440 BLOOD AND LYMPH. 



pathological conditions. The details of the histological changes 

 will be found in the text-books of histology. It is sufficient here 

 to state simply that groups of nucleated, colorless cells, erythro- 

 blasts, are found in the red marrow. These cells multiply by 

 karyokinesis and the daughter-cells eventually produce hemo- 

 globin in their cytoplasm, thus forming nucleated red corpuscles. 

 The nuclei are subsequently lost, either by disintegration or by 

 extrusion, and the newly formed non-nucleated red corpuscles 

 (erythrocytes) are forced into the blood-stream, owing to a gradual 

 change in their position during development caused by the growing 

 hematopoietic tissue. When the process is greatly accelerated, as 

 after severe hemorrhages or in certain pathological conditions, 

 red corpuscles still retaining their nuclei (normoblasts) may be 

 found in the circulating blood, having been forced out prematurely. 

 Such corpuscles may subsequently lose their nuclei while in the 

 blood-stream. In the embryo, hematopoietic tissue is found in 

 parts of the body other than the marrow, notably in the liver and 

 spleen, which at that time serve as organs for the production of 

 new red corpuscles. In the blood of the young embryo nucleated 

 red corpuscles are at first abundant, but they become less numerous 

 as the fetus grows older.* It is interesting to note that in the 

 adult after severe anemias e. g., pernicious anemia and in rabbits 

 after the injection of saponin the spleen may again take on its 

 hematopoietic function. The venous sinuses become crowded with 

 cells of the marrow type.f 



Variations in the Number of Red Corpuscles. The average 

 number of red corpuscles for the adult male, as has been stated 

 already, is usually given as 5,000,000 per c.mm. The number 

 is found to vary greatly, however. Outside pathological con- 

 ditions, in which the diminution in number may be extreme, dif- 

 ferences have been observed in human beings under such conditions 

 as the following: The number is less in females (4,500,000) ; it varies 

 in individuals with the constitution, nutrition, and manner of life; 

 it varies with age, being greatest in the fetus and in the new-born 

 child; it varies with the time of the day, showing a distinct diminu- 

 tion after meals; in the female it varies somewhat in menstruation 

 and in pregnancy, being slightly increased in the former and di- 

 minished in the latter condition. 



Variation with Altitude. Perhaps the most interesting of the 

 conditions that may influence the number of the blood corpuscles 

 is a change in altitude. Attention was first directed to this point 

 by Bert,J who believed that the diminished supply of oxygen in high 



*Howell, "Life History of the Blood Corpuscles," etc., "Journal of 

 Morphology," 1890, vol. iv.; Bunting, "Univ. of Pennsylvania Medical 

 Bulletin," 1903, xvi., 200. 



t See Bunting, "The Journal of Experimental Medicine," 1906, viii., 625. 



J Bert, "La pression baroe"mtrique," 1878, p. 1108. 



