42 PRACTICAL HISTOLOGY. 



a. Shape. Are the milk corpuscles globules, or are they 

 circular discs like coins? To determine this, touch the 

 edge of the cover-glass with a needle, and cause the cor- 

 puscles to roll over and over, or effect this by inclining the 

 stage. Farther, attend to this : when the focus is altered 

 a globule comes into and goes out of focus gradually, while 

 a flat corpuscle passes into and out of focus suddenly. 



b. Size. Attend in this case to the relative, but omit 

 the absolute size. 



c. Border. Is it smooth and regular, or notched ? 



d. Upper and under surf ace. Elevate the objective, then 

 slowly lower it, and carefully watch the upper surface as it 

 comes into view. Is it smooth or rough ? The character 

 of the lower surface is unimportant in this case, but when 

 a membrane such as the mesentery is under observation, it 

 is important. 



e. Colour. When exactly in focus, are the corpuscles 

 coloured or colourless ? 



f. Transparency. Bodies may be opaque, transparent, 

 or semi-transparent. The first transmits no light ; through 

 the second the outlines of subjacent objects are visible ; the 

 third transmits light, but the outlines of subjacent objects 

 cannot be distinctly seen. 



g. Consents. Does the corpuscle appear to be homo- 

 geneous, or can included particles be seen on carefully 

 focussing through its whole depth ? 



h. Effects of re-agents. Add acetic acid and observe its 

 effects on the corpuscles. For this purpose place a drop 

 of the acid on the slide close to the edge of the cover-glass. 

 If it fail to pass under the cover, dip a strip of bibulous 

 paper into the milk at the margin of the cover-glass 

 opposite the acid. The acid will then speedily penetrate 

 and irrigate the corpuscles (see 257). After it has 

 affected them, those that lie near each other no longer 

 remain distinct, but coalesce and form irregular masses, 

 from which it is inferred that each milk globule has an 

 envelope that is soluble in acetic acid. From the results 

 of the chemical analysis of the milk, this envelope is 

 believed to consist of casein, a substance soluble in an 

 excess of acetic acid. 



