OF MICROSCOPIC OBJECTS. 175 
CHAPTER VII. 
INJECTION. 
1, Insection 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 appa- 
ratus, 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 a 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 soft leather 
well oiled or greased, in order to free it from all danger of 
any liquid penetrating it, 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 aceurately that there is no escape 
when the pipes are tightly placed upon it dry. 
3. The pipes are usually about an inch long, to their 
ends 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 
the artery 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 
of a very fine needle (and this will need now and then to be 
cleaned, or to be freed from any chance obstruction), to 
