150 
IOWA ACADEMY OF SCIENCES. 
which is very deceptive. It may, however, be combined with 
other reagents with superior results. When added to picric acid 
there is given one of the most delicate fixing agents yet imag- 
ined; one which appears to faithfully preserve every detail of 
structure, and which also permits of subsequent treatment in 
any desired way. Mixtures of formaldehyde, chromic acid, and 
acetic acid; or of formaldehyde, platinic chloride, and acetic 
acid are also very desirable. The principle involved here 
appears to be that formaldehyde may often be advantageously 
substituted for osmic acid in such mixtures on account of its 
superior penetration and the absence of a tendency to over- 
fixation. In all these cases formaldehyde is to be used pure, 
not diluted. 
III. FORMALDEHYDE IN NEUROLOGICAL WORK. 
I have been impelled to make a critical examination of neuro- 
logical methods in connection with a certain line of investigation 
in which I am engaged. Of course the technique employed in 
the study of any nervous system is necessarily highly special- 
ized, but the following notes have a general application. For- 
maldehyde may justly claim a place in neurological methods. 
Its chief uses are: 
First — It is an excellent hardening agent for the brain, where 
anatomical methods alone are to be employed. It hardens with 
surprising rapidity, so that after a week or ten days a fairly 
large brain can be thoroughly studied. 
It also preserves the form and color of the several parts. Its 
only undesirable effect lies in the increase in volume which is 
given by a solution of just moderate strength. 
This tendency to swell the parts may be lessened by the use 
of a strong solution, one containing 10 to 20 per cent of the 
commercial article. It has also been recommended by various 
workers that a mixture of formaldehyde and alcohol be used, 
the tendency of the latter to shrink tissues, offsetting the swell- 
ing action of the former. Messrs. Parker and Floyd believe 
that they have struck the proper balance in the following mix- 
ture: 
95 per cent alcohol 6 volumes, 
2 per cent formaldehyde. 4 volumes, 
in which a barely perceptible increase in the size of the brain 
occurs. I believe that it is well to double the strength of the 
formaldehyde in this mixture, and I am accustomed to do so in 
my own work. 
