THE BLOOD. 165 



and in the mammalian body dead cells and organic foreign substances, 

 such as buried catgut ligatures, are ingested by the same cells. On 

 account of this property of " eating up " bacteria and dead matter, poly- 

 morphonuclear leucocytes are included under the term phagocytes. 



The leucocytes consist largely of proteins, cell-albumin, cell-globulin, 

 and nucleo-protein ; they also contain a little glycogen, with some 

 neutral fats, and lecithin and cholesterol. The inorganic salt which 

 is most abundant in their composition is, as in the case of the red 

 corpuscles, potassium phosphate. 



In the embryo leucocytes are derived from cells which resemble the 

 erythroblasts but are colourless. In post-natal life the polynuclear 

 and eosinophile cells, as also the basophile cells when they occur, are 

 derived from the special cells of red bone marrow called myelocytes ; 

 the lymphocytes are derived from lymph glands and lymphoid tissue 

 generally, including that of the Malpighian bodies of the spleen. The 

 nuclear changes which accompany cell division can be seen in definite 

 areas, known as gerrn centres, in the lymphoid tissue of the lymphatic 

 glands and elsewhere. 



The condition called leucocytosis, or increase of the number of 

 leucocytes in the blood, occurs normally during the digestion of a 

 protein meal. It also takes place in many infective diseases, being 

 accompanied by overgrowth of the red marrow, in which the poly- 

 morphonuclear cells are formed. The increase in the number of 

 polymorphonuclear leucocytes in the blood is part of the process by 

 which the body resists and overcomes infection by micro-organisms. 



THE BLOOD PLASMA. 



When blood is shed, it rapidly becomes viscid and in a few minutes 

 sets to form a clot. It is therefore necessary to use means to retard 

 or prevent clotting in order to obtain plasma for examination and 

 analysis. The various methods which are used for this purpose will be 

 described in connection with coagulation. 



Plasma is a pale yellow fluid, and has a specific gravity of about 

 1030, considerably lower than that of blood as a whole. The red 

 corpuscles have a specific gravity of about 1090, and therefore sink 

 if blood which is prevented from coagulating is allowed to stand. It 

 is found that the proportion of H and OH ions in plasma or blood is 

 about equal, and therefore its reaction is neutral. It must be noted, 

 however, that although blood gives a neutral reaction with phenol- 

 phthalein, it is alkaline to litmus, because litmus acid is strong enough 

 to displace the acid from sodium bicarbonate, which is an important 

 constituent of plasma, and to combine with the sodium. 



