BLOOD. 407 



same way as medullated fibres, can be obtained from the vagus 

 of a dog or an ox. 



31. The crosses of Ranvier can be demonstrated by treating 

 the fibres for from one-half to one hour with 0.5 per cent, silver 

 nitrate solution (in the dark), and then exposing them, after 

 washing in water for some hours, to the sunlight in a little 



glycerin. 



6. BLOOD. 



32. A drop of fresh blood placed under a cover glass will 

 show rouleaux. On such specimens various chemical tests may 

 be made (e. g., the influence of water, strong salt solution, 

 tannic acid, potassium hydroxide, etc.). Blood may be obtained 

 by pricking the tip of the finger after washing it thoroughly 

 with soap and water and then with ether. The first drop of 

 blood should be removed, and specimens made from subse- 

 quent drops. The finger should not be squeezed. Cover 

 glasses should be absolutely clean (see technique for mount- 

 ing paraffin sections). 



33. For the preparation of so-called dried blood specimens, 

 thin smears of blood are made on cover glasses. A small drop 

 of blood is placed on a clean cover glass and another is placed 

 over it. The two are then drawn apart in such a way that their 

 surfaces are always parallel. In this way two smears of blood 

 are obtained, which should consist of a single layer of red 

 blood-cells, etc. These are allowed to dry in the air for fifteen 

 minutes, and then heated on a copper bar at a temperature of 

 120 C. for two hours (Ehrlich); or they may be left in a mixt- 

 ure of equal parts of absolute alcohol and ether for two hours 

 (Nikiforoff). They are then dried and stained. Another 

 method of fixing the blood cells is to immerse the slide in 

 Zenker's fluid for fifteen minutes, and wash in running water 

 for from one to two hours. 



The various, kinds of granules contained in leucocytes 

 may be stained in the following ways: 



a-granulations, in acidophilic or eosinophilic cells, are well 

 stained in eosin (aqueous solution for twenty-four hours, or 

 saturated glycerin solution for twelve hours) ; or in a saturated 



