80 THE BLOOD 



up one molecule of oxygen. Both oxy- and ordinary hemo- 

 globin are crystallizable. A good method is to shake the blood 

 in a test-tube until it becomes laky, and then place it on ice 

 until the crystals form. They have different forms in different 

 animals for example, those of man and most mammalia 

 are rhombic prisms; of the squirrel, rhombic plates; and those 

 of the guinea-pig, tetrahedra. These crystals are soluble in 

 water, but do not dialize. 



Identification of Blood Stains. Hematin unites with hydro- 

 chloric acid (HC1) to form hemin, the crystals of which are 

 of the greatest importance in the identification of blood stains. 

 Scrapings from the stain are placed on a glass slide, and a drop 

 of a 1 per cent, solution of sodium chloride (NaCl) is added. 

 Heat over a gentle flame, avoiding ebullition until the water 

 has nearly evaporated. Then quickly add one or two drops 

 of glacial acetic acid, cover with a cover-glass, and again warm 

 until the acid has nearly disappeared. When cool, micro- 

 scopic, characteristic, brown crystals are deposited. Together 

 with the spectroscopic test they indicate positively the presence 

 of blood. The presence of cells of certain size and non-nucleated 

 ones will exclude the blood of certain animals, but it is not 

 sufficient evidence of human blood. Mammals have non- 

 nucleated cells. 



A method of considerable medicolegal value in the detection 

 of human blood rests upon the formation of specific precipitins. 

 The technique of this method is as follows: The blood stain 

 is macerated in iso tonic saline and filtered. The clear filtrate 

 is divided into two parts, each of which is placed in a test- 

 tube. To one of these is added rabbit's serum immunized to 

 human serum; the other is left as a control. Into a third 

 test-tube is placed some of the precipitating serum alone, and 

 into a fourth tube a mixture of precipitating serum and some 

 indifferent serum of an ox in physiological saline. These four 

 tubes are placed in an incubator at body temperature for an 

 hour. If the stain be due to human blood a cloudiness will 

 develop in the first tube, while the others remain clear. This 

 method fails to differentiate human blood from ape's blood, 

 and must be used with great caution, owing, in some cases, 

 to the apparent formation of antiprecipitins. 



Solutions of hemoglobin and compounds derived from it 



