588 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. 217, 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. JE/ects of He-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 their 

 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. 



Many, if not all, animal structures require examination 

 in several ways before an accurate idea of their general 

 structure can be obtained. It is of much importance 

 to examine a body 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, 

 with the application of distilled water and of chemical 

 re- agents. The most scrupulous cleanliness must always 

 be observed in microscopical examinations ; many errors 

 have arisen in consequence of a want of sufficient care not 

 having been taken to prevent the admixture of various 

 accidental substances. The better way of avoiding errors 

 from this cause, is to become familiar with the characters 



