458 



THE BLOOD 



[Cli. XXIX. 



This is called phagocytosis (see also p. 298). The polynuclear leuco- 

 cytes appear to be the most vigorous phagocytes. The drawings in 

 fig. 322 show some stages in this phenomenon; the cells represented 

 there, however, are not normal leucocytes, but certain large amoeboid 

 cells found in connective tissues, which congregate specially in 

 inflamed parts. 



The Blood-Platelets. Besides the two principal varieties of 

 blood-corpuscles, a third kind has been described under the name 

 blood-platelets (Blut-pldtcJien). 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-protein 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-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 in a cell of known capacity, and the number of 

 corpuscles in a given area of the cell, is counted. Having 

 ascertained the number of corpuscles in the diluted blood, 

 it is easy to calculate the number in a given volume of normal 

 blood. 



The hsemacytometer most frequently used at the present 

 time is known as the Thoma-Zeiss haemacytometer. It 

 consists of a carefully graduated pipette, in which the 

 dilution of the blood is done; this is so formed that the 

 capillary stem has a capacity equalling one - hundredth of 

 the bulb above it. If the blood is drawn up in the capillary 

 tube to the line marked 1 (fig. 323) the saline solution may 

 afterwards be drawn up the stem to the line 101 ; in this way we 

 have 101 parts, of which the blood forms 1. The blood and the 

 saline solution are well mixed by shaking the pipette, in the bulb 

 of which is contained a small glass bead for the purpose of aiding 

 the mixing. The other part of the instrument consists of a 

 glass slide (fig. 324) upon which is mounted a covered disc, m, 

 accurately ruled so as to present one square millimetre divided 

 into 400 squares of one-twentieth of a millimetre each. The 

 micrometer thus made is surrounded by another annular cell, c, 

 which has such a height as to make the cell project exactly 

 one-tenth millimetre beyond m. If a drop of the diluted 

 blood is placed upon m, and c is covered with a perfectly flat 

 cover-glass, the volume of the diluted blood above each of the 

 squares of the micrometer, i.e. above each 4^, will be ^VT. of 

 a cubic millimetre - An average of ten or more squares is then 

 taken, and this number multiplied by 4000 x 100 gives the 

 number of corpuscles in a cubic millimetre of undiluted blood. The average 

 number of corpuscles per cubic millimetre of healthy blood, according to Vierordt 



