16 



34. Potassium dichromate. 2%, 3%, and 5% aqueous solutions. This 

 is quite similar to Miiller's fluid in its action, and may be employed in the same 

 cases. It is generally used for the central nervous system. 



Harden in an abundance of the solution for 2 to 8 weeks, beginning with 

 the 2% solution for 2 to 6 days, 3% solution 1 to 3 weeks, 5% solution 1 to 3 

 weeks. Wash out in running water 24 to 48 hours. Place in 67% and 82% 

 alcohols several days, keeping in the dark meanwhile, changing when the alcohol 

 is colored. 



ISOLATION. 



35. One of the simplest ways of examining the structure of 

 a tissue is the separation from one another of the structural elements 

 composing it, thus permitting its analysis. Likewise, for a correct 

 conception of the forms of the cells and fibers of the various tissues 

 of the body, one must see these elements isolated and thus be able 

 to inspect them from all sides. It frequently occurs also that isola- 

 tion is not quite complete and one can see in the clearest manner 

 the relations of the cells or fibers to one another. 



In the employment of this method the tissue may be taken 

 fresh and isolation accomplished by teasing with needles or similar 

 instruments; or it may be treated with media which will serve to 

 render teasing partially or entirely unnecessary. In such cases 

 simply shaking or gently tapping the preparation will often suffice. 

 In many instances it is desired to examine the tissue while the 

 elements are still alive, as, for example, in the study of ciliated 

 cells, and recourse must be had to some "normal," "indifferent" 

 medium. Best of all is the medium with which they are bathed dur- 

 ing life in the case of tissue from the animal body, blood serum, 

 the aqueous humor of the eye, liquor amnioticus, or, as an artificial 

 substitute often more convenient if reagents are to be used subse- 

 quently, physiological or normal salt solution, being a 6/10 to 9/10% 

 solution of common salt (sodium chlorid) in distilled water. 6 /10% 

 is suitable for use with Amphibian tissue; 75/100% is normal for 

 reptiles and 9/10% best for mammals. Other normal physiological 

 solutions may in some cases be preferable, such as Ringer's solution, 

 Locke's solution, etc. 



36. The use of chemical solutions to facilitate isolation of the 

 elements is called Maceration. The chemical agents or solutions for 

 isolating are, in general, the same as those used for fixing and hard- 

 ening. But the solutions are only about one-tenth as strong as for 



