CHAP, i.] BLOOD. 35 



not formed wholly anew in the body but is manufactured in some 

 way or other out of haematin derived from haemoglobin. This 

 must entail a daily consumption of a considerable quantity of 

 haemoglobin, and, since we know no other source of haemoglobin 

 besides the red corpuscles, and have no evidence of red corpuscles 

 continuing to exist after having lost their haemoglobin, must 

 therefore entail a daily destruction of many red corpuscles. 



Even in health then a number of red corpuscles must be 

 continually disappearing ; and in disease the rapid and great 

 diminution which may take place in the number of red corpuscles 

 shews that large destruction may occur. 



We cannot at present accurately trace out the steps of this 

 disappearance of red corpuscles. In the spleen pulp, red corpuscles 

 have been seen in various stages of disorganisation, some of them 

 lying within the substance of large colourless corpuscles, and as it 

 were being eaten by them. There is also evidence that destruction 

 takes place in the liver itself, and indeed elsewhere. 



27. This destruction of red corpuscles necessitates the birth 

 of new corpuscles, to keep up the normal supply of haemoglobin ; 

 and indeed the cases in which after even great loss of blood by 

 haemorrhage a healthy ruddiness returns, and that often rapidly, 

 shewing that the lost corpuscles have been replaced, as well as 

 the cases of recovery from the disease anaemia, prove that red 

 corpuscles are, even in adult life, born somewhere in the body. 



In the developing embryo of the mammal the red CDrpuscles 

 of the blood are not haemoglobin-holding non-nucleated discs of 

 stroma^ but coloured nucleated cells which have arisen by the 

 development of haemoglobin and stroma in the ' undifferentiated 

 protoplasmic ' cell substance of certain cells. 



Still later on in the life of the embryo the nucleated red cor- 

 puscles are replaced by ordinary red corpuscles, by non-nucleated 

 discs composed almost exclusively of haemoglobin-holding stroma. 

 How the transformation takes place, and especially how the 

 nucleus comes to be absent is at present a matter of considerable 

 dispute. 



In the adult as in the embryo the red corpuscles appear to be 

 formed out of preceding coloured nucleated cells. In the interior 

 of bones is a peculiar tissue called marrow, which in most parts, 

 being very full of blood vessels, is called red marrow. In this 

 rel marrow the capillaries and minute veins form an intricate 

 labyrinth of relatively wide passages with very thin walls, and 

 through this labyrinth the flow of blood is comparatively slow. 

 In the passages of this labyrinth are found coloured nucleated 

 cells, that is to say, cells the cell-substance of which has under- 

 gone more or less differentiation into haemoglobin and stroma. 

 And there seems to be going on in red marrow a multiplication of 

 such coloured nucleated cells, some of which transformed, in some 

 way or other, into red non-nucleated discs, that is into ordinary 



