OF MICROSCOPIC OBJECTS. 123 



less weight may be used ; if, however, the piston does not move, 

 more must be added. 



21. Such is the method recommended by the " Micrographic 

 Dictionary," and perhaps it is as good as any mechanical plan 

 could be; but where the operator is willing to undergo the labour 

 of performing all this with the hand, he has a much better chance 

 of succeeding, because the pressure can be regulated so accurately, 

 and changed so 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 commence. Ano- 

 ther 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, the artery when cut across contracts considerably, and is 

 lost in the adjoining substance; but where the opening is made 

 longitudinally all danger of this contraction is obviated. 



23. The different systems of vessels are often injected with 

 various colours, so that their relative positions, &c., may be shown 

 most clearly. In some specimens, the veins are injected with 

 \vhite, and the arteries with red; in the kidney, the urinary tubes 

 are often filled with white, and the arteries with red. Then, 

 again, the liver affords tubes for three or four colours. But no 

 written instructions on the 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 in- 

 jected 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 regu- 



