22 4 



Relative count 



Large and small mononuclear 2 to 4 per cent. 



Polynuclear - - 90 to 95 



Eosinophil - rare. 



THE ISOTONIC POINT OR TONICITY OF THE BLOOD 



If a drop of blood is allowed to drop into a one 

 per cent, solution of salt in a small test tube and stirred 

 up, the uniformly turbid solution will eventually 

 become clear when the corpuscles have settled at the 

 bottom and the supernatant fluid will be unchanged ; 

 if, on the contrary, we add another drop of blood to a 

 little water in a test tube the whole drop is immediately 

 laked, and we have resulting a solution of haemoglobin. 

 The former solution of salt is called hypertonic, the 

 latter solution of water hypotonic. Now, if we start 

 with such a hypertonic solution, one per cent, salt, 

 and proceed gradually to dilute it, we shall eventually 

 reach a strength where the hypotonic, i.e., haemolysing 

 effect begins to appear. The strength of salt solution 

 just above this where no change occurs is the isotonic 

 point for the particular blood in question. This point 

 then gives us information as to the resistance of the 

 corpuscles to a haemolytic agent. The blood in 

 various diseases is found to vary in regard to the 

 strength of salt required to prevent haemolysis. So 

 that if a normal blood is unchanged by a 0*5 per cent, 

 salt solution, whereas an abnormal requires 0*6 per 

 cent, to protect it, the latter blood is described as 

 having a less resistance than the former, but it has a 

 higher isotonic point. 



The determination of the isotonic point then 

 gives us a more 'definite notion of the state of the 

 blood in disease than does a mere determination of 



