118 



Inside the Living Cell 



demonstrated by putting electrodes on two parts of an exposed nerve 

 and connecting them to an electrometer. These impulses can even 

 be detected outside the body using the sensitive electronic amplifiers 

 now available, yet it has been found that the analogy with electric 

 currents in metallic conductors is not at all close. The nerve fibres 

 themselves are rather compHcated structures. When examined in the 

 living state, or recently excised, they consist of long cylindrical 

 threads, usually 2-20 fx in diameter, although much larger ones are 

 sometimes found. Every nerve has grown out of a nerve cell and as 

 a kind of extension of it (Plate 15). Each fibre of a typical nerve con- 



Fat 



Molecules 



Protein 



\ / Protein 



A single 



lipo-protein 



layer 



Core or axon 



Myelin sheath 

 (concentric layers 

 of lipo-protein) 



FIG. 21. A nerve with myelin sheath 



sists of a *core* or axon, which is enclosed in a membrane. In the 

 case of 'myelinated' nerves, the core is covered by a glistening sheath 

 of a white substance called myelin which has been shown by electron 

 microscope pictures to consist of a large number of concentric layers 

 of material. This material is a combination of protein with a fatty 

 substance, which together constitute a 'lipo-protein'. It has been 

 found that the molecules of the fat are enclosed between thin layers 

 of protein and the long hydrocarbon chains of the fatty substance 

 are arranged with their lengths pointing outwards from the centre 

 of the fibre (see Fig. 21). Inside this myelin sheath is a soft jeUy- 



