If I had to venture a guess:
They're referring to fixation [0], or preservation of the tissue, and not to affixation, the attachment of something.
So, basically, they fixed the tissue prior to removing it from the skull, causing it to keep form. Whereas, if they removed the tissue from the skull and then chemically fixed it, the tissue would have already lost form and the lymph would be undetectable (no longer be in original form).
Think about a cake in a pan. Baking = fixing. Remove the cake before "fixing" it, and it's a plot of mess. "Fix" it first, and you have a free-standing cake, it'll hold shape. Bad analogy, sorry. :)
[0] http://en.wikipedia.org/wiki/Fixation_(histology)
Edit: forgot reference.