BLOOD AND LYMPH. 8j 



On being drawn from the vessels, if the plasma 

 be allowed to evaporate, or if certain fluids such as 

 strong solutions of common salt or bichromate of 

 potassium be added, a part of the red blood-cells 

 lose their regular shape, their edges become cren- 

 ulate and jagged, and they sometimes seem to 

 become smaller; finally, they assume the form of 

 irregular globular masses, beset with short, blunt 

 spines. Various other irregular forms are produced 

 under the same circumstances which it is not neces- 

 sary to describe here. The addition of water causes 

 the spines to disappear, and the cells swell up and 

 assume a globular form. These changes in form, 

 produced by chemical agents and by change in the 

 density of the fluid in which the cells lie, should not 

 be mistaken for an expression of vitality, or re- 

 garded as analogous with the amoeboid movements 

 of the white blood-cells. 



The red blood-cells are much more abundant than 

 the white, there being in normal human blood about 

 350 to 500 of the former to one of the latter. It is 

 estimated that in man there are between four and 

 five million red blood-cells in one cubic millimetre 

 of blood. The diameter of the average cell is 

 about y^-g-th of a millimetre, or about 7.9 JJL.* Not 



* The Greek letter jit is frequently employed to represent the 

 micro-millimetre, or the one-thousandth part of a millimetre, this 

 having been widely adopted as the unit for microscopic measurement. 

 In English measurement the average red blood-cell has a diameter of 

 about ^V^th of an inch. 



