CIRCULATORY ORGANS. 265 



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, 

 haemoglobin, 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 (o.ooi mm.). Where two dimen- 

 sions are given they are the length and breadth of the oval corpuscles. Musk 

 deer, 2.5^; man, 7.7^; hen, 7x12^; carp, 9x15^; frog, 16x25;*; Necturus, 

 31x58.5/4; Amphiuma, ?X75/*. 



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 



