GENERAL ZOOLOGY 



ingest solid particles like bacteria and other foreign bodies into their cyto- 

 somes and function in this way in the event of infections or wounds in any 

 part of the body. These white blood cells are known as microphages. In con- 

 nective tissue other white blood cells, the macrophages, are the chief local 

 defenders in what is known as the inflammatory reaction, or first line of bodily 

 defense, at the site of an infection. There are only about 8,000 to 10,000 leuco- 

 cytes in a cubic millimeter of blood in a healthy individual, but the number may 

 be greatly increased in illness. Enormous numbers of leucocytes are held in 

 reserve in the bone marrow and can be quickly released to the blood stream 

 in response to stimulation. Blood counts are effective aids in the diagnosis 

 of disease. Two main classes of white blood cells are distinguished, those in 

 which the cytoplasm does not contain granules and those in which the cyto- 

 plasm is granular. The most abundant kind of non-granular leucocyte is the 

 lymphocyte (Fig. 3. 16/)), which is about the size of an erythrocyte and is 

 differentiated in the lymph nodes and in the spleen. Another much less 

 frequent and larger non-granular leucocyte is the monocyte. Granular leu- 

 cocytes, all of which are larger than the other blood cells, have nuclei 

 conspicuously irregular in shape and are called polymorphonuclear leucocytes 

 (Fig. 3.16C')- There are three kinds, distinguished by the staining reactions 

 of the cytoplasmic granules. Granular leucocytes differentiate in the red 

 bone marrow. 



It has proved very difficult to obtain information concerning the life span of 

 white blood cells. Lymphocytes apparently rupture or dissolve in great 

 numbers soon after they leave their places of differentiation, contributing 

 their globulins, a kind of protein, to the blood plasma. Gamma globulin is 

 concerned in the formation of antibodies against certain diseases. In cats and 

 dogs it appears that the total population of white blood cells is replaced 

 several times a day. 



Slightly more than half the volume of the blood is normally plasma, which 

 is the carrier for all substances, except the oxygen, transported by the blood. 

 Plasma contains, in addition to the blood cells, the blood platelets, which 

 appear to be fragments of cytoplasm of uncertain origin and function. It also 

 contains many submicroscopic constituents which are very important. One of 

 these is a protein known as fibrinogen; when blood clots, this becomes changed 

 to fibrin, in the meshes of which the cells are held. The clotting of blood 

 acts to seal damaged blood vessels and prevent further loss of blood. Blood 

 does not clot unless a vessel is ruptured, except when there has been tissue 

 damage. In circulating blood there is a substance known as prothrombin, 

 the formation of which depends on the presence of vitamin K (p. 32). In 

 the presence of calcium salts, prothrombin is transformed into thrombin, 

 which, in turn, conditions the change of fibrinogen to fibrin. This reaction 

 does not occur in a closed blood vessel because of a substance called heparin, 

 or antithrombin. When a vessel is broken, the damaged tissue cells and also 

 disintegrating platelets release a substance known as thromboplastin, or throm- 

 bokinase, which neutralizes the effect of heparin. Then thrombin is formed 



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