834 PHYSIOLOGY 



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 is very similar. In the red marrow are a number 

 of nucleated cells containing hsemoglobin, which are thought by Lowit 

 to be themselves derived from colourless nucleated cells. In the confused 

 medley of colourless cells which exists in the bone- marrow and are pre- 

 cursors of all the varied corpuscles found in the blood, it is difficult to be 

 certain of the identity of the colourless erythroblasts and to distinguish 

 them from the smaller colourless cells engaged in bone formation or in the 

 production of leucocytes. The haemoglobin-containing cells are often to 



mr 



FIG. 366. Section of red marrow of pigeon. (DENYS.) 

 Ic, eosinophile leucocytes ; eg, fat cells ; e, nucleus of endothelial cell of 

 blood-vessel ; ca, blood-capillary ; er, erythroblasts lying within vascular 

 endothelium ; glr, fully formed red corpuscles. 



be seen in process of division, and the nucleated daughter- cells appear to 

 undergo a process of nucleolysis, the nucleus being extruded or dissolved. 

 When blood formation is quickened as the result of previous destruction 

 or loss, some of these immature nucleated blood-discs may make their 

 way into the circulation and be found in the blood, where they are spoken 

 of as normoblasts. 



How long a corpuscle continues to exist in the circulating blood is not 

 known. The experiments made to determine the length of time during 

 which foreign corpuscles such as those of birds can be recognised after 

 injection into the circulation of a mammal are evidently beside the mark, 

 since these foreign cells will be destroyed by the serum and rapidly taken 

 up by the phagocytes of the body. Sooner or later, however, every cor- 

 puscle undergoes disintegration, a process which is generally ushered in 

 by the ingestion of the corpuscle by some phagocytic cells. Thus in the 

 haemolymph- glands and in the spleen we find large cells which have englobed 

 red corpuscles and in which we can recognise pigment- granules obtained 

 from their destruction. The chief place of disintegration of the haemoglobin 

 is certainly the liver, i.e. the organ where the haematin is converted into bile- 



