02 WILLIAM TOWNSEND PORTEE. 



must influence our conceptions of the form and the purpose 

 of these structures in the peripheral fibres. 



In order to prove that constrictions are present in the 

 central nervous system^ it is necessary to demonstrate them in 

 silver and in osmium preparations, in isolated fibres and in 

 sections. 



Silver Preparations — teased. — Small pieces of the 

 white sustance of the spinal cord treated with silver-osmium 

 solution give these results : the colour ranges from pale grey 

 to deep black, according to the size of the piece, the strength 

 and quantity of the osmic acid, the duration of its action, the 

 distance of the fibre from the surface of the piece, and the 

 thickness of the myelin. Beneath the outer darker parts of 

 the fragment used is a portion whose pale grey colour and soft 

 consistence show that the osmic acid has affected it but slightly. 

 It is here that the silver crosses are seen at their best, for the 

 silver solution readily penetrates the entire piece, and its deep 

 brown impregnations contrast strongly with their grey sur- 

 roundings. The teasing should not be too thorough ; the 

 nerves are easily broken at the constricted points, and even in 

 the most careful preparations many will be found with the 

 silvered constriction torn in two. In the grey fibre-groups are 

 numerous axis-cylinders cross-striped with Frommann's lines, 

 and sticking out in all directions from the edges of the group. 

 Small granular silver masses, of a great variety of forms, may 

 at first be taken for constriction stains ; some of these are 

 probably the remains of broken crosses, and others are pre- 

 cipitations between the fibres. The medullary sheath is poorly 

 marked, often quite unrecognisable, and again only indicated 

 here and there by faint lines, more or less parallel to the axis- 

 cylinder. Well-shaped crosses are hard to find, but crosses 

 whose sharpness of outline is somewhat blurred by the silver 

 granules lying about them are common. A sceptical observer 

 will sooner or later be convinced by finding an unmistakable 

 cross in a fibre whose medullary sheath is still recognisable, and 

 whose isolated position guards against deception. The lines of 

 Frommann have in my preparations been seen to best advan- 



