82 



For C. are generally necessary : 



1. Fixing the tissue ( 18-38). Hardening. 



2. Sectioning by one of the following methods : 



(a) Free-hand, without an imbedding mass, or 



(b) With an imbedding mass, as 



(1) By the Paraffin method ( 42), or 



(2) By the Collodion method ( 50), or 



(3) By the Freezing method ( 61). 



ISOLATION. 



2. 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, normal salt solution, being a iV~A^ solution of common 

 salt (sodium chlorid) in distilled water. 



The chemical agents or solutions for isolating are, in general, 

 the same as those used for fixing and hardening. But the solutions 

 are only about one-tenth as strong as for fixing and the action is very 

 much weaker and requires from one or two hours to as many days. 

 In the weak solution the cell cement or connective tissue is softened 



