CH. XXVI.] 



H^IMACYTOMETERS 



423 



The Blood-Platelets. 



Besides the two principal varieties of blood-corpuscles, a third 

 kind has been described under the name blood-platelets (Blut-platcheri). 

 These are colourless disc-shaped or irregular bodies, much smaller than 

 red corpuscles. Different views are held as to their origin. At first 

 they were regarded as immature red corpuscles ; but this view has been 

 discarded. Some state that* they are merely a precipitate of nucleo- 

 proteid which occurs when the plasma dies or is cooled. There is, 

 however, no doubt that they do occur in living blood, and have been 

 seen to undergo amoeboid movement ; some observers state that they 

 are nucleated. 



Enumeration of the Blood-Corpuscles. 



Several methods are employed for counting the blood-corpuscles ; most of them 

 depend upon the same principle, i.e., the dilution of a minute volume of blood with 

 a given volume of a colourless saline solution similar in specific gravity to blood 



FIG. 355. Hsemacytometer. (Gowers.) 



plasma, so that the size and shape of the corpuscles is altered as little as possible. 

 A minute quantity of the well-mixed solution is then taken, examined under the 

 microscope, either in a flattened capillary tube (Malassez) or in a cell (Hayem & 

 Xachet, Gowers) of known capacity, and the number of corpuscles in a measured 

 length of the tube, or in a given area of the cell, is counted. The length of the tube 

 and the area of the cell are ascertained by means of a micrometer scale in the micro- 

 scope ocular ; or in the case of Gowers' modification, by the division of the cell 

 area into squares of known size. Having ascertained the number of corpuscles in 

 the diluted blood, it is easy to find out the number in a given volume of normal 

 blood. 



