THE CIRCULATORY SYSTEM. 



121 



radicles (Fig. 104). The veins leaving the bones are abundantly 

 supplied with valves. 



FIG. 1 04. 



Section of small venous radicle in marrow of the fowl. (Bizzozero.) Just within the vascular 

 wall is a zone of leucocytes, one of which contains a karyokinetic figure. Within this 

 zone is a second zone of erythroblasts, foururidergoing division, and in the centre of the 

 lumen are a number of matured red blood-corpuscles (containing nuclei in the case of 

 birds). The cytoplasm of the leucocytes contains no haemoglobin, while that of the 

 erythroblasts does. In birds and, probably, in other classes of animals the marrow 

 of the bones is one of the sites for the production of leucocytes as well as red corpuscles. 

 The latter are not produced from the former, but only from the erythroblasts, which con- 

 stitute a distinct variety of cell. 



Throughout life the cancellated portions of the flat bones and of 

 the bodies of the vertebrae contain red marrow, but the shafts of 

 the long bones are occupied by the yellow variety, which has lost 

 its power of producing red blood-corpuscles and leucocytes, and 

 has, therefore, become functionally passive. 



