490 GENERAL CONSIDERATIONS 



middle layer, assisted by the wandering cells or by the passage of material 

 from cell to cell. In coelenterates, ctenophores, and fiatworms the 

 only circulation is that of food carried in the branches of the gastrovas- 

 cular cavity and also passed from one cell to another. In the higher forms 

 circulation is carried on by means of a blood-vascular system. 



The circulating medium of lower animals up to and including the 

 echinoderms is watery in character; in the annelids it is a hemolymph 

 containing protein in solution; but in the mollusks, arthropods, and 

 chordates it is Mood, consisting of fluid plasma and corpuscles. In 

 the blood of invertebrates and the lower chordates the cells in it are 

 generally ameboid and are similar to the white corpuscles in the verte- 

 brates. In the latter red corpuscles are also present. Hemoglobin, 

 which is the main carrier of oxygen in the blood, is dissolved in the 

 plasma of forms which do not have red blood corpuscles. In the mollusks 

 and crustaceans there is another similar substance present known as 

 hemocyanin. Instead of iron, which is an essential constituent of hemo- 

 globin, copper is contained in hemocyanin. Some poisons produce their 

 effects by dissolving the red blood corpuscles, a phenomenon known as 

 hemolysis; this destroys most of the oxygen-carrying power of the blood. 



The circulatory system is the transport system of the body, carrying 

 food and oxygen, wastes of all kinds, and internal secretions from one 

 point to another. The loss of the watery circulatory medium in lower 

 animals is not serious, since replacement is easy, but injury to the blood 

 vessels involving loss of blood is very serious to the higher types, in which 

 replacement takes considerable time. They have, however, a safeguard 

 in the coagulation of the blood. In the lowest animals which possess 

 corpuscles this consists only of a massing together of the corpuscles at 

 the point of injury, but later coagulation includes the formation of a 

 clot which closes the injured vessels and stops the hemorrhage. The 

 clot is composed of fibrin formed from fibrinogen, which is a protein in 

 solution in the plasma. It is produced by the action of thrombin, which 

 is in turn derived from the cellular elements in the blood. The precise 

 nature of thrombin and the exact method of its origin are not known. 



The pressure necessary to maintain the circulation in higher animals 

 is due to the dilation of the arteries by blood forced into them by the 

 heart, the elasticity and muscle tonus of the arterial walls, and the 

 peripheral resistance due to the narrow capillaries. By tonus is meant 

 that the muscle fibers are constantly shghtly stretched and tense, which 

 causes them to react more quickly when stimulated. This condition 

 is partly nervous in origin. 



512. Respiration. — Respiration is necessary to furnish the oxygen that 

 the animal organism needs and to rid the body of waste carbon dioxide. 

 The absorption of gases into the body and their passage outward both 

 take place in obedience to the laws of diffusion of gases. Among 



