to try his hand on. In small animals, such
as mice, bats, and frogs, the whole circulation
of the system may be injected from the aorta,
and the pulmonary vessels from the
pulmonary artery. But, amateurs who do not
follow medical science as a profession, will
purchase better specimens of professional
preparers than they are likely to produce.
If several sets of vessels in the same
preparation (as the arteries, the veins, and
the gland-ducts), are required to be displayed
by injection, differently coloured substances
are employed. A white injection is
prepared from the carbonate of lead. Blue
injections do not answer well, because they
reflect light badly; to avoid that inconvenience,
Prussian-blue is sometimes largely
mixed with white, and so is vermilion
also. It should be remembered that these
preparations are mostly viewed as opaque
objects, and not by transmitted light. Small
portions of the injected organ are mounted
in cells, either dry or in fluid, according as
circumstances allow. Still, thin sections of
organs in which the capillaries are imperfectly
injected, may be mounted as transparent
objects, when they are better seen
than such as have been completely filled. In
general anatomy, the main point is to fill the
capillaries, and to try and make the injections
in such a way as that the several colouring
matters may be seen forced into the arteries
and the veins, touching each other, and more
or less mingled in the finest parts of the
organic network.

Injected preparations are the dearest to
purchase, the most difficult to make, and
the most difficult to study and interpret.
They demand the skilful exercise of the
anatomist's art; but, those who turn out
good injections are wrong in fancying, as
some seem to fancy, that nobody else can
produce equally good ones. The same
remark applies to the secrets of the composition
of the matter injected. With the precautions
which experience alone can teach, the practitioner
will succeed in making good injections
with whatever colouring-matter he habitually
uses in preference to others. The main point
of success is to employ the amount of time
and patience which the conditions necessary
for the work require. Whatever be the organ
injected, an hour and a-half or two hours
must be allowed to each set of vessels.
By hurrying the work, either the injection
fails to have the several colouring-matters
in contact with each other in the
capillaries, or ruptures take place. The
dissection of injections intended for microscopic
observation, like almost all dissections
effected by the aid of that instrument, are
performed under water. The exceptions are,
such tissues as are affected by the action of
water; thus, the retina is rendered white
and opaque by the action of water, instead
of semi-transparent; also tissues, as that of
the placenta and certain glands, which ought
to be examined while charged with blood.
It requires a lengthened study of an injection
to ascertain whether it has succeeded or no;
and several injections of the same tissue must
also be inspected. As in the study of the
anatomical elements by the aid of the microscope,
an observer must go through a certain
course of education before he can distinguish
in an injection what is of importance from
what is of none. Practice alone will enable
the learner to recognise the bundles of the
tissues, the follicles or little bags of the
glands, and the distribution and windings of
the vessels which accompany or cover them.
The same of the mucous membranes; the
undulations and anastomoses or
inter-communications of the capillaries, their
distribution around the glandular orifices; and
these orifices themselves cannot be properly
studied without devoting several hours,
sometimes several days, to their examination.
Consequently, injections shown to passing
observers are rarely well interpreted, unless
the persons to whom they are exhibited are
in the habit of looking at objects so prepared.
It is rare that they remember more than a
general idea of an elegant piece of coloured
network.

"But what is the use of attending to such
minutiæ?" an inexperienced reader may ask.
It is difficult to explain briefly the full
application of such elementary studies; but
one instance may be cited. That dreadful
disease, cancer, is known to most by name.
Now, there are other diseases of less gravity,
which resemble cancer so nearly, that the
practitioner cannot decide whether to operate
or not. The microscope distinguishes true
cancer from false, easily and infallibly.

Interesting anatomical preparations are
the pigment-cells from the iris of the eye—
the pigment-cells from a negro's skin,
resembling those in the tail of a tadpole;
transverse sections of hairs, human and
others, sliced like a cucumber, to show their
internal structure; transverse and perpendicular
sections of teeth, comprising a representative
of each great group in zoology;
fibrous membranes, commencing with those
of egg-shells; muscular fibre separated into
fibrilias; the capillaries in various organs;
sections of bone; preparations of morbid
tissues, for comparison with healthy ones;
and many others, which will naturally
present themselves to the student. One
object recommended for study will startle
many. Dr. Carpenter philosophically tells
us, "The nerve-fibres are readily seen in the
fungiform papillae of the tongue, to each of
which several of them proceed. These bodies,
which are very transparent, may be well
seen by snipping off minute portions of the
tongue of the frog, or by snipping off the
papillae themselves from the surface of the
living human tongue, which can be readily
done by a dexterous use of the curved
scissors, with no more pain than the prick of a