CLASS AMPHIBIA 503 



sary for metabolism aud remove from them to the excretory organs, 

 waste products of body activity. They differ in several respects; 

 the lymph neither contains red blood corpuscles for transporting 

 oxygen nor moves in a continuous closed vascular circuit as does 

 the blood. Other differences will be noted in the discussion. 



The Blood Vascular System. — The blood moves through a closed 

 system of tubelike vessels of various sizes which distribute it to 

 all parts of the body. The pump is the heart, which, by its con- 

 tractions, forces the blood to flow to the tissues. Since the system 

 is a closed one, the blood eventually returns to the heart. 



The blood vessels leading away from the heart are the arteries. 

 When these reach the tissues, they break up into very small vessels, 

 the capillaries. The vessels leading back to the heart are the veins. 

 The arteries and veins are connected by the capillaries. 



Blood is comprised of a clear liquid called the 'plasma, suspended 

 in which are blood corpuscles of three kinds, the red blood corpuscles 

 or erythrocytes, the white blood corpuscles or leucocytes, and the 

 spindle cells or thrombocytes. In addition, the blood may contain 

 dissolved nutritive substances from the digestive system, waste 

 products from tissue repair aud destruction, hormones being trans- 

 ported from organs of one part of the body to another, or foreign 

 substances accidentally introduced. 



The capillaries are very small vessels, the walls of which are made 

 up of endothelium continued from the linings of arteries and veins. 

 The}^ connect the distal ends of the arteries with the proximal ends 

 of the veins, but in so doing they branch extensively and anastomose 

 to form fine networks in the tissues invaded. Through their thin 

 walls, acting as semipermeable membranes, food products brought 

 by the arterial blood pass into the tissues, oxygen is unloaded from 

 the red blood corpuscles, and carbon dioxide and waste products 

 are taken up to be conducted into the veins. Leucocytes are able 

 to get out of the capillaries, squeezing their way between the cells 

 of the capillary walls, and thus become free in the surrounding tis- 

 sue to engulf bacteria or other harmful objects. 



The abundance of the capillaries varies with the activity of the 

 organ ; the greater the rate of metabolism the greater their abundance. 

 Examples of such are the various glands and the mucous membrane 

 of the digestive tract, in contrast, a tendon has few capillaries. 



