THE RED BLOOD-CORPUSCLES 833 



good by a continual regeneration of fresh haemoglobin and new red cor- 

 puscles. The seat of the formation of red corpuscles in the higher verte- 

 brates is the 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. 365). That the 

 marrow is the tissue 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 corpuscles by the injection of toxic agents. Under such 



Wlsmast *%& 



^^t*y^4^pst/fc . 



, *"/ "-^v^ 1 , / 





-^ 



'49*Slm 



FIG. 365. Section of red marrow of mammal. (B6HM and DAVIDOFF.) 



a, e, erythroblasts ; &, recticulum ; c, myeloplax ; d, g, marrow cells ; 



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



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 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 red blood-corpuscles, form a sort of inner lining 

 to the dilated capillaries of the marrow (Fig. 366). Here we can see all 

 grades between the colourless nucleated corpuscle which lies nearest the 

 periphery and the fully formed red oval corpuscle containing haemoglobin, 

 lying next the lumen and ready to be carried away in the blood- stream. 

 If blood formation 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 erythroblasts, and the fully formed erythrocytes are 

 almost entirely wanting. In the frog this process of blood- corpuscle 



27 



