46 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



These cells multiply by karyokinesis, and the daughter-cells eventually pro- 

 duce haemoglobin in their cytoplasm, thus forming nucleated red corpuscles. 

 The nuclei arc subsequently lost, either by disintegration or, more likely, by 

 extrusion, and the newly-formed non-nucleated red corpuscles are forced into 

 the blood-stream, owing to a gradual change in their position during develop- 

 ment caused by the growing haematopoietic tissue. When the process has 

 been greatly accelerated, as after severe hemorrhages or in certain pathological 

 conditions, red corpuscles still retaining their nuclei may be found in the circu- 

 lating blood, having been forced out prematurely as it were. Such corpuscles 

 may subsequently lose their nuclei while in the blood-stream. In the em- 

 bryo, haematopoietic tissue is found in parts of the body other than the mar- 

 row, 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. 1 



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 cubic mm. The number is found to vary greatly, 

 however. Outside of pathological conditions, in which the diminution in 

 number may be extreme, differences 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 diminution 

 after meals; in the female it varies somewhat in menstruation and in preg- 

 nancv, being slightly increased in the former and diminished in the latter 

 condition. Perhaps the most interesting example of variation in the number 

 of red corpuscles is that which occurs with changes in altitude. Residence in 

 high altitudes is quickly followed by a marked increase in the number of red 

 corpuscles. Viault 2 has shown that living in the mountains for two weeks at 

 an altitude of 4.°>!)2 meters caused an increase in the corpuscles from 5,000,000 

 to over 7,000,000 per cubic mm., and in the third week the number reached 

 8,000,000. The accuracy of this observation has been demonstrated since by 

 many investigators. Some very careful work done under the direction of 

 Miescher 3 has shown that a comparatively small increase in altitude, 700 

 meters, causes a marked increase in the number of red corpuscles and in the 

 amount of haemoglobin, while return to a lower altitude quickly brings the 

 blood back to its normal condition. From these observations it would seem 

 that a diminished pressure of oxygen in the atmosphere stimulates the hema- 

 topoietic organs to greater activity, and it is interesting to compare this result 

 with the effect of an actual loss of blood. In the latter case the production of 

 red corpuscles in the red marrow is increased, because, apparently, the anaemic 

 condition causes a diminution in the oxygen-supply to the haematopoietic tissue, 



1 Howell : "Life History of the Blood-corpuscles," etc., Journal of Morphology, 1890, vol. iv. 



2 La s, maine m&dicale, 1890, p. 4G4. 



s Archiv fib- erp. Pathol, u. Pharmakol., 1897, Bd. 39, S. 426-464. 



