We have now considered most of those objects which require any _peculiar_ treatment in section-cutting, &c.; but in no branch of microscopic manipulation is experience more necessary than in this. CHAPTER VI. INJECTION. 1. Injection is the filling of the arteries, veins, or other vessels of animals with some coloured substance, in order that their natural arrangement may be made visible. This is, of course, a delicate operation, and needs special apparatus, which I will now attempt to describe. 2. _Syringe._--This is usually made to contain about two ounces. On each side of the part next to the handle is a ring, so that the finger may be thrust through it, and the thumb may work the piston as in an ordinary syringe. The plug of the piston must be “packed” with wash-leather, and fit so closely as to be perfectly air-tight; and if, when it has been used awhile, it is found that some of the liquid escapes past the plug into the back part of the body, it must be _repacked_, which operation will be best understood by examining the part. These syringes are made of various sizes, but in ordinary operations the above will be all that is needed. The _nozzle_ is about an inch long, and polished so accurately that there is no escape when the _pipes_ are tightly placed upon it _dry_. 3. The _pipes_ are usually about an inch long, to the end of which are affixed thicker tubes so as to fit the nozzle, as before mentioned, whilst a short arm projects from each side of these, so that the silk or thread which is used to tie this artery, &c., upon the thin pipe may be carried round these arms, and all danger of slipping off prevented. The _pipes_ are made of different sizes, from that which will admit only a very fine needle (and this will need now and then to be cleaned, or to be freed from any chance obstruction), to that which will take a large pin. These sizes must always be at hand, as the vessels of some subjects are exceedingly minute. 4. _Stopcock._--This is a short pipe like a small _straight_ tap, which fits accurately upon the end of the syringe like the pipes, and also takes the pipes in the same manner. The use of this is absolutely necessary when the object is so large that one syringe full of liquid will not fill it. If no preventive were used, some part of the liquid would return whilst the syringe was being replenished, but the stopcock is then turned as in an ordinary tap, and all danger of this effectually removed.