6 Neurological Technique 



as to which set of changes have most to do with the result 

 obtained. Chemical changes must involve physical changes, and 

 vice versa. 



A fixing agent may produce a chemical, and physical, change 

 without itself entering into the composition of the tissue. Alco- 

 hol, for example, is supposed to fix by producing a coagulation 

 or precipitation of the substances without further taking part in 

 the resulting changes. Its action is thought to be similar to 

 that of heat, which is a fixing agent much used for the lower 

 forms of animals. The resulting differences no doubt consist in 

 molecular changes as well as physical changes. 



Again, that the nucleus and nucleoli of the cell, in all the 

 more general methods, react to stains differently from the other 

 parts of the cell, is no doubt due to inherent chemical as well as 

 to evident physical differences which may or may not be effected 

 by the reagents. The nucleus of the cell in the fresh state exer- 

 cises a selective action toward the so-called nuclear stains. It 

 stains more deeply, or takes more of the stain, than the other 

 organs of the cell. It cannot be said with certainty whether 

 this is due more to the greater compactness or denser accumu- 

 lation of substance in the nucleus (physical difference), or to its 

 inherent chemical difference. 



That the more highly differential staining methods depend 

 solely upon physical differences, produced or inherent, between 

 the different structural elements of the tissue, is doubtful. 

 Many of the methods devised to bring' out certain structures in 

 especial prominence are thought to depend upon pertain chem- 

 ical compounds existing in those structures and not in others, 

 causing them, to react differently toward the reagents. An 

 example of this is the selective affinity which the axone of the 

 nerve cell exhibits toward methylen blue, one of the basic aniline 

 dyes. This affinity, so strongly manifested by the fresh tissue, 

 is almost entirely lost if the stain is applied after the action of 

 a fixing agent. Osmic acid is reduced by the medullary sheath 

 of the axone before it is affected at all by the other structures. 

 This reducing power of the normal structure is lost after treat- 

 ment with potassium bichromate or chromic acid. 



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