Histology and Embryology. By G. S. Minot. 99 
indispensable. It may be acquired by patient practice even by 
those who have no special manual skill, just as we are all able to 
write. There are many things which cannot be cut with a mac! line. 
The razor for cutting should be of the best quality, and when used 
always drawn towards the body, while the surface, which looks 
downward in cutting, must be flat. The edge must be perfect, the 
slightest notch being sufficient to tear a section to pieces, and so 
sharp that a human hair can be split with it. The sections them- 
selves must be as thin as possible. 
Since all parts of the body, with few exceptions, such as the 
skeleton, &c., are soft and permeated by water, besides possessing 
great elasticity, they cannot be cut in their natural condition ; it 
becomes necessary, therefore, to harden the organs. Now protoplasm 
is the main constituent of cells, and itself consists chiefly of albumen. 
This substance can be coagulated by the action of various agents, 
some of which can be applied to the tissues without injuring them, 
to produce a coagulation of the albumen in its natural form within 
the cells. 
Alcohol is one of the most valuable agents for this use. It 
produces its effect by its strong affinity for water, which it can 
withdraw from the tissue, thus causing the albumen, which requires 
an abundance of water to maintain its semi-fluid state, to solidify. 
It may be employed for the majority of tissues with perfect success. 
The volume of alcohol should be from twenty to thirty times that of 
the object to be hardened ; weaker alcohol, say of 80 per cent., should 
be used first ; after a sojourn of an hour or two, or even longer, if 
large, the object may be transferred to stronger (96 per cent.) 
spirit, and there left for twenty-four hours, more or less, according 
to the size of the piece. The great difficulty in the use of alcohol 
is to prevent the shrinkage which naturally follows upon the ab- 
straction of the water from the tissues. This may be avoided by 
using first weak, and then strong, and finally very strong spirit. 
In some cases the action is not even then sufficient, and recourse 
must be had to absolute alcohol, which generally produces the 
desired result. 
"When even that does not succeed, the specimens may be put in 
picric acid (concentrated cold aqueous solution) for twenty-four 
hours, then in a syrupy solution of gum arabic for twenty-four 
hours, and finally in strong alcohol again for the same length of 
time. The picric acid removes the alcohol, and allows the gum to 
penetrate the object, within which it is finally coagulated by the 
last dose of spirit. The sections when made must be left in water 
for a day, to dissolve out the gum which they still contain, and 
which renders them quite opaque. A very few drops of strong 
carbolic acid may be added to the water to prevent the develop- 
ment of bacteria, &c., which w r ould quickly ruin the preparations. 
