68 The Vertebrate Organ Systems 



The blood from the legs can follow another pathway back to the 

 heart. Some of the blood in the femoral veins does not enter the 

 renal portal. This blood passes into the ventral abdominal vein which 

 is formed by the junction of branches from the two pelvic veins. The 

 ventral abdominal picks up blood from the bladder and from the body 

 wall and carries it to the liver. 



The hepatic portal vein is formed from the various veins that 

 drain the intestinal tract and its associated organs. The hepatic portal 

 vein ends in the capillaries of the liver; thus dissolved foods from the 

 digestive tract are first filtered through the liver and some are re- 

 moved from the blood for temporary storage. Blood passes by means 

 of the hepatic vein to the posterior vena cava and thence into the 

 sinus venosus. 



The Blood. — The blood consists of two main portions : the Hq- 

 uid or plasma and the formed bodies consisting of the erythrocytes, 

 (Fig. 3) thrombocytes, and leucocytes. The erythrocytes or red blood 

 cells contain the respiratory pigment hemoglobin which carries oxygen. 

 There are approximately 400,000 of these cells per cubic millimeter 

 of blood, and they are elliptical in shape and possess nuclei. 



The white blood cells or leucocytes number about 7,000 per cubic 

 millimeter of blood. They are colorless, nucleated, and in general 

 move by amoeboid motions. There are several different types of leuco- 

 cytes which have separate functions. Some of them destroy invading 

 organisms, others remove dead cells. 



Thrombocytes, which are small spindle-shaped cells, aid in the 

 clotting or coagulation of blood. This latter process is very important 

 for the prevention of excessive bleeding after injury. 



The Lymphatic System. — From the thin-walled capillaries of 

 the circulatory system, a certain amount of the blood plasma with con- 

 tained foods, minerals, etc., escapes into the surrounding tissues. This 

 material, known as tissue fluid, brings the essential elements of the 

 blood to the individual cells. Once this fluid has lost its load of food 

 materials and has picked up wastes, it is known as lymph. 



A series of lymph ducts and lymph spaces provide the means for 

 circulating the lymph through the body and finally returning it to the 

 main portion of the circulatory system. The lymph enters either the 

 internal jugulars or one portion of the iliacs through special ducts. | 



Beneath the skin is a large subcutaneous sinus. At the dorsal 

 portion of the body cavity is another large lymph space separated from 



