192 MANUAL OF ANIMAL BIOLOGY 



proportion of the oxygen is conveyed to the tissues of the body. 

 Here the unstable oxyhemoglobin in the red corpuscles is broken 

 down, and the free oxygen passes through the walls of the capil- 

 laries and is taken up by the tissue cells. The red cells containing 

 the hemoglobin are then returned to the lungs and the process is 

 repeated. 



3. Vascular, or Circulatory, System 



The vascular system of the Frog and of the Vertebrates in gen- 

 eral has the general structural features of the vascular systems that 

 were noted in the various higher Invertebrates. Thus we find that 

 it consists essentially of a connected, closed system of muscular- 

 walled tubes, or blood vessels, together with a circulating fluid 

 medium, the blood. The blood is continuously propelled through 

 the vessels by the rhythmic contractions of the heart. The lat- 

 ter is to be regarded as a blood vessel which has become adapted 

 for this particular function by the development of a special type of 

 muscular tissue. It is the dynamic and all important center of the 

 system. The blood vessels are divided into arteries, which carry 

 blood away from the heart; veins, which carry blood to the 

 heart; and capillaries, which connect the arteries and veins 

 together in the tissues, through an amazing network of thin-walled, 

 microscopic vessels. Blood leaving the heart must always make 

 the complete circuit involving certain arteries, capillaries, and 

 veins before it again reaches the heart. (W. pp. 168-179.) 



In addition to the closed circulatory system, as just noted, and 

 of fundamental importance is the open lymphatic system which 

 consists primarily of various sized lymph spaces, channels, and 

 thin-walled lymph vessels through which a fluid derivative of the 

 blood, the lymph, slowly moves. The system is so arranged that 

 every cell in the organism is bathed by this lymph, so that in the 

 transfer of materials to and from the cell, the lymph stands as the 

 final agent. 



Blood. We may now consider certain structural features of the 

 vascular system in more detail. The blood may be regarded as a 

 liquid tissue, that is, a tissue in which the intercellular material is 

 of a liquid rather than a solid nature. In all other tissues of the 

 organism, the intercellular material holds the cells rigidly in place. 

 The blood is composed of a liquid substance, or plasma, which con- 

 tains enormous numbers of various types of highly developed blood 



