292           THE MICROTOMIST' S VADE-MECUM
For the purposes of interstitial injection the method proves    sew
particularly valuable. The skin of the frog may be inter-          t
stitially injected from  the aorta by using sufficient pressure  Pth
to force the oil out of the capillaries. On preparing a por-
tion of the skin by corrosion it will then be found that the     O
lymphatics are more or less completely injected, and successful
preparations further demonstrate the system of fine tortuous
canals, leading from the blood-vessels to the lymphatics,
described by Arnold. The best preparations are those afforded.
by the skin of the inner surface of the hind leg, and it is
desirable to set up a state of cedema before injecting. A
ligature is placed between the abdomen and the thigh just
tight enough to cause hindrance to the return-flow     of the
lymph and venous blood, without suppressing the arterial
circulation. In two or three days the skin will be found suffi-
ciently hyperemic. The ligature is then removed, the apex        bt
of the ventricle cut off, and the frog thoroughly bled, one
aorta tied and oil injected from the other (the veins not being
tied) until pure oil alone flows from the open ventricle; the
veins are then tied and the injection continued. (For further
details and for the precautions it is desirable to take, as well 5tsses
as for the description of the results, I refer the reader to the
interesting paper from which I am abstracting.)
Good preparations of this sort may also be obtained from thefol
the mesentery of the frog and of Salamandra maculata (these
need not be corroded, as the membranes are thin enough to       to add
be transparent); and from Triton tcniatus, by corrosions of
the end of the tail.
The corneal corpuscles of Triton cristatus may sometimes      daYsiE
be injected from  the aorta, and in the rabbit excellent pre-   :teri
parations may be obtained from the cranial periosteum, the atyO
oil being injected (by the pressure obtained from a flask of tush
oil raised three metres high) into the carotid artery. (See       he
the details as above.)                                          8toodo
465. Impregnation.-The principle of this process is that      e1ithd