130 ESSENTIALS OF ZOOLOGY 



stitutes approximately 60 per cent of the volume of blood. It is 

 itself about 90 per cent water and 10 per cent dissolved solids, chiefly 

 proteins and salts. Sodium chloride is the most abundant salt here. 

 Calcium and phosphorus salts are present in small proportions but, 

 nevertheless, are very important. An interesting suggestion has even 

 been made: that salts of the plasma are the same in kind and pro- 

 portion as those which were present in the prehistoric sea. The 

 primitive marine animals are thought to have had body fluids based 

 on sea water. Their descendants seem to have carried this composition 

 down through time. However, modern sea water has become about 3 

 times as concentrated. The plasma carries also carbon dioxide col- 

 lected from tissues, hormones, and antibodies. The hormone materials 

 serve to regulate the metabolism of different tissues over the body, 

 and the antibodies bring about immunity to certain diseases. 



The corpuscles, or formed elements, as they are often called, are 

 of two classes, the red corpuscles, or erythrocytes, and the white cor- 

 puscles, or leucocytes. The red corpuscles are small, nonnucleated 

 cells measuring about %2oo i^^ch across the face of each in man, and 

 approximately the same in the rat. These cells contain hemoglobin, 

 a red pigment, which is apparent in the mass but not evident in a 

 single corpuscle. Single corpuscles have a greenish amber cast, and 

 are in the shape of biconcave discs. There are about 5,000,000 red 

 corpuscles in each cubic millimeter of blood in the male human 

 (4,500,000 in female). This furnishes an idea of the number which 

 may be present in the rat. Hemoglobin is capable of combining 

 readily with oxygen in such a way that the oxygen may be given up 

 easily to cells with lower oxygen content. Red cells, then, are the 

 oxygen carriers of the blood. In adult animals these corpuscles are 

 formed primarily in the red bone marrow. In earlier life the liver 

 and spleen supplement this. The total surface area of all the red 

 corpuscles in the body of a rat has been estimated as the equivalent 

 of about 3 square rods. 



The leucocytes are nearly colorless in their natural condition. They 

 are quite variable in size and shape. The majority of the white cells 

 are of the amoeboid type and many have different types of granules 

 in the cytoplasm. These cells are classified according to nuclear 

 condition or shape, size of cell, and staining reaction of these cyto- 

 plasmic granules. These serve the body in devouring foreign mate- 

 rial, including bacteria. In this way they protect the animal's body 

 against toxins and disease. 



