23 
direction ; and that it is more scientific to examine the secreting 
structure of the kidney as a whole, than to make a pretty specimen 
showing only the red vessels with their malpighian tufts. Not that 
I intend to depreciate for one moment the value of injections of 
these tufts, but that I would impress upon you the necessity of 
studying them in conjunction with the tissues in which they are 
embedded ; for their purpose is to keep these tissues in repair, and 
they are, therefore, inferior to them—inferior, not only in function, 
but, if you will believe me, in beauty also. But it behoves me to 
enter, without loss of time, more immediately upon the plain facts 
of my subject. First, however, let me explain, that I have supposed 
the study of compound animal tissues to be far from general, and 
therefore propose to enter into elementary details concerning their 
preparation. If, however, I have been mistaken in this, I shall 
have to regret a sad failure in bringing before you this paper, which 
is only intended as a guide to those who have not studied, but are 
willing to study a subject, perhaps more than all others, replete 
with scientific interest. 'To commenee, then, at the very beginning, 
let us suppose we have obtained a portion of some solid organ— 
the kidney for instance. We must, in the first place harden it. 
Now, for hardening tissues, as wen as for cutting, staining, and 
_ mounting them, there are a great variety of methods; so many, in 
fact, that were I to attempt a short description of each, I should 
_ ‘leave no time for the rest of my subject. Therefore I propose in 
_ this, as in other steps of preparation, to describe only those which, 
__ having seemed to me the best, I have usually adopted. First, then, 
we place our piece of kidney, which should be about the size of a 
_ filbert, in equal parts of methylated spirit and water. In this it 
should remain for about 24 hours. Then transfer to a weak solu- 
tion of chromic acid, 1 per cent., or only half this strength for 
nerve tissues. In the course of a day or two, the chromic acid 
y solution must be changed, and again in two or three days more, 
and even a third time in the course of a fortnight, should the 
me hardening not progress satisfactorily. At the end of this time the 
_ tissue should have become hard and tough, for both these pro- 
_ perties are indispensable, since it is easier to cut soft substances 
_ than those which are hard and brittle. Well »* Supposing the 
chromic acid stage to have been satisfactory, we place it in methy- 
_ lated spirit for a few days to complete the process of hardening ; 
this final change adding very considerably to the firmness of the 
tissue. It is well, perhaps, to explain the object of these various 
_ changes during the process of hardening. First, it is intended, by 
_ placing the tissue in spirit and water, to avoid any softening 
_ through decomposition in the centre of the substance, since chromic 
acid does not permeate the tissue sufficiently rapidly to preserve it 
_ from decomposition in the centre of a thick piece. Moreover, the 
spirit and water washes out a quantity of blood and serous fluid 
