PRINCIPLES OF STRUCTURE 



57 



63.5^^-- 



increasingly pronounced. Finally, the positive intrinsic double refrac- 

 tion of the molecules is overcompensated by the negative double 

 refraction due to the lamellar texture (see p. 87,) and the sign of the 

 myelin birefringence is reversed (Nageotte, 1936). 



The absorption of water can be followed by means of X-rays. The 

 dry myehn substances 

 obtained from evapor- 

 ated benzene solu- 

 tions give X-ray in- 

 terferences which cor- 

 respond to twice the 

 chain length (lecithin 

 and cephalin 44A, ster- 

 ol 34 A, sphingomyelin 

 and cerebroside 65- 

 67 A; ScHMiTT and 

 Palmer, i 940) . If water 

 is added to these lipids, 

 the X-ray periods are 

 enlarged and so allow 

 of an evaluation of the 

 thickness of the water 

 lamellae formed. It can 

 be seen from Fig. 48 

 that the original period 

 of 63.5 A of mixed 

 nerve lipids has be- 

 come 150 A at a water content of 75%. This implies a water layer 

 of 86 A between the bimolecular lipid layers. 



The myelin forms offer a good example of the manner in which 

 complicated microscopic structures can result from a simple arrange- 

 ment of submicroscopic entities. They show, however, that no co- 

 ordinated growth is possible as a result of such a process, for the 

 myelin forms "grow" at random aimlessly in the substrate and the 

 final outcome is a chaos rather than an illustration of organized life 

 (Fig. 46). 



Fig. 48. Water intercalation between bimolecular lipidic 

 films. Size of the adsorbed water layer with increasing 

 water content. The black points correspond to o%, 25%, 

 50%, 6-j°o and 75% of water content (from Schmitt 

 and Palmer, 1940). 



