158 THE PHYSIOLOGY OF EARTHWORMS 



the lesion. He therefore concluded that the median giant fibre 

 conducts from anterior to posterior and that the lateral fibres 

 conduct in the reverse direction. 



The first electrophysiological experiments carried out on these 

 fibres (Eccles, Granit and Young, 1933) showed that conduction 

 occurs equally well in both directions in both median and lateral 

 giant fibres. When the fibres were stimulated electrically there was 

 an all-or-none response above threshold that gave rise to two 

 impulses, one travelling faster than the other. The faster of these is 

 that of the median giant, the slower is in the lateral fibres. The 

 observation that only one impulse occurs in the lateral giants 

 indicates that the transverse connections are physiological as well 

 as anatomical. 



This result posed a problem that even now has not been properly 

 resolved. Stough (1926, 1930) considered that polarization of the 

 giant fibres was the function of the oblique septa which cross the 

 fibres at each segment. They slope antero-posteriorly in the median 

 giant, and postero-anteriorly in the lateral giants. They are histo- 

 logically complete and divide the giant fibres into a series of blocks 

 of tissue which are segmentally arranged. These synapses, if such 

 they are, do not conduct in one direction as was shown by Eccles, 

 Granit and Young (1933). This is an unusual situation and even 

 more unusual is the fact that the delay time taken for an impulse to 

 cross from one side of the synapse to the other is very short 

 indeed and considerably less than normal vertebrate synapses, 

 since the delay involved in crossing 50-100 synapses is only 5 

 msec (Bullock, 1945). 



The action potentials of the giant fibres are extremely large and 

 Rushton and Barlow (1943) were able to record them from the 

 external surface of the animal without any need for dissection. The 

 speed of conduction has been estimated as 17-25 m/sec in the 

 median giant, (Eccles, et al, 1933; Bullock, 1945; Rushton, 1945) 

 and 7-12 m/sec in the lateral giants, at a temperature of 10- 

 12 °C. 



The conduction velocity of these fibres can be changed by 

 alteration in the temperature. A rate of 18-8 m/sec in the median 

 giant at 22-3 °C can fall to 10-8 m/sec at 8-2 °C. In the lateral giant 

 fibres the comparable rates were 9-6 m/sec and 5-7 m/sec respec- 

 tively, a decrease of similar proportions. The mean Qio is 1-52 for 



