ME. J. L. CLAEKE ON THE OBEY SUBSTANCE OF THE SPINAL COED. 451i 
period varying from two or three to ten minutes, according to their thickness. They are 
then washed in pure spirit, after which, if thin, they are floated on the surface of spirit 
of turpentine, where they remain until they are quite, or nearly transparent, when they 
are removed to glass slides on which a little Canada balsam has been previously dropped. 
If now examined under the microscope, they frequently show but little or no traces of 
cells or fibres,-~a ch’cumstance which seems to have at first caused Scheodbe van der 
Kolk and some others to abandon the method ; but if the sections be set aside for some 
time and treated occasionally with a little turpentine and Canada balsam, the cells and 
fibres reappear and present a beautiful appearance. Before they are finally covered with 
thin glass, they should be examined at intervals by the microscope. If the sections be 
thick, I find it best to place them in a shallow vessel, the bottom of which is kept simply 
wet with turpentine, which can therefore ascend from below while the spirit evaporates 
from their u'pper surfaces ; for the 'principle of the method is this, — to replace the spirit 
by tm’pentine, and this by Canada balsam without drying the sections. The method at 
first presents some difficulties, and practice is necessary to ensure complete success. 
Experience, also, will suggest, according to cu’cumstances, many little deviations from 
the exact rules here given, which to a certain extent must be considered as general. 
For the last three years I have used chromic acid instead of spirit in the process of 
hardening. This is one of the modifications mentioned in my memoir “ On the Medulla 
Oblongata”*. The medulla of Man and the higher Mammalia is steeped in a solution 
of one part of crystallized chromic acid in 200 parts of water, for two or three weeks, 
and then kept in a solution of about one part of bichromate of potash in 100 or 200 
parts of water f. Spuit is used to wet the knife in making the sections, which are first 
placed in spirit for a few minutes, and then (with or mthout the previous use of acetic 
acid) transferred to the turpentine and Canada balsam, as before. 
Lenhossek, Gerlach, and quite recently Schroder van der Kolk, have adopted this 
method of rendering sections transparent. Lenhossek uses spirit for hardening the 
medulla, with some slight modification in other stages of the process. An entire series 
of very beautiful preparations of the cord and medulla oblongata have been purchased 
of him by the Eoyal College of Surgeons of London. Gerlach uses bichromate of 
potash in the process of hardening; then acetic acid, spirit, and Canada balsam J. He 
does not mention turpentine ; but if this be dispensed with, the Canada balsam must be 
very thin, and the section must be placed on its surface to allow of the evaporation of 
the spirit, which will not mix with the balsam, but in contact with it becomes turbid 
and opake. 
In his Essay on the Spinal Cord published in 1854, Schroder van der Kolk says that 
my (second) method, “ which appears to have been extremely successful,” did not suc- 
* 1857. Philosophical Transactions for 1858, Part I. 
t For the Eodentia, Birds, Beptiles, and Fishes, it is necessary to use the solution much weaker — about 
one part to 600 of water, and gradually increase the strength at the end of a week . 
X Mikroskopische Studien, p. 2, 1858. Erlangen. 
