286 COMPARATIVE MORPHOLOGY OF VJ ITI IIATES 



the activator is formed, to act. This subject is a new one and much 

 may be expected from it in the future. 



The structures concerned in the circulation are two fluids, the 

 blood and the lymph ; and the vessels (vascular system) in which the 

 fluids circulate, certain parts of the vessels being specialized (hearts) 

 for the propulsion of the blood and lymph. A blood heart occurs in 

 all vertebrates in connexion with the blood circulation; most verte- 

 brates have lymph hearts in connexion with the lymph-vessels, but 

 in the higher groups the flow of the lymph is due to the blood pressure 

 and also to the motion of the parts through which the lymph-vessels 

 course. 



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 sefum, which is, under ordinary circumstances, 

 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 (except cyclostomes) and the camels, circular biconcave 

 discs in the other mammals, 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 to a less extent with 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 binocave shape after leaving them. This account has been disputed. 



