154 ISOLATION OF ELEMENTS. [CH. VII. 



ISOLATION OF HISTOLOGICAL ELEMENTS. 



§ 244. For a correct conception of the forms of the cells and fibers of 

 the various organs of the body, one must see these elements isolated 

 and thus be able to inspect them from all sides. It frequently occurs 

 also that the isolation is not quite complete, and one can see in the 

 clearest manner the relations of the cells or fibers to one another. 



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

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

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

 shorter, that is, from one or two hours to as many days. In the weak 

 solution the cell cement or connective tissue is softened so that the cells 

 and fibers may be separated from one another, and at the same time the 

 cells are preserved. In fixing and hardening, on the other hand, the 

 cell cement, like the other parts of the tissue, are made firmer. It is 

 better also to dilute the fixing agents with normal salt solution (§ 313) 

 than merely with water. 



$ 245. Isolation by Means of Formaldehyde. — Formaldehyde in 

 a -fV/i solution in normal salt solution is one of the very best dissociat- 

 ing agents for brain tissue and all the forms of epithelium (§ 308). It 

 is prepared as follows : 5 cc. of formal, formol, formalin or formalose, 

 that is, a 40 r /, solution of formaldehyde, are mixed with 995 cc. of 

 normal salt solution. This acts quickly and preserves delicate struct- 

 ures like the cilia of ordinary epithelia, and also of the endymal cells of 

 the brain. It is very satisfactory for isolating the nerve cells of the 

 brain. For the epithelium of the trachea, intestines, etc., the action is 

 sufficient in two hours ; good preparations may also be obtained after 

 two days or more. The action on nerve tissue of the brain is about as 

 rapid. For the stratified epithelia, like those of the skin, mouth, etc., 

 it may require two or three days for the most satisfactory preparations. 

 See Figs. 130 and 131. 



§ 246. Example of Isolation. — Place a piece of the trachea of a 

 very recently killed animal, or the roof of a frog's mouth, in the form- 

 aldehyde dissociator. After two hours or more, up to two or three 

 days, excellent preparations of ciliated cells may be obtained by scrap- 

 ing the trachea or roof of the mouth and mounting the scrapings on a 

 slide. If one proceeds after two hours, probably most of the cells will 

 cling together, and in the various clumps will appear cells on end show- 

 ing the cilia or the bases of the cells, and other clumps will show the 

 cells in profile. By tapping the cover gently with a needle holder or 

 other light object the cells will be more separated from one another, and 

 many fully isolated cells will be seen. 



