THE BLOOD AND LYMPH. 131 



bones, where they are situated either in the tissues of the marrow 

 itself, whence their descendants, while still cellular, pass into the 

 vessels, or in the large venous channels of the marrow, where the 

 blood-current is sluggish and the erythroblasts remain close to the 

 vascular walls. In some anaemic conditions the erythroblasts ap- 

 pear in the circulating blood, where they may be distinguished from 

 the normal red corpuscles by the presence of their nuclei and, fre- 

 quently, also by a difference in size (see Fig. 110, c). 



In the reptilia and birds the red corpuscles are normally nu- 

 cleated ; but, though morphologically resembling cells, they are 

 incapable of multiplication or spontaneous movement, and have 

 undergone such modifications that they are not cells in a physiolog- 

 ical sense. 



The functional value of the red corpuscles is dependent upon the 

 haemoglobin they contain, which is said to constitute 90 per cent, of 

 their solid matter. It is readily oxidized and reduced again, and 

 serves to carry the oxygen of the air, obtained during the passage 

 of the blood through the pulmonary capillaries, to all parts of 

 the body. The red corpuscles, therefore, subserve the respiratory 

 function of the blood, as the plasma subserves its nutritive func- 

 tion. 



The leucocytes, or white blood-corpuscles, are cellular elements 

 closely resembling the amoeba in their structure, which are present 

 in the blood in much smaller number than the red corpuscles, the 

 usual proportion being about one to six hundred. They vary some- 

 what in size and structure, either because of differences in their origin, 

 or because they are in different stages of development. The majority 

 of them are capable of amoeboid movements ; but while they are cir- 

 culating in the more rapid currents of the blood the constant shocks 

 they receive through contact with other corpuscles or with the vascu- 

 lar walls keep their cytoplasm in a contracted state and they maintain 

 a globular form. If, however, through any chance they remain for 

 some time in contact with the wall of a vessel, they are able to make 

 their way between the endothelial cells and pass out of the circulation 

 into the surrounding tissues. Here they creep about, and for this rea- 

 son have been called the migratory or wandering cells of the tissues. 

 They ultimately either suffer degenerative changes and disappear, 

 or find their way back into the circulation through the lymphatic 

 channels. During these excursions they may incorporate stray 



