364 FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES III 



However, in the electron microscope much finer filaments with 

 100-200 A diameter are visible (Fernandez-Moran, 1952b). 



Like other fibrous protein substances, the neurofibrils are posi- 

 tively uniaxial, but their birefringence is very weak and is pushed into 

 the background by the very strong anisotropy of the myelin sheath 

 (Fig. 179a). The axoplasm shows form birefringence (Bear, Schmitt 

 and Young, 1935). When heated, it shrinks lengthwise, like col- 

 lagen fibres (Schmitt and Wade, 1935). 



The intrinsic birefringence of neuronin is 0.005, which is near to 

 that of myosin (0.008). The mean refractive index as indicated by the 

 minimum of the form birefringence curve amounts to 1.57-1.60^ 

 a value which coincides with that of neurokeratin 1.58 and muscle 

 myosin 1.576 (H, H. Weber, 1934). 



Fine-stnicture of nerves. Thin sections have yielded very instructive 

 electron micrographs which settle several controversial points of 

 nerve cytology. A distinct neurolemma which envelopes the myelin 

 sheath is visible. At the nodes of Ranvier the axon is constricted but 

 not intercepted (RozsA, Morgan, Szent-Gyorgyi and Wyckoff, 

 1950a, b). 



Fernandez-Moran (1950, 1952a) has compiled the results of his 

 electron microscopic studies in a diagrammatic outline which is re- 

 produced in Fig. 180. 



There is a 200 A thick granular neurolemma (N) with dark smooth 

 fibres (E) which resemble elastic fibres, and adhering cross-striated 

 collagen fibres (C). The sheath (M) consists of about 50 thin con- 

 centric lamellae with an average periodicity of 80 A. The interlamellar 

 spaces are locally inflated. The sheath is separated from the axon by 

 a reticulate membrane, the axolemma (Ax), formed by beaded 

 filaments 100-200 A in width. In the axis cylinder a verv fine reticulum 

 is visible. 



g. Fibrillar Proteins. Recapitulation. 



The important frame proteins are of the fibrillar type. Their poly- 

 peptide chains have a strong tendency to crystallize by forming chain 

 lattices. X-ray diffraction studies have disclosed two types of axial 

 spacings in these lattices, which have been classified as the keratin- 

 myosin and the collagen group (Astbury, 1947; Marks, Bear and 

 Blake, 1949). 



