MICROSCOPIC EXAMINATION OF BLOOD. 



713 



fluid of the higher Invertebrata, such as the Crab, which resemble 

 the Colourless corpuscles of Vertebrated blood rather than its 

 Red corpuscles, — these last, in fact, being altogether peculiar to 

 the circulating fluid of Vertebrated animals. 



557. In examining the Blood microscopically, it is, of course, of 

 importance to obtain 



as thin a stratum of Fig. 373. 



it as possible, so that 

 the Corpuscles may 

 not overlie one an- 

 other. This is best 

 accomplished by 

 selecting a piece of 

 thin glass of perfect 

 flatness, and then, 

 having received a 

 small drop of Blood 

 upon a glass slide, 

 to lay the Thin-glass 

 cover not upon this, 

 but with its edge 

 just touching the 

 edge of the drop ; 

 for the blood will 

 then be drawn-in by 

 capillary attraction, so as to spread in a uniformly-thin layer 

 between the two glasses. The inexperienced observer will be sur- 

 prised at the very pale hue which the Red corpuscles exhibit 

 beneath the Microscope, when seen in a single stratum ; but this 

 surprise need no longer be felt, when it is borne in mind that the 

 thickness of the film of colouring fluid which they contain is pro- 

 bably not more than 1-20, 000th of an inch ; and if a drop of Ink, 

 or of almost any coloured liquid, however dark, be pressed-out 

 between two glasses into an equally thin film, its hue will be 

 lightened in the same degree. The red hue of the corpuscles, 

 however, becomes obvious enough, when two or more layers of them 

 are seen-through at once. The Colourless corpuscles in Human 

 blood are usually not more than 1 : 350 of the Red, so that no 

 more than one or two are likely to be in the field at once ; 

 and these may generally be recognized most readily by then- 

 stand ing- apart from the rest ; for whilst the Red corpuscles have 

 a tendency to adhere to eac'i other by their discoidal surfaces, the 

 Colourless show no such disposition. Thin films of blood may 

 be preserved in the liquid state, with little change, by applying 

 Gold-size or Asphalte rouad the edge of the thin-glass cover before 

 evaporation has had time to take-place ; but it is in some respects 

 preferable to dilute the liquid with a small quantity of Groadby's 

 solution, its strength being so adjusted as not to produce any 



AHered Cnlnnrless (or White) Corpuscle of 

 Blood, an hour after being drawn from the 

 finger. 



