CIRCULATION IN THE FROG'S FOOT. 389 



pigment matter, or crystals : occasionally their contents present definite 

 moving currents, as in the cells of some vegetables ; or trembling rota- 

 tary 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-formations to swell out from endosmosis ; while 

 syrup, gum-water, and concentrated saline solutions cause them to col- 

 lapse from exosmosis. Acetic acid possesses the valuable 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 disap- 

 pearance. The mineral acids dissolve most of the mineral constituents 

 that are met with ; so that in this way we are enabled to tell with tole- 

 rable certainty, at all events, the group of chemical compounds to which 

 any particular structure may be referred. 



Should it be desirable to make an examination of the vital fluid, 

 blood, the smallest drop, caused by the prick of a fine needle, may be 

 placed on a strip of glass, and waved backwards and forwards, that the 

 blood may dry as quickly as possible ; in this way the corpuscles or 

 blood -disks will retain their form ; and if the preservation of the speci- 

 men is wished, a thin glass-cover must be placed over it, and cemented 

 down in the way directed at page 77. 



To the advanced observer, the examination of the mucous mem- 

 brane will afford some instruction. Should the specimen be small, it 

 will be better to pin it to a piece of cork ; then well wash it by means of 

 a small syringe. If the investing epithelium be required for examina- 

 tion, a portion may be detached from the surface by a knife, placed on 

 a glass slide, and viewed as a transparent object with a -inch power. 

 Villi and papilla? are best seen upon injected specimens. 



TO VIEW THE CIRCULATION OF BLOOD IN THE FROG. 



The part most commonly employed for this purpose is the trans- 

 parent web of the hind foot ; and in order to secure the animal, and 

 keep its web open, various contrivances have been had recourse to. 

 The older microscopists, Baker, Adams, and others, were in the habit of 

 tying the frog to a frame of brass with some fine cord ; in the present 

 day the entire body of the animal, with the exception of the foot to be. 



