382 THE BLOOD AND THE BLOOD-FORMING ORGANS. 



Methods of Examination of the Blood. 



The blood may be examined fresh on the warm stage without the addition of any 

 fixative, simply surrounding the cover with oil or vaselin to prevent evaporation. 

 This method is especially useful in the examination of blood for the plasmodia malarise. 

 For most purposes, however, the cells should be treated the instant the blood leaves 

 the vessels in such a way as to retain their normal form. This fixation may be accom- 

 plished by the use of chemical agents (wet method) or by quick drying on the cover 

 glass or slide (dry method). 



WET METHOD. Among the chemical fixative agents are osmic acid and a solution 

 of corrosive sublimate. Osmic acid : A drop or two of blood drawn from the cleansed 

 finger-tip by a needle prick is allowed to fall into a cubic centimetre of from one- to 

 two-per-cent osmic acid. After an hour the blood cells may be transferred by a pipette 

 to a solution of acetate of potash, in which they may be preserved. 



Sublimate may be used in the form of Hayem's solution, consisting of 



Chloride of sodium 1 gm. 



Sulphate " 5 " 



Corrosive sublimate 0.5 " 



Water, distilled 200 " 



The blood is received directly into this solution, in which it is studied. The wet 

 method of fixation is especially to be recommended for studies on the minute structure 

 of blood cells. 



DRY METHOD. It has been found that if the freshly drawn blood from a finger 

 prick be immediately dried on a glass in a very thin layer, the cell forms are quite well 

 preserved and may be exposed to the action of staining agents. 



For this purpose square cover-glasses of medium size should be cleaned in strong 

 nitric acid, rinsed in alcohol and ether, carefully dried, and kept free from dust. A 

 drop of blood may be expressed by very light pressure only from the finger tip, previ- 

 ously cleansed with alcohol and ether, and for the best results the drop must be spher- 

 oidal and about one-sixteenth of an inch in diameter. One cover-glass should be held 

 in the forceps, or between the fingers if thoroughly dry, and its central point touched 

 to the drop of blood. After contact with the blood this cover-glass should be instantly 

 laid upon a second glass so as to cover all but an eighth of an inch along one side, and 

 as soon as the blood has spread to the edges, the cover-glasses should be quickly sepa- 

 rated by sliding without pressure and dried in the air. If, instead of drying in the air, 

 the specimens are rapidly dried high over an alcohol flame, the fixation will be more 

 successful, and many artificial changes in the red cells will be avoided. 



Another method, more successful in many hands, consists in touching the drop 

 with the smooth edge of a glass slide, applying this edge with its adherent blood 

 obliquely to a slide, and when the blood has spread along the edge of the slide, drawing 

 it rapidly across the surface of the second slide. 



For the permanent fixation of the cells and to prevent their solution by strong dyes, 

 one of two methods may be recommended : 



1. Heat Fixation. The specimens are heated in a hot-air bath or on a copper plate, 

 for from five minutes to two hours at a temperature of 110 C. to 120 C. 



2. Chemical Fixation. The specimens are placed for from one to thirty minutes in 

 strong methyl alcohol. 



Various staining agents are to be employed according to the object in view. The 

 triacid mixture of Ehrlich ' gives an excellent stain for the neutrophile and eosinophile 

 granules, but fails to show basophile granules or parasites, for which reason it is being 

 supplanted for general work by combinations of methylene blue and eosin. The com- 

 position of the Ehrlich stain is as follows: 



1 Great care must be used in selecting these dyes. Those made by Griibler, of Leip- 

 sic, are reliable. It is far better to purchase the solution already mixed as sold by Grub- 

 ler under the name of "triacid mixture for neutrophile granules." 



