HOW ZOOLOGISTS DO THEIR WORK 183 



light placed behind them so as to shine through 

 them, i.e. by transmitted light. If the object is 

 net extremel-* thin, it will shut out too much light, 

 and thus it cannot be clearly seen, therefore all 

 objects, except the most minute, must be divided 

 into thin slices, technically known as "sections." 



If we want to know not only the microscopic 

 structure of organs, but also their shape and 

 position in the body, and their relations with 

 other parts, we must have every successive section 

 carefully preserved, and the whole row arranged 

 in correct successive order; the physiologist may 

 often content himself with single sections ; the 

 zoologist must have rows and rows of them. 

 What a task this was, a quarter of a century ago, 

 for scientists who cut their sections by hand ! 



Let us, however, describe first the way in 

 which objects are prepared for section-cutting 

 whether by hand or by machine. It has already 

 been noticed that animal substances contain a 

 quantity of water, and therefore will not keep. 

 The same circumstance renders them soft and 

 squashy, so that the sharpest razor in the world, 

 in cutting a section, must necessarily do more or 

 less damage to the structure of the delicate tissues. 

 The water is held in the meshes of the tissues 

 just as it is held, for example, in the meshes of a 

 sponge. Now, if we were dealing with the sponge, 

 we could get it to absorb any other fluid substance 

 besides water; we might choose one that would 

 prevent decomposition ; we might choose one that 

 would go harder by cooling; so as to change the 

 sponge into a strong solid block that could be 

 knocked about without sustaining any damage. 

 This is exactly what we must do with our animal 

 tissue to prepare it for section-cutting ; and the 



