59 



228. Put a drop of blood on a slide. Heat it slowly over 

 a flame, so as to evaporate the water. Then add a small crys- 

 tal of common salt and a few drops of glacial acetic acid ; put 

 on a cover-g-lass, and again heat slowly till the liquid just 

 begins to boil. Take the slide away from the flame for a few 

 seconds, then heat it again for a moment, and repeat this pro- 

 cess for two or three times. Now let the slide cool and examine 

 with the microscope (high power). The small black or 

 brownish-black crystals of hemin will be seen. This test is 

 often important in some medico-legal cases where only a trace 

 of blood is available for examination. If the blood stain be 

 upon a piece of cloth, it may be soaked in a little distilled 

 water and examined by the spectroscope or micro-spectroscope. 

 The liquid may then be evaporated to dryness on the water 

 bath and the hemin test made. Or perform the hemin test 

 directly on the piece of cloth. 



229. In the blood saved for clotting, note that in a few 

 minutes the blood congeals, and when the vessel is tilted the 

 blood no longer moves as a fluid, but as a solid. After an hour 

 or so, pale yellow colored drops of fluid- — the serum — are seen 

 on the surface, having been squeezed out of the red mass, the 

 latter being the clot and consisting of fibrin. 



Note in the clot of horse's blood the upper light colored 

 layer of leucoytes — the buffy coat. Coagulation is slow in this 

 animal and the red and white corpuscles on account of the 

 difference in their specific gravity have time to separate. 



230. Salted Plasma. Note that in the flask containing 

 the mixture of blood and sodium sulphate, no coagulation has 

 occurred. Place some of this fluid in the centrifuge to separ- 

 ate the corpuscles and plasma, or let the mixture stand until 

 the corpuscles" sink ; the plasma mixed with the saline solution 

 is known as the salted plasma. 



231. Oxalate Plasma. Note also that the potassium oxa- 

 late blood mixture does not coagulate. Centrifuge the mix- 

 ture or let stand until the corpuscles fall, to obtain the plasma. 

 The oxalate precipitates (as the oxalate of lime) the calcium 

 which is necessary for coagulation. 



232. To a portion of the oxalate plasma add a few drops 

 ot a.2% calcium chloride solution. Coagulation results (more 

 quickly at 40°). 



