MICROSCOPIC STRUCTURE OF THE BLOOD. 287 



nucleated about the end of the fourth month, but it is still a moot 

 question whether any of the nucleated cells are actually concerned 

 in the formation of the non-nucleated. 



Non-nucleated blood-corpuscles are also formed in the medulla 

 or marrow of bones, and in the spleen. In the red marrow of 

 bones are found nucleated cells possessing the power of ameboid 

 movement, the true " marrow-cells " of Kolliker. In the proto- 

 plasm of these cells hemoglobin is formed, and this portion of the 

 protoplasm becomes converted into the non-nucleated corpuscle. 

 There are, besides these marrow-cells, others of smaller size, 

 erythroblastSy likewise ameboid, nucleated, and colorless, which 

 later undergo karyokinesis ; the daughter-cells become colored, lose 

 their nuclei, and are converted into the mature non-nucleated red 

 blood-corpuscles. It is to the red marrow that is to be principally 

 attributed the formation of the red corpuscles in adult life. This 

 process is called hematopoiesis, and is a constant process, new 

 corpuscles taking the place of the old which have outlived their 

 usefulness. It occurs to a .much greater extent than usual after' 

 hemorrhages. 



In the spleen are to be found cells somewhat resembling the 

 erythroblasts just described, and it is the opinion of some authori- 

 ties that these are also sources of the red blood-corpuscles. 



Destruction of Red Corpuscles. The duration of a red blood- 

 corpuscle is undetermined, but it is doubtless limited. Some 

 authorities place its life at from three to four weeks. Old corpus- 

 cles constantly undergo disintegration and new ones appear. The 

 fact that fewer corpuscles are found in the blood of the hepatic than 

 in that of the portal vein, and the additional fact that biliary 

 pigment is formed from the coloring-matter of the blood, indicate 

 that in the liver a part, at least, of these destructive changes takes 

 place. 



The spleen is also regarded by some authorities as being an 

 organ in which red corpuscles are destroyed. The argument 

 advanced in favor of this theory is that some of the susten- 

 tacular or supporting cells of the splenic pulp contain colored 

 granules which resemble the hematin of the blood ; in others red 

 corpuscles are found in various stages of disintegration. The 

 explanation is that these large cells are engaged in the process of 

 destroying used-up corpuscles. Opposed to this theory is the fact 

 that the blood coming from the spleen contains no hemoglobin in 

 solution, which it certainly would do if red corpuscles were 

 destroyed in that organ ; besides, after removal of the spleen the 

 destruction of corpuscles apparently goes on much the same as 

 before. 



There seems to have been an idea in the minds of some that 

 it was essential that some organ or organs should be charged with 

 the duty of destroying the red corpuscles. This does not, how- 



