181 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 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, 

 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 with 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 beeu 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 



