726 THE MICROSCOPE. 



7. Contents. The contents of those structures which 

 consist of envelopes, as cells, or of various kinds of tubes, 

 are very important. These may consist of included cells or 

 nuclei, granules of different kinds, pigment matter, or 

 crystals : a fair illustration of the changes effected by dis- 

 ease is given in fig. 256, from a cyst in a diseased liver : 

 occasionally their contents present definite moving cur- 

 rents, as in the cells of some vegetables; or trembling 

 rotatory molecular movements, as in the ordinary globules 

 of saliva in the mouth. 



8. Effects of Re-agents. These are most important in 

 determining the structure and chemical composition of 

 numerous tissues. Thus water generally causes cell- for- 

 mations to swell out from endosmosis ; while syrup, gum- 

 water, and concentrated saline solutions, cause them to 

 collapse from exosmosis. Acetic acid possesses the valu- 

 able property of dissolving coagulated albumen, and in 

 consequence renders the whole class of albuminous tissues 

 more transparent. Thus it operates on cell-walls, causing 

 them either to dissolve, or become so thin as to display 

 their contents more clearly. Ether, on the other hand, 

 and the alkalies, operate on fatty compounds, causing theii 

 solution and disappearance. The mineral acids dissolve 

 most of the mineral constituents that are met with ; .so 

 that in this way we are enabled to tell with tolerable cer- 

 tainty, at all events, the group of chemical compounds to 

 which any particular structure may be referred. 



All animal tissues should be stained, as previously 

 directed, and cut into sections, otherwise an accurate 

 idea of their general structure cannot be obtained. It 

 is also of importance to examine specimens by reflected 

 light, transmitted light, and by polarised light ; when 

 immersed in water, or in a highly refracting fluid, such 

 as glycerine, oil, turpentine, and Canada balsam ; with 

 a cover and without a cover. The most scrupulous 

 cleanliness should always be observed in microscopical 

 examinations ; many errors of interpretation have 

 arisen in consequence of a want of sufficient care 

 in preventing the admixture of various accidental 

 substances. The better way of avoiding errors from 

 this cause is to become familiar with the characters 



