56 FUNDAMENTALS OF SUBMI C ROSCOPI C MORPHOLOGY I 



lecithin causes the molecules to arrange themselves in layers which 

 are similar to surface films, except that there are no mono- or oligo- 

 molecular layers but huge, microscopically visible structures consisting 

 of bimolecular lamellae. If the length of the pair of overlapping lecithin 

 molecules is about 50 A (Trillat, 1925/27), a wall of a myelin tube 5 (i 

 in thickness consists of some 1000 double layers (Fig. 47b). Water 

 continues to be absorbed until all the hydrophilic groups are saturated. 



T 



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o ° °- ° 



t 



> 



to o o r 

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 o _ o 00 L 



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o o 



o o 



!f^?^„° "o^'^tC 



c O O g "o O o 



o o o o o o o o 

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10 o o o o " 



B ----.1 



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Lecithin 



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Fig. 47. Myelin forms of lecitiain 



a) Submicroscopic structure. Hatched, hydrophilic; black, lipophilic part of the fork- 

 shaped lecithin molecule, b) Microscopic image and optics. 



thus causing further growth of the tubes. In course of time the myelin 

 forms therefore traverse the whole field of view under the cover glass 

 of the microscopic preparation. 



It can be proved by optical means that the lecithin molecules in the 

 myelin tubes are perpendicular to the surface. For, in a flowing solu- 

 tion (see p. 90) the lecithin molecules appear to be optically positive. 

 The myelin tubes, however, are optically negative with respect to 

 their long axis. From this it follows that the lecithin chains must be 

 oriented perpendicular to the tube axis. Bear and Schmitt (1956) 

 mention a formula (p. 86) from which the double refraction n^ — n^ of 

 the cylindrical myelin tube with its optical axis in radial direction can 

 be computed. For the myelin forms of lecithin in Ringer solution the 

 authors fijid n^ — n^ = 0.0039 (Schmitt and Bear, 1937). On further 

 absorption of water the lamellar structure of the myelin forms becomes 



