54 STAINING. 



catch the eye, which is then able to follow out with ease the 

 contours and relations of the elements to which the nuclei 

 belong; the extra-nuclear parts of these elements being 

 expressly left unstained in order that as little light as possible 

 may be absorbed in passing through the preparation. Pos- 

 sibly this may be an irrational procedure, but it has hitherto 

 been found in practice to be the most efficient for general 

 work. 



To these two chief kinds of selective stains may be added 

 a third group, the plasmatic stains, consisting of those few 

 stains that take effect on cytoplasm, or formed tissue, or 

 ground substance only, leaving nuclei unstained. In this 

 book, therefore, stains are looked upon as being (1) General 

 or Ground stains ; (2) Selective stains ; the latter group 

 being subdivided into (a) Nuclear, (6) Plasmatic, (c) Histo- 

 logically Selective, or Specific. This classification, however, 

 is not followed in the arrangement of the special paragraphs, 

 it being more practical to follow an order based on the 

 chemical nature of the staining agents, and on convenience 

 of exposition. 



Some writers have divided stains into nuclear, general, and selective. 

 This arrangement appears to me faulty because every nuclear stain is eo ipso 

 selective, and because it ignores the subdivisions of selective stains. 



92. The Methods of Staining. Colouring matters possessing 

 so great an affinity for certain elements of tissues that they 

 may be left to produce the desired electivity of stain, without 

 any special manipulation on the part of the operator, are un- 

 fortunately rare. In practice, selective staining is arrived at 

 in two ways. In the one, which may be called the direct 

 method, you make use of a colouring reagent that stains the 

 element desired to be selected more quickly than the elements 

 you wish to have unstained ; and you stop the process and fix 

 the colour at the moment when the former are just sufficiently 

 stained and the latter not affected to an injurious extent, or 

 not affected at all, by the colour. This is what happens 

 for instance, when you stain the nuclei of a preparation by 

 treatment with very dilute haematoxylin ; you get, at a certain 

 moment, a fairly pure nuclear stain ; but if you were to pro- 

 long the treatment, the extra-nuclear elements would take up 



