ANIMAL STRUCTURES MODE OF INVESTIGATING. 727 



of common substances which are likely to be mixed up 

 with preparations, as the following : oil globules, air 

 bubbles, portions of worsted, cotton, and linen, silk and 

 wool fibres, hairs from plants, vegetable tissues, human 

 hair, portions of feathers, and the starches wheat and 

 potato starch from bread crumbs. In taking fluids from 

 bottles and vessels, the possibility of mixing small portions 

 of their contents must be avoided ; the pipette should be 

 washed immediately after it has been used. Another 

 fallacy arises from the great transparency of some struc- 

 tures ; a membrane may appear perfectly clear and trans- 

 parent when in reality it is covered with a delicate layer 

 of epithelium, which only becomes visible after the appli- 

 cation of some chemical re-agent \ on the other hand, by 

 the action of re-agents, a fibrous appearance is produced. 

 Acetic acid, when added to many preparations, frequently 

 produces a swelling of the tissue, which looks like base- 

 ment membrane, but which in reality has been formed by 

 the action of the acid. The mechanical pressure of the 

 thin glass, if pressed down tightly, alters structures very 

 much; the appearance of the blood discs are, at times, 

 much distorted in this way, and lead to false conclusions 

 and erroneous descriptions. 



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-discs 

 retain their form j and if the preservation of the specimen 

 is wished, a thin glass-cover carefully placed over it, and 

 cemented down, in the way directed at page 217. 



To the advanced observer, the examination of the mucous 

 membrane will afford some instruction. Should the speci- 

 men 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 is required for examination, a por- 

 tion can be detached from the surface by a knife ; when 

 placed on a glass slide, add a drop of iodine solution, then 

 view it with a J-inch power. Villi and papillae are best 

 made out in injected specimens, as will be seen on reference 

 to Plate VII. 



