340 THE VERTEBRATE BODY 



We hope that we are not discovered to meet a similar fate. We realize 

 that these are the leucocytes, white blood cells, which are the watch dogs 

 of the blood, and the object that they engulfed was a group of staphylo- 

 coccus bacteria, which are responsible for many infections. 



Finally, there are spindle-shaped cells, the thrombocytes, floating past 

 in the plasma. We watch some of these as they flow through the break 

 in the capillary and see a remarkable thing. When they touch the outer 

 skin they burst like a balloon liberating a fluid that was contained within 

 them. This fluid, upon mixing with the plasma, forms a tough, fibrous 

 clot which is rapidly sealing the break and the flow of blood through 

 it is soon stopped. We realize that these cells are responsible for the 

 important process of blood clotting. 



We decide to explore this marvelous system further, so we hop on an 

 erythrocyte as it floats past and go swirling along in the stream of 

 plasma. We notice that the size of the vessel in which we flow becomes 

 larger as it is joined by many other small vessels and we are soon floating 

 slowly in a huge vessel that appears miles across. All of the erythro- 

 cytes are now a deep dark red in color and we realize that we have 

 passed into a vein which our guide book tells us is the femoral vein. We 

 pass by the large opening of the pelvic vein and are in the renal portal 

 vein. As we approach the kidney, the vessel gives off many branches 

 which become smaller as they rebranch and we soon find ourselves again 

 in a capillary. Here the pressure is very great and almost crushes us, 

 for the vessels bringing the blood in are larger than those carrying it 

 away. Under this great pressure we can see some of the plasma with its 

 dissolved minerals being squeezed through the capillary walls into the 

 tubules of the kidney. A little later the tremendous pressure is dimin- 

 ished and much of the water and useful minerals are reabsorbed into the 

 blood. We next flow into the large postcaval vein and on into a huge 

 cavity, the sinus venosus. Beneath us we can feel the mighty pulsa- 

 tions of the heart, which cause this entire region of the body to heave 

 in an alarming manner. Suddenly, directly beneath us, valves open up 

 and we are engulfed in a huge whirlpool that sucks us through an open- 

 ing into the right auricle. We have hardly had time to regain our com- 

 posure when the walls of the auricle come crushing in on us and we are 

 forced through another valve into the ventricle. We know what is to 

 follow so we cling to our raft as the ventricle closes in with a mighty 

 force and are thrown out through another valve into the conus arteriosus. 

 This soon branches into three parts, but most of the first blood to leave 

 the ventricle goes into the first branch, which is the pulmocutaneous 

 artery. As it forks again, we take the pulmonary artery to the lungs. 



