IO2 



PRACTICAL HISTOLOGY. 



tl. 



mucilage and shake it up in a test-tube with air until it forms fine 

 bubbles. Place a drop of it with its included bubbles on a slide, 

 cover, and examine. 



(a.) Observing larger and smaller bubbles, especially note how 

 the appearance of the bubble varies with the elevation or depres- 

 sion of the lens. Sketch these appearances. When the tube is 

 depressed, the bubble has a small, clear centre, and a wide, black, 

 sharp, refractile margin (fig. 55, A), because so much of the light 

 is refracted by the air, and does not pass through the bubbles of 

 air into the lens. Study the appearance of the bubble when the 

 centre and then the upper surface are in focus (B, C). 



The student will have an opportunity by-and-by of observing 

 the appearance of a bubble of air in Canada balsam (fig. 55, 2), and 

 an oil globule in water (fig. 55, 3). 



8. Cotton Fibres (H). Place a few fibres of cotton-wool in 

 water, cover, and examine. 



(a.) Observe the fine translucent flattened threads, which are 

 really tubes, each looking as if twisted on itself at intervals 



(%. 56). 



(l>.) Remove the cover-glass and 

 the water, add a drop of iodine ; 

 cover and examine ; note that the 

 fibres are yellowish ; add a drop of 

 strong sulphuric acid or a drop of the 

 following mixture : Glycerine 2, 

 water i, and sulphuric acid 3 parts. 

 At the edge of the cover-glass suck 

 it through \vith blotting-paper, and 

 note that the fibres become bine. 

 They are composed of cellulose, 

 which does not give a blue with 

 iodine alone, but with iodine and 

 sulphuric acid. 



9. Linen Fibres (H). -Examine 



/*.. 



Linen. Cotter* 



n 



FIG. 56. Fibres of Silk, Wool, Cotton, 

 and Linen. 



(a.) Observe the cylindrical or 

 flattened translucent fibres with no 

 twist ; but they have a few markings on them here and there, and 

 at these points the fibre is generally slightly thicker. They are in 

 reality tubes with" thick walls (fig. 56). 



10. Wool (H).-^Examine in water. 



(a.) Observe the cylindrical fibres with numerous zigzag trans- 

 verse lines due to the epithelial covering of the fibre (fig. 56). 

 It is convenient to examine dyed wools also, as some of these 

 exhibit the zigzag imbricate scales even better tharwundyed wool. 



