

 43 o- to 92 cr. It is suggested that these involve the action of the lower 

 parts of the brain. 



Spinal reflexes are recorded from the decerebrate frog, the average time of 

 the heteronymous reflex being 18-5 cr, and of the homonymous 10-7 o-, giving 

 an excess delay for the crossed reflex of 7*8 a. There is evidence that in 

 some same-side reflexes the delay is longer than the average mentioned, 

 being 14'4 a, and the excess delay of the crossed reflex is then about 4 a. 



It is considered that, normally, with a strong mechanical stimulus, the 

 intraspinal path of the homonymous reflex in the decerebrate frog involves 

 one synapse (although in some cases it may involve two), while the 

 heteronymous path contains three synapses. 



The path followed in the reflex activity of the hind limb, evoked by 

 stimulation of the fore limb, both homonymous and heteronymous, contains 

 three synapses. 



The electrical variation recorded from the hind limb muscles of the 

 decerebrate frog shows a series of oscillations at its commencement, at rates 

 reaching 277 per second. It is suggested that these are the expression of 

 irradiation within the spinal centres. 



I have to express my indebtedness to Dr. C. Lawrence Herman for his 

 kindness in preparing a considerable number of frogs by decerebration and 

 cord section for use in the investigation, and to my students, Mr. C. A. 

 Oosthuizen and Mr. H. Zwarenstein, for assistance from time to time in the 

 experiments. 



