876 



PHYSIOLOGY 



bone marrow. Here we have a structure protected from pressure where 

 the capillaries and veins are dilated and thin-walled, and allow a slow passage 

 of blood and the entry of newly formed corpuscles through the imperfect 

 walls into the blood stream (Fig. 376). That the marrow is involved 

 in the process is shown by the fact that it is the only tissue of the 

 body which undergoes an alteration in appearance when blood formation 

 is stimulated by such means as repeated bleeding or destruction of cor- 

 puscles by the injection of toxic agents. Under such conditions the red 

 marrow, which in adult mammals is present only in the epiphyses, is found 

 to have increased in extent and in many cases to occupy the greater part 





^ 1^ 



r ~^~ 



*2 



*.c 



/- 



o 



h 



Fia. 376. Section of red marrow of mammal. (Bonn and DAVIDOFF.) 



a, e, ery throblasts ; 6, recticulum ; c, myeloplax ; d, g, marrow cells ; 

 /, a marrow cell dividing ; h, a space which was occupied by fat. 



of the shaft of the bone, having taken the place of the yellow marrow. It 

 is in the red marrow therefore that we must seek the precursors of the red 

 blood corpuscles. In the bird the erythroblasts, i. e. the precursors of the 

 nucleated red blood corpuscles, form a sort of inner lining to the dilated 

 capillaries of the marrow (Fig. 377). Here we can see all grades between 

 the colourless nucleated corpuscle which lies nearest the periphery and 

 the adult red oval corpuscle containing haemoglobin, lying next the 

 lumen and ready to be carried away in the blood stream. If blood forma- 

 tion has been stimulated by repeated bleeding, this blood-forming tissue is 

 found to occupy the greater part of the lumen of the marrow capillaries. 

 If however blood formation has been reduced to its lowest extent by a 

 process of chronic starvation, the erythroblasts form a single layer of cells 

 just inside the dilated capillaries, and intermediate stages between the ery- 

 throblasts and the fully developed erythrocytes are almost entirely wanting. 

 In the frog this process of blood-corpuscle formation occurs only at one 

 period of the year, namely, in the early summer, and it is only at this time 

 that the bones are found to contain red marrow. In mammals the process 



