554 DR JAMES W. DAWSON ON 



Three groups of staining methods were used : — 



( 1 ) Methods for fibre-tract degeneration. The Weigert myelin sheath stain and 

 the Marchi method are of special value in giving a topographical distribution of the 

 areas, but they permit of very limited histological observations. 



(2) The elective staining methods. These render possible the isolated repre- 

 sentation of certain tissue constituents, but for this reason they do not give a 

 complete histological picture. The methods for representing fibre-tract degeneration 

 must also be regarded as elective staining methods. 



(3) The diffuse stains. These, on the other hand, e.g. Van Gieson's stain and 

 hsematoxylin and eosin, do not sharply distinguish between the individual elements 

 of the central nervous system, yet are of great value in giving an insight into the 

 detailed structural changes, especially when controlled by elective staining methods. 



The Kulschitsky-Pal modification was found to be the best of the many modifica- 

 tions of Weigert' s classical method of demonstrating the medullated sheath. This 

 has been used by numerous recent workers as being that which allows sufficient 

 differentiation of the medullary layers without decolorising the finest fibres of the 

 cortex. Weigert used as a test of a satisfactory differentiation the supraradial net- 

 work which lies between the tangential fibres and the pyramids of Ferrein : any stain- 

 ing which did not show this clearly he regarded as unsuitable. In the preparations 

 through the whole hemisphere it was found impossible to differentiate the medullary 

 layer sufficiently without decolorising this supraradial network, and an endeavour 

 was made to have two sets of preparations in different stages of differentiation. 



As a substitute for the Weigert method, with unmordanted celloidin sections 

 and frozen sections, Heidenhain's iron-hsematoxylin stain was used. This stain 

 brings out the finest fibres in the grey matter of the cord, and in the cerebral 

 cortex the fine fibre network is stained. Frozen sections thus stained are extremely 

 brittle, but sufficient may be left of the section for comparative examination. 



The Marchi method used was Orr's modification, in which the penetration of the 

 osmic acid is assisted by a small proportion of acetic acid. It was found, however, that 

 instead of bringing the thin pieces of tissue direct from Miiller's fluid into the Marchi 

 fluid, it was preferable to begin with a proportion of 3 : 1 of Miiller's fluid and Marchi 

 fluid, and, later, increasing the proportion of Marchi fluid to finish with Marchi fluid 

 of full strength. The results in tissues thus treated were invariably constant. 

 The Marchi-stained sections were frequently counter-stained with safranin. 



For the staining of the axis cylinders two specific staining methods were 

 employed. Cajal's silver impregnation method (for myelinated axis cylinders) was 

 used when the tissue could be fixed directly in 96 per cent, absolute alcohol : when 

 the tissue, however, had been already placed in formalin or in Miiller's fluid, the 

 Bielschowsky method, or, in the latter case, the Bielschowsky- Williamson method, 

 w;is used. These staining methods depend on the reducing action of formaldehyde, 

 which acta on an ammoniated silver solution and deposits metallic silver. It stains 



