CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
929 
the quicker the hardening process goes on. It appears that it is not desirable that 
the specimens should be kept in the same solution more than one month. 
This method is a modification of that proposed by Hamilton/" and is much more 
convenient than the process which I formerly used, which was to place the brain first 
in absolute alcohol and iodine and then in chromic acid. 
The Muller’s fluid and spirits of wine has very great penetrating power, so that it 
is not necessary to remove the brain entirely from the skull, but two or three holes 
made in the latter are quite sufficient. So far both methods are equal; but the latter 
has a great advantage inasmuch as it does not cause any contraction in the nervous 
tissue ; on the contrary, the brain swells and becomes enlarged so as partially to 
project through the holes in the skull. The hardening process takes a long time to 
be effected. The brain requires at least six months to become sufficiently firm to be 
submitted to the section knife; but it remains in that state for twelve months 
longer, more or less. 
Here a curious thing is to be noticed, indicating, probably, different chemical com¬ 
position in brains of various species ; for instance, the brain of Mormyrus is still in a 
good condition for making thin sections after having remained eighteen months in the 
hardening fluid, while those of some other Teleostei, Gurnard or Ballan Wrasse, for 
instance, are too friable to make satisfactory sections after twelve months. 
In consequence of this slow method of hardening the nervous tissue, the histological 
elements are not contracted or distorted, but show much better the form which they 
possessed during life; so that we see none of those angular nuclei or cells which 
MEYNERTt reports that he has seen in sections of the brain of Fishes. 
Another advantage especially in travelling is, that there is no necessity, as there is 
with absolute alcohol and chromic acid, to make sections soon after they have been put 
into the hardening fluid on account of their becoming too friable; but they can be 
safely left until a convenient opportunity occurs of attending to them. 
When the brains have become sufficiently hard, sections of any degree of fineness 
can be made in the microtome ; and after having been stained with rosaniline or 
carmine, they can be cleared, with oil of cloves and preserved in Canada balsam. 
RanvierJ and other writers on histology recommend that before the application of 
the oil of cloves, the sections should be immersed in alcohol; this is not at all 
necessary for the nervous system, since the oil of cloves renders the sections quite 
transparent even when they are taken direct from the watery fluid; the only precaution 
required being to absorb all superfluous moisture with blotting paper. This makes one 
operation less to perform, and it is obvious that when a hundred sections or more have 
to be placed in consecutive order, the fewer processes they have to go through the 
better. 
* Journal of Anatomy and Physiology, vol. xii., 1878. 
t Stkickeb’s Handbook, Sydenham Society, vol. ii. 
| Traite Technique d’Histologie, p. 112. Paris, 1875-78. 
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