THE BLOOD CORPUSCLES. 143 



just what the process is or in what part of the body the cells origi- 

 nate. Nor is it definitely known just where the worn out cells 

 are dealt with. In the embryo certain cells are set apart to 

 develop the vascular system. Some of these form the blood ves- 

 sels and some the red corpuscles, but later in foetal life, the latter 

 come from cells in the spleen, liver and red bone-marrow. At 

 first the red corpuscles are nucleated, but towards the end of 

 foetal life they begin to lose their nuclei, so that at birth there 

 are very few nucleated red corpuscles remaining in the blood. 

 After birth, the red corpuscles are formed in the red bone-mar- 

 row of the flat bones. In these places special nucleated cells are 

 found, which are called erythroblasts, and from these the ery- 

 throcytes develop. After severe haemorrhage nucleated red cells 

 may appear in the blood for a short time; the same is true in 

 some forms of anaemia in which there occurs a Very rapid destruc- 

 tion accompanied with a very rapid formation of red cells. 



Since the life of a erythrocyte is necessarily limited, provision 

 must be made for the destruction and elimination of the sub- 

 stances of which they are composed. In the pigments of the bile 

 we find the remains of part of the haemoglobin. The bile is 

 secreted by the liver into the intestine (see p. 71), and in case 

 the free outflow of bile is interfered with, the blood absorbs the 

 pigment and the individual becoir.es yellow or is said to be jaun- 

 diced. The bile pigments do not, however, contain all the ele- 

 ments of the haemoglobin, for the iron is not excreted by the bile. 

 It is, on the contrary, stored up by the liver to be used again 

 in the formation of fresh haemoglobin. Some have thought that 

 the function of the spleen is to destroy the red blood cells, the 

 waste products of which are sent to the liver through the splenic 

 vein. The evidence for this is the presence of pigment and iron- 

 containing substances in the blood of this vein. 



Iron is an essential constituent in the haemoglobin molecule, 

 and it is necessary that some be constantly supplied to the body 

 in the food. But this amount need not be large, since the iron- 

 containing substance can be used time and again in the manu- 

 facture of new haemoglobin, and once the body has the requisite 

 amount, little more need be added (see p. 120). Indeed, it is 



