CLASS AMPHIBIA 513 



pressure is increased by a constriction of the smaller arteries or 

 arterioles. Their muscular walls may contract from stimuli received 

 from the nervous system or from hormones. 



Blood corpuscles, which are of three kinds, float in the plasma. 

 The erythrocytes are flattened and elliptical, with an oval nucleus 

 in the center. They contain a pigment, hemoglobin, which has the 

 property of absorbing oxygen. The colorless thrombocytes or 

 spindle cells are not as large as the erythrocytes but resemble them 

 except for their tapering ends. When these cells contact certain 

 foreign bodies, they break up, releasing a substance that causes, 

 upon contact with air, the coagulation of certain proteins in the 

 blood plasma in which blood corpuscles become entangled, forming 

 a clot. The insoluble protein strands thus formed are called fibrin 

 (see chapter on The Vertebrate Animal). After the frog has been 

 injured, the formation of a clot prevents indefinite bleeding and 

 makes it possible for the tissues to begin repair. 



The white blood corpuscles or leucocytes are of three kinds: lym- 

 pJiocytes, monocytes, and granulocytes. Their outline is irregular, 

 due to their amoeboid movement, and the shape of their nuclei 

 varies greatly. They are much less numerous in the blood stream 

 than are the red blood corpuscles and spindle cells. Leucocytes 

 may escape from blood capillaries and engulf bacteria and other 

 harmful substances in the tissues. They are finally returned to the 

 venous system by lymphatic vessels. Worn out corpuscles are re- 

 moved from the blood stream by the liver and spleen. The spleen 

 seems to be the primary organ concerned in supplying new blood 

 corpuscles except for a period in the spring when the bone marrow 

 may produce some. Leucocytes may also increase by fission. 



Ljnnphatic System. — The lymphatic system of the bullfrog is an 

 open system comprised of a series of large irregular sinuses in vari- 

 ous parts of the body. It collects lymph from the tissues and 

 eventually returns it to the veins. The lymph is a colorless fiuid 

 containing leucocytes but no erythrocytes. It is derived from seep- 

 age of plasma from the capillaries. It bathes all of the cells, col- 

 lects wastes, and distributes food products. In the region of the 

 intestinal tract, lymphatics absorb a considerable amount of fat 

 and are called lacteals. Lymph removes cellular debris and trans- 

 ports leucocytes which engulf harmful material and cleanse the tis- 

 sues of the body. 



