420 COLLOIDS IN BIOLOGY AND MEDICINE 



are often presented by the fusing together of a metal wire and a glass 

 tube because of the different degree of contraction (shrinking) upon 

 cooling. Cracks frequently occur. Now imagine how very compli- 

 cated the problem becomes whenever three, four or perhaps a dozen 

 fused ingredients are subjected to a manipulation to which they react 

 differently. This simple consideration teaches us that we can never 

 expect a piece of tissue that is fixed and hardened to show the same 

 appearance as when it is alive. Only by comparing pieces of tissue, 

 treated in different ways, can we recognize what is normal and what 

 is due to fixation, but even in these distorted pieces we can see the 

 places of least resistance where inequalities of staining exist, and by 

 careful consideration we can learn much even from such pieces as 

 are regarded as spoiled by histologists. 



Of course, a fixative must not block its own path. If massive 

 organs, e.g., the brain or liver, are to be fixed, the fixative solutions 

 have a great distance to travel before they reach the center; if 

 shrinkages or precipitates are formed at the periphery, the diffusion 

 paths are closed at the outset and the fixative could never reach the 

 center even if such objects should lie in the fluid for weeks. It is 

 quite reasonable to expect that in such large organs the central 

 portion will show a different kind of fixation than the periphery. 



We can well understand how temperature plays an important role, 

 conditioning not only the rate of diffusion but also governing the 

 processes of coagulation. 



Wherever feasible the objects are cut into small pieces and placed 

 in very large quantities of fixative fluids (50-100 times the volume of 

 the object) so that too great a dilution shall not occur and the action 

 shall be quite uniform; if the fixation is prolonged the fluid must be 

 renewed from time to time. 



Among the fixatives an important role is played by certain elec- 

 trolytes (chromic acid, bichromate, mercuric chlorid, picric acid, etc.) ; 

 they cause swelling in solutions that are too dilute, shrinking in too 

 concentrated solutions. On this account C. DEKHUYSEN and W. 

 STOELTZNER prepared "isotonic" solutions which cause neither swell- 

 ing nor shrinking. These authors, as may be seen from their method 

 of expression (hypertonic, hypotonic), evidently proceeded from prem- 

 ises which depend upon osmotic pressure, a factor involved only to a 

 very limited extent. By treating the whole practice of fixation from a 

 colloid standpoint, doubtless a whole series of valuable new methods 

 of fixation would be evolved for histologists. Such a study would 

 furnish us with more definite rules for knowing why, on the one 

 hand, one solution is more suitable for marine animals, and on the 

 other, why other solutions are more suitable for mammalian organs. 



