PROTEINS 



363 



substantiated by electron micrographs (Sjostrand, 1949); and in this 

 monograph (Fig. 151b, p. 259) such an arrangement has been shown 

 to be probable in the microstructure of the chloroplasts. It looks, 

 therefore, as if submicroscopic lamellar protein-lipid systems of the 

 kind are fairly common in biological material. 



The axon. The protein of the nerve axon has been termed neuronin 

 (Bear, Schmitt and Young, 1937). In the living nerve it constitutes 

 only 3-4% of the fibre weight, the rest being an aqueous solution. 





r 



Fig. 180. Submicroscopic structure of an internodal segment of a myelinated nerve fibre 



(from Fernandez-MorAm, 1950, 1952a). N neurilemma, M myelin sheath, Ax axolemma, 



A axon, C collagen fibrils, E dark smooth fibrils. 



It is for this reason that X-ray absorption micrographs record 5 to 8 

 times less mass in the nerve axon (0.05 X io~'^'^ gj fji^) than in the 

 myelin sheath (0.3 to 0.4 X lo'^^g^^s. Engstrom and Luthy, 1949, 

 Engstrom and Lindstrom, 1950). This high dilution makes it un- 

 certain whether the axoplasm exists as a sol or as a gel in the living 

 state. Flaig (1947) reports that its viscosity is considerably increased 

 during nerve activity, indicating a sol — gel equilibrium similar to 

 that involved in protoplasmic flow. 



In fixed axoplasm, neurofibrils become visible. Their diameter 

 ranges down to the resolving limit of the ordinary microscope. 



