224 
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 o*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 
