no 



DISPERSE SYSTEMS 



parallel to one another and at right angles to the myelin sheath 

 (Fig. 26 (/) ). If now the surrounding water be made slightly 



alkaline, say, altered from pH 7 to 7-4, 

 the coil will steadily unroll (Fig. 26 (b) ) 

 till it assumes the appearance in Fig. 

 26 (c) ). Fig. 26 (d) indicates that it is 

 still a double structure. This stretching or 

 unrolling, due in the first instance to in- 

 creased alkalinity on the surface of water 

 and soap, is carried out by the in- 

 creased internal tension developed in the 

 myelinated soap by the imbibition of 

 water. The addition of a quantity of a 

 weak acid just sufficient to ensure that 

 the water is merely acid (e.g. pH 6-9) will 

 cause the processes to retract (Fig. 26 (e)). If a greater acidity 

 is developed the sheath will be ruptured and the crystals dis- 

 seminated and dissolved. Leathes has demonstrated that lecithin 

 and cholesterol oleate, compounds very widely distributed in the 

 body, readily show the development of myelin forms. (See also 

 Chap. XL, Membranes.) 



Bragg and others have shown that by means of X-rays, diffrac- 

 tion patterns of crystals can be obtained. For instance, a single 



Fig. 25. — Myelin outgrowths from 

 lecithin after 24 hours in equal 

 parts of n'lOO calcium and sodium 

 hydro.Kides. (Courtesy of Trofessor 

 Leathes.) 



Fig. 26. — Myelin forms of Ammonium Oleate, viewed in convergent polarised light. 

 See text. After J. H. ( lark, AiiiiTiain Journal of Physiology. 



solid crystal gives a regular interference pattern of sharply defined 

 spots round a central image. If. now, we have a large number of 

 small crystals regularly arranged (as in (/) Fig. 26), the diffraction 



