184 PREPARATION AND MOUNTING 
quickly when requisite, that no mere machine can compete 
with it, however well contrived. 
22. When the beginner attempts to inject a subject, one 
of his difficulties is finding the vessel from which to com- 
mence. Another consists in distinguishing the arteries 
from the veins; but this is partly removed by making a 
longitudinal incision in the vessel, and with a blunt thick 
needle probing a little distance into the tube. The artery 
will be found thicker in the coating than the vein, and the 
difference is easily perceived by this mode of testing: the 
vein is also of a bluer colour than the artery. I say above, 
a “longitudinal incision ” must be made: the reason for this 
is, that an artery when cut across contracts considerably, and 
is lost in the adjoining substance; but where the opening 
is made longitudinully all danger of this contraction is 
obviated. 
23. The different systems of vessels are often injected 
with various colours, so that their relative positions, 
may be shown more clearly. In some specimens, the veins 
are injected with white, and the arteries with red; in the 
kidney, the urinary tubes are often filled with white, and 
the arteries witn red. Then, again, the liver affords tubes. 
for three or four colours. But no written instructions on 
this point can benefit the young student, and he must be 
content for a while to employ himself with single colours 
until he has gained the mechanical skill and the primary 
knowledge which are necessary before he can make any 
advance. 
24. We will now consider the best methods of mounting 
injected objects. They must always be well washed in 
water after they have been kept in any preservative liquid, 
using a camel-hair pencil to clean the surface if necessary. 
Many parts when injected are in masses, such as the lungs, 
liver, &c., of animals, and consequently sections of these 
must be cut. For this purpose Valentin’s knife is very 
convenient, as the thickness can be regulated so easily; but 
where the injections are opaque, there is no need to have the 
