NERVOUS SYSTEM 165 



neuropile extrusion of these vesicles through small pores in the 

 axoplasmic membrane w^as noted. This finding of vesicles only on 

 the pre-synaptic side suggested that some degree of polarization is 

 present, despite the electrophysiological evidence to the contrary, 

 but Hama (1959) demonstrated that such vesicles are to be found 

 on both sides of the septa, pre-synaptic and post-synaptic. Thus on 

 histological grounds there is no evidence for polarization. Hama 

 (1959) also considers that the myelin-like lamellae figured by 

 earlier workers are absent, and indeed the layers of the septa are 

 extremely thin (Fig. 52). 



Although the discontinuities in the axons, known as septa, are 

 composed of thin layers and appear to have a similar construction 

 on both sides there is little knowledge yet of their effect upon the 



L ateral 

 giant 



High 



1 

 Septum 



Fig. 53. Possible explanation of functioning of septa in giant 

 fibres of L. terrestris (based on ideas of Kao and Grundfest, 1957). 



transmission of the impulse along the axon. They must allow the 

 conduction across the gap to occur with the utmost rapidity and 

 pose little in the shape of a barrier to the potential. 



A possible explanation of this phenomenon comes from Kao and 

 Grundfest (1957) by analogy with another giant fibre system, that 

 of the crayfish. Because of the relatively short length of the 

 individual segments electrotonic spread can account for the spread 

 along the individual sections. The septa are considered to be 

 ephapses, not true synapses, that is the simple physical adjunction 

 of nerve fibres from two segments. In the crayfish there is a low 

 resistance in one direction and a high resistance in the other along 

 the length of a giant fibre so that conduction proceeds in one 

 direction only. If a similar set of resistance conditions are present in 

 the earthworm it may account for functional polarity in the living 



