BLOOD AND H^EMOPOIETIC ORGANS 73 



red bone marrow. Is the marrow in defect ? Then the 

 cells of the blood are also in defect. Is the marrow 

 diseased ? Do its cellular constituents no longer pre- 

 serve their due relative proportion to one another? 

 Then the microscopic picture of the blood faithfully 

 mirrors such disease and such alterations. It is be- 

 coming probable, indeed, that the blood is never itself 

 the seat of a primary pathological process, but simply 

 exhibits the consequences of disease in the marrow or 

 elsewhere. 



We must now turn to a closer study of the properties 

 of the blood as outlined above, and we shall begin by 

 looking at it as a tissue containing living cells. 



CELLULAR CONSTITUENTS OP THE BLOOD. 



The cells which constitute the population of the blood- 

 stream are of three kinds : the red cells, the white cells, 

 and the blood platelets. We shall endeavour to trace 

 the origin, life-history, and fate of these separately. 



The red cells in the adult with their origin in the 

 foetus we are not concerned arise in the red marrow of 

 the bones. They are produced from mother cells which 

 are at first colourless, but in which haemoglobin gets 

 deposited, and at first, like all cells, they contain a 

 nucleus. How the nucleus is got rid of whether by 

 extrusion (as is most probable) or by absorption has 

 been much discussed, but is a subject of little practical 

 interest. At any rate, by the time they reach the 

 general circulation they are non-nucleated, and for that 

 reason are by some denied the title of ' cells ' at all. 

 Everything in them, indeed, seems to have been 



