402 BLOOD AND LYMPH. 



on continually, the process being much increased after hemorrhages 

 and in certain pathological conditions. The details of the histo- 

 logical changes will be found in the text-books of histology. It is 

 sufficient here to state simply that a group of nucleated, colorless 

 cells, erythroblasts, is found in the red marrow. These cells 

 multiply by karyokinesis, and the daughter-cells eventually produce 

 hemoglobin in their cytoplasm, thus forming nucleated red cor- 

 puscles or normoblasts. 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, hema- 

 topoietic 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.* 



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,f who believed that the diminished supply of oxygen 

 in high altitudes may be compensated by an increased amount of 

 hemoglobin, and subsequently ViaultJ demonstrated that living for 



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

 Morphology," 1890, vol. iv. 



f Bert, "La pression barometrique," 1878, p. 1108. 



J Viault, "Comptes rendus de l'academie des sciences," 1890 and 1891. 



