28 The Preparation of the Brain and 



it be more convenient to cut it at a future time, the tissue should 

 be transferred to methylated spirits and kept in it till sections can 

 be cut. 



It is important not to allow the tissues to remain in the harden- 

 ing fluid too long ; because they finally lose the toughness which 

 they acquired under the influence of the acid, and become brittle. 

 Moreover, it does not colour readily if it be too long in the chromic 

 acid. The action of the chromic acid on the tissues is, as Dr. 

 Eutherford observes, somewhat analogous to the process of tanning, 

 with which all of you are familiar. In the process of tanning the 

 tannic acid forms a solid substance with the gelatine. In the case 

 of the hardening of nerve tissue by chromic acid there is probably 

 an analogous change produced. 



The cord then is placed in chromic acid, the cerebrum and cere- 

 bellum in the mixture of chromic acid, potassium bichromate, and 

 water, well surrounded with fluid and kept free from dust. After 

 a time, varying with the density of the tissues, they become of a 

 yellowish tinge, quite hard and not yielding when gently pressed 

 between the fingers. 



The hardening fluid in the case of the cord must be changed 

 after the first twenty-four hours, and fresh fluid added, and also 

 changed during the process of hardening once or twice. 



In the case of the cerebrum and cereljellum the strength of the 

 solution must be doubled after the first fortnight. 



Secondly, with regard to making sections of the hardened 

 tissues. 



The sections are sliced by means of a razor, with or without a 

 section machine. 



Some profess to make sections of tissues as well, or better, 

 without machines as with them. 



If the piece of tissue be as small as a " hean" and if it be 

 embedded in a hard substance, such as parafiin or wax, a machine 

 may be dispensed with, although even in this case the use of the 

 machine is advantageous. If, however, we desire to make uniformly 

 thin sections of the entire spinal cord, or brain, say of a rabbit or a 

 cat, then the machine is of great service, for it enables one to cut 

 slice after slice the same thickness with a rapidity and a precision 

 which contrasts very agreeably with the repeated failures, the waste 

 of time and tissue, which even accomplished histologists experience 

 when they do not use a machine in these cases. 



The indicator for graduating the thickness of the sections, and 

 the freezing apjmratus, which act perfectly, are Dr. Eutherford's 

 modifications of Mr. Stirling's machine. The machine made by 

 Hawksley has been materially altered by Dr. Eutherford, and the 

 only machine approved by him is made by Baker. 



The tissue is embedded in the machine in a mixture of lard 



