1 64 ESSENTIALS OF PHYSIOLOGY. 



lie free in the hollowed interior of the cell. Adjacent cells have 

 meanwhile become united to form a syncytium, and the hollows 

 become continuous* along the connecting branches, so that a system 

 of blood-vessels is formed. 



(2) In post-natal life nucleated red corpuscles or erythroblasts are 

 found in the red marrow of bone (fig. 53). These are constantly 

 undergoing mitotic division ; the cells thus formed lose their nuclei 

 by atrophy or extrusion and pass into the blood capillaries, which 

 in the marrow probably have incomplete walls. Nucleated red cor- 

 puscles may pass into the blood stream after birth in certain diseases 

 in which a rapid destruction of blood corpuscles is taking place. 



The duration of the existence of a single red corpuscle is not known, 

 but there is evidence that large numbers of these cells are destroyed 

 daily to form the pigment of the bile. Fragments of broken-down 

 erythrocytes are also of constant occurrence in the cells of the spleen. 

 Moreover, the pigment of hair and of the coloured parts of the skin is 

 believed to be derived from haemoglobin. Blood corpuscles are also 

 lost by accidental haemorrhage, in disease, and, in the female, by 

 menstruation. The deficiency brought about in all these ways, normal 

 or abnormal, is as a rule rapidly and completely made good by the 

 activity of the bone marrow, which, in post-natal life, is the only source 

 of the red corpuscles. When an unusually large and rapid formation 

 of red corpuscles is required, for instance after severe haemorrhage, the 

 red marrow increases in amount, and replaces the yellow marrow to 

 some extent. 



THE WHITE BLOOD CORPUSCLES. 



The distinguishing feature of the polymorphonuclear leucocytes is 

 their power of amoeboid movement, a power which is shared by the 

 eosinophile cells, and to a much less extent by the lymphocytes. The 

 leucocytes are able to make their way through the capillary walls 

 between the epithelial cells and wander out into the tissue fluids of the 

 body. They are especially susceptible to certain chemical stimuli 

 (chemiotaxis}, and are found to concentrate in large numbers round 

 various chemical substances placed in their neighbourhood. For 

 example, they appear in force where pathogenic bacteria are active, and 

 serve a useful purpose in surrounding and destroying these germs, and 

 thus constitute an important protective mechanism for the body. When 

 the leucocytes fail to overcome and ingest tjie bacteria, they may them- 

 selves be destroyed by the bacterial toxins. The same function of the 

 removal of useless or harmful material is shown in other ways. For 

 example, the removal of the tail of the tadpole is effected by leucocytes, 





