Il8 HOW TO WORK 



it very useful to place a piece of hair or hog's bristle, between the 

 thin glass and the glass slide, by which means too great pressure will 

 effectually be prevented. The same effect is obtained by using a glass 

 cell, but it will be found, I think, that it is more convenient to pursue 

 the plan just described in the mere examination of most tissues than 

 to place them in a glass or other cell. 



The student may also refer to 136, 137, 138, and 142. 



Whenever a specimen is to be preserved permanently in fluid, it 

 should be immersed in the solutiou in which it is intended to remain 

 for several hours or days previous to being mounted, so that it may 

 be thoroughly saturated with it in every part. The fluid maybe placed 

 in a moderately deep cell, in a watch-glass, or in a cup of one of the 

 palates used by artists, from which it may afterwards be removed to 

 the slide. The thin glass having been applied, and all superfluous 

 fluid removed, a thin layer of Brunswick black is to be carefully 

 placed round the edge so as to cement the thin glass to the slide. 

 When this is dry other layers are to be applied successively until the 

 joint is considered quite tight. The cement adheres better to the 

 glass slide if it is roughened previously by grinding in this part, or it 

 may be scratched with the writing diamond just where the cement is 

 to be placed. All objects, except the very thinnest, if preserved per- 

 manently in fluid should be placed in a cell, because there is a much 

 better prospect of their being kept permanently, than when placed 

 upon the glass slide in the manner employed for examining the speci- 

 men temporarily. The chance of air getting into the cell is much 

 diminished if the cement which is used possesses slight elastic power, 

 so as to admit the alteration which necessarily takes place in the 

 volume of the fluid under variations of temperature. For cements, 

 see page 48. 



Examination of the Simple Tissues. 



I propose now to refer very briefly to the methods of demon- 

 strating the structure of some of the most important tissues of the 

 higher animals, and at the same time I shall have to allude to their 

 general characters. 



2i. Areolar Tissue can always be obtained from beneath the 

 skin, and mucous membranes, or from the external coat of the 

 arteries. In some situations it is lax and very abundant. It may be 

 blown up with air, and dried to show the areolae or spaces in which 

 it is disposed. If the vessels be injected with plain size, the areolas 

 become distended with it, and when cold, very thin sections may be 

 easily cut which show the arrangement of the fibres in the most 



