THE RED BLOOD CORPUSCLES 197 



rapidly in the spring, while, at the beginning of the period of hiberna- 

 tion, the yellow marrow is present in especially large amounts.^ 

 In the frog, lymphoid red marrow appears only in the early summer, 

 which fact indicates that this animal obtains a considerable supply of 

 new red cells at this time of the year.^ 



The precursors of the red cells are called erythroblasts, while the 

 process by means of which these cells are converted into mature red 

 corpuscles, is known as hematopoiesis. Their migration into the 

 blood-stream is greatly facilitated by the circulatory conditions exist- 

 ing in the marrow. In the first place, it is to be noted that these 

 channels are protected by unyielding bony walls, while their cellular 

 lining is thin and rather imperfect. And besides, as the rapidity of 

 the blood flow is slight and the pressure low, a certain traction is 

 brought to bear upon them, but naturally, the quickness with which 

 they are formed and are forced into the circulation, depends in a large 

 measure upon how greatly the system is in need of them. Thus, it is 

 possible to retard the production of these elements in such a degree 

 that the lumen of the vessels becomes practically free from them, 

 while the region close to their wall is filled with cells in all intermediary 

 stages of development. It is also possible to stimulate the hemato- 

 poietic process by causing a greater destruction of the circulating red 

 cells. This end may be attained either by bleeding, or by the adminis- 

 tration of toxic substances. The histological picture then obtained 

 is quite different from that just given, because the lumen of the chan- 

 nel is now filled with young erythrocytes, many of which are still in 

 possession of a nucleus. Some of these nucleated cells find their way 

 into the general circulation, where they are recognized as normoblasts. 

 Under certain pathological conditions the liver and the spleen seem 

 to regain the corpuscle-forming power which they possessed during 

 embryonic life. 



While the duration of the life of the red cells has been estimated at 

 about four weeks, it cannot be said that this point has been definitely 

 settled. The attempt has been made to arrive at a conclusion by 

 introducing a limited number of elliptical corpuscles into the circula- 

 tion of a mammal. It seems, however, that the length of time during 

 which the cells of the lower forms or of birds continue to live in the 

 mammalian blood, cannot be regarded as a safe guide, because as they 

 are thus placed into a medium which is foreign to them, they may go 

 to pieces much sooner than they would otherwise. Another method 

 to which brief reference should be made here, depends upon the deter- 

 mination of the number of red cells which must be destroyed daily 

 in order to permit of the excretion of the usual amounts of bile pig- 

 ment. If the quantity of bile is 15 grams per kilo of the body weight 

 and the percentage of its pigment 0.2, the daily output of pigment 

 must amount to 1.95 grams. But in order to obtain this quantity of 



1 Pappenheim, Zeitschr. fiir klin. Med., xliii, 1901, 363. 



2 Marquis, Dissertation, Dorpat, 1892, 



