CIRCULATORY ORGANS. 26 5 
BLOOD AND LYMPH. 
The two circulating fluids, blood and lymph, are much alike. 
Each consists of a fluid portion, the plasma, in which float numer- 
ous solid particles, the corpuscles. The plasma is colorless or 
slightly yellow and can be separated by clotting into a solid part, 
fibrin, and a fluid, the serum, which is, under ordinary circum- 
stances, incapable of clotting again. The lymph plasma contains 
-less of the fibrin-forming substances (fibrinogen) than does the blood 
plasma. The composition of the plasma is very complex. Besides 
water it contains proteids, extractives, salts, and a number of less- 
known substances, internal secretions, enzymes, etc. The plasma 
can also absorb a considerable amount of carbon dioxide. It serves 
to carry nourishment to the tissues and takes away from them the 
waste of metabolism. 
The corpuscles are of three kinds, erythrocytes, leucocytes and 
blood plates. Only the leucocytes occur in the lymph while the 
blood contains all three. 
The erythrocytes, or red corpuscles give the blood its color. 
They have fixed outlines and are flattened oval discs in the non- 
mammals and the camels, circular biconcave discs in the other mam- 
mals, and in all except the mammals they are nucleated throughout 
their existence. They owe their color to an iron-containing proteid, 
hemoglobin, which readily combines with oxygen and carbon dioxide 
and as readily gives up these gases in places where they are scanty. 
This renders the erythrocytes the respiratory elements of the blood. 
It has recently been stated that the erythrocytes of the mammals are hat- 
shaped, (hollow cones) while inside the blood-vessels and that they assume the 
biconcave shape after leaving them. This account has been disputed. 
The size of the erythrocytes varies in different vertebrates, being the largest 
in the amphibia (Amphiuma) and smallest in the vertebrates (musk deer). A 
few measurements are giving here in microns (0.001 mm.). Where two dimen- 
sions are given they are the length and breadth of the oval corpuscles. Musk 
deer, 2.54; man, 7.74; hen, 7x12p; carp, gxi5#; frog, 16x25; Necturus, 
31x58.54; Amphiuma, ?x75H. 
In the higher vertebrates the red corpuscles arise by division of giant cells 
(erythroblasts) in the red bone marrow, but in the young and at times of great 
depletion of the blood new red corpuscles may be formed in the spleen and the 
liver. At first all are nucleated but in the mammals the nucleus is soon lost. 
The leucocytes or white corpuscles (divided accordingly as they 
