CHAPTER XVI. 



TFA'USiqVK. 



The destructive reactions within living cells are counterbalanced 

 by synthesis of elements for repair and continued life. When cells 

 die, syntheses cease and the enzymes involved in vital processes 

 become acti\e in breaking down the products they formerly helped 

 to build. Such disintegration processes working within the dead 

 or dying cell constitute autolysis. In addition, bacteria present, 

 or soon entering dead and dying tissues, rapidly add to the disinte- 

 gration processes and the accompanying distortion or loss of char- 

 acteristic cellular structure. 



To prevent these changes, and also to make cells and intercellular 

 substance insoluble and stainable, tissues from freshly killed animals 

 are immersed in fluids called fixatives. Such fixing fluids shoidd 

 accomplish certain definite things in addition to preventing autolytic 

 and bacterial decomposition. They should penetrate tissues rapidly 

 and coagulate or otherwise preserve protoplasmic substances, leav- 

 ing intra- and extracellular structures preserved in the same 

 relative spatial relations as during life. They should prevent sub- 

 sequent changes, such as shrinkage, hardening, and dissolution, and 

 should not hinder later staining. With these requirements for a 

 fixati\'e, let us consider the properties of some chemicals in common 

 use as components of various fixing fluids. 



FIXATION. 



Fixing Reagents. — .4 ce^ic Acid.— This precipitates nucleoproteins 

 and, therefore, fixes chromosomes very well, but does not affect the 

 proteins or fats. It causes permanent swelling and distortion of 

 collagenous fibers of connective tissue. Due presumably to its 

 failure to precipitate or otherwise change most of the substance of 

 protoplasm so as to retard its penetration, it diffuses throughout 

 tissues very rapidly. It causes some swelling and leaves the tissue 

 soft. 



Picric Acid. — Sa,tuTSited aqueous solutions of this chemical 

 precipitate all proteins by forming protein picrates, and its pene- 



