6 PRACTICAL HISTOLOGY. 



reagent is caused by some difference in the age or physical properties of the various cor- 

 puscles. (Indicate this in PI. I., Fig. 12.) 



(e) Exposure to Air. If a drop of blood be exposed to the air before the cover is applied, 

 in some cases crenation occurs, but this phenomenon is not constant, nor when it takes 

 place does it always occur to the same extent. 



None of these preparations of human blood make good permanent preparations. 



2. Observe the colourless corpuscles (H). In a fresh drop of blood they are few in number 



from five to eight being usually in the field at once. They are best seen between the 



rouleaux of coloured corpuscles, and resemble in general characters the colourless corpuscles 

 of the newt or frog. They are, however, smaller and do not exhibit amoeboid movements 

 unless when kept at the temperature of the blood (38 C.) on a hot stage (p. xxviii). Some are 

 larger than and others are about the same size as the coloured corpuscles, and for the most 

 part they are all finely granular, though now and again a coarsely granular one is met with. 

 If the glass cover be touched with the point of a needle the colourless corpuscles are seen 

 to adhere to the glass, unlike the coloured corpuscles, which freely roll over one another ; and 

 even when the coloured corpuscles impinge on a colourless one applied to the slide, they 

 seldom displace it, so firmly do these adhere to any object. (Indicate the characters of the 

 colourless corpuscles in PI. I., Fig. 1 3.) 



EFFECT OF REAGENTS. The actions of Acetic Acid, Water and Magenta on the colourless 

 corpuscles of human blood are identical with the effects of the same reagents on the corre- 

 sponding corpuscles of the blood of the newt or frog. (Indicate the effect of acetic acid in 

 PI. I, Fig. 14, and magenta in PI. I., Fig. 15). 



3. FEEE GRANULES (H). Besides the two varieties of corpuscles just described (i and 

 2) a number of very fine granules may be observed in the human blood. 



4. FIBRIN (H). Place a cover on a drop of blood on a slide and allow it to stand for 

 a quarter of an hour. At the end of this time, on careful examination a number of delicate 

 threads, which form an open network between the rouleaux, may be detected. These threads 

 consist of fibrin, and are still better seen if the blood be irrigated with a drop of magenta or 

 iodine solution. The former colours the threads red, the latter colours them yellow. (Indicate 

 the appearance of the fibrin threads in PI. I., Fig. 9.) 



(C.) BLOOD-CRYSTALS. 



5. HAEMOGLOBIN. The colouring matter of blood, haemoglobin, may be obtained in a 

 crystalline form, though it does not occur in that form inside the coloured corpuscles. In 

 the corpuscles it seems to lie in the meshes of a stroma. 



PREPARATION. Kill a guinea-pig and take a drop of blood from the heart or else- 

 where, mix it on a slide with a drop of distilled water and cover. The haemoglobin diffuses 

 from the blood-corpuscles into the water, and, as evaporation takes place, needle-shaped 

 crystals form at the margin of the cover-glass. They cannot, however, be preserved for any 

 length of time, so as to make permanent preparations. Or use the blood of a white rat, from 

 which haemoglobin crystals form in a few minutes. 



6. HJEMIN. Besides haemoglobin, which exists normally in the coloured blood-cor- 

 puscles, and from which it may be obtained in a crystalline form, as above described, certain 



