NERVOUS SYSTEM 159 



the conduction velocity of the median giant, and 1 -46 for the lateral 

 giants. The giant fibres continue to conduct impulses over a 

 temperature range of —5 °C to + 36 °C, both of which tempera- 

 tures are beyond the survival limits of the intact animal (Hogben and 

 Kirk, 1944). Temperatures above +40 °C or below -5 °C 

 produce irreversible changes and conduction ceases, failing in the 

 median fibre before the laterals in the majority of cases (Turner, 

 1955). 



The early results of Eccles et al. (1933) on the electrical proper- 

 ties of the giant fibres were confirmed and expanded by Bullock 

 (1945). Two waves representing the median and lateral giant 

 fibres were observed after stimulation. Rushton (1945) showed that 

 the fast wave is characteristic of the median giant by applying a 

 pressure block to this fibre and noting the disappearance of the 

 fast response ; he dealt similarly with the lateral fibres and noted 

 the absence of the slow wave. On cutting the ventral nerve cord 

 laterally, alternately left and right at intervals of ten segments he 

 found that both waves were still functioning but the conduction 

 time of the slower wave increased by 0-8 msec for each section after 

 the first, and the potentials in each lateral get out of phase with one 

 another despite the cross connections between them. But if a 

 sufficiently long length of nerve is examined, say over 100 segments, 

 the wave which should be slowed down in fact is able to catch up 

 and fuse with its fellow. 



Bullock (1945) was able to show that although the giant fibres 

 may not be polarized and can conduct impulses in either direction 

 they are unable to do so while in the intact animal. Mechanical 

 sensory stimulation of the anterior segments, back to and just 

 beyond the clitellum, gives rise to potentials only in the median 

 giant. Stimulation posterior to this region and as far back as the 

 rear extremity elicits activity only in the lateral fibres (Fig. 50). 

 Adey (1951) found that essentially the same results were obtainable 

 from Megascolex sp., the critical switch of conduction in the giant 

 fibres occurring at about segment 65. This distinction of direction 

 of conduction, not a function of the giant fibres themselves, can 

 therefore only be due to the anatomical connections between the 

 sensory input and the giants. Anterior sense organs make contact 

 with the median giant and posterior sense organs contact the 

 lateral giants (Rushton, 1946). 



