120 PHYSIOLOGY CHAP. 



which are identical with those circulating in the blood ; these, he 

 maintains, are not simply deposited there, but originate in situ. 

 According to Foa the platelets are autonomous elements, and 

 since they are composed of protoplasm and nuclear substance, are 

 real cells sui generis, capable of multiplying by direct division in 

 the circulating blood. 



In view of the importance assigned to the platelets in respect 

 of blood coagulation, we shall return to them after considering the 

 chemical constitution of blood plasma. 



The microscopic examination of the blood can be made with fresh or fixed 

 preparations. It is essential to use slides and cover-glasses that have been 

 scrupulously cleaned (first in alcohol containing HC1, and then in ordinary 

 alcohol) and well dried with a linen cloth. The blood required is obtained 

 by pricking the ball of the finger or lobe of the ear with a needle (better, a 

 lancet), so that the blood wells out in drops without employing compression. 

 In order to examine fresh preparations microscopically, it is only necessary to 

 take up a drop of blood on the cover- glass and lay this on the slide with the 

 drop downwards. If the glasses are clean, the blood spreads uniformly between 

 them, and the preparation only needs a gentle tap on the cover-glass to 

 distribute the morphological elements in an even layer and make it ready 

 for observation. 



In order to keep the formed constituents of the blood alive for prolonged 

 observation, a drop of blood must be gently compressed between two cover- 

 glasses, which are then separated by drawing one across the other. One of 

 these films is laid over the central depression of a special slide, such as is used 

 for the observation of bacteria in hanging drops. The margin of this de- 

 pression is previously filled with vaseline to prevent the intrusion of air, which 

 would cause the preparation to dry up making a minute moist chamber. 

 The preparation is then placed on Schultze's warm carrier and kept at the 

 required temperature. 



The fixing of the blood for microscopic study is performed in two different 

 w r ays, by the wet or the dry method. To fix it by the wet method the blood 

 is collected in a watch-glass and the various fixing solutions added. Such are 

 solutions of osmic acid, corrosive sublimate, palladium chloride, Kleinenberg's 

 picro-sulphuric acid, Flemming's osmic - chrom - acetic mixture, etc. When 

 completely fixed, the solution is removed, and the preparation can be examined 

 immediately or after staining. Fixing by the dry method is effected by 

 warming the film preparation. It must be dried in the air, and then passed 

 6-10 times through the flame of a spirit lamp, care being taken not to scorch 

 it ; or it may be laid for about an hour on a copper plate, warmed to 120. 

 Fixation is also effected by placing the air-dried blood film for an hour in a 

 mixture of equal parts olabsolute alcohol and ether. 



Excellent results are obtained by warming, and then dipping into alcohol 

 and ether. 



Staining is necessary in studying the detailed structure of the formed 

 elements of the blood. To stain fresh preparations, weak solutions of iodine, 

 methyl- violet, methylene-blue, eosin, etc., must be used. For staining dry 

 preparations, countless methods are described in special text-books, but we 

 must here confine ourselves to the most ordinary, which are also the most 

 practical for the doctor. The film-preparations are passed 6-10 times through 

 the flame of a lamp, and then placed for about half-an-hour in equal parts of 

 absolute alcohol and ether. They are then dried again in the air, and stained 

 in a watch-glass with Ehrlich's acid haematoxylin (haematoxylin 2 grins., 

 absolute alcohol 60 grms.). To this first solution is added the following mixture, 

 which has previously been saturated with alum : glycerin 60 grms., distilled 



