CIRCULATION AND RESPIRATION 



133 



Lymphatic System. — Among invertebrate animals the blood, 

 lymph, and body fluids are in many cases the same fluid but in 

 vertebrates lymph is distinct from blood and a portion of its 

 circulatory pathway is composed of lymphatic vessels, which are 

 distinct from blood vessels. Lymphatic vessels are best devel- 

 oped in mammals. In the frog the lymphatic system is charac- 

 terized by numbers of large communicating lymph spaces, 

 especially in the subcutaneous region. The cisterna magna, one 

 of the largest lymph spaces, is formed 

 in the subvertebral region, as a large 

 sac in the dorsal wall of the body 

 cavity. Lymph spaces are also found 

 within and between the organs, the 

 larger spaces being lined with flat- 

 tened endothelial cells. 



There are two pairs of lymph hearts 

 in the frog. The two anterior hearts 

 are located on the posterior side of 

 the transverse processes of the third 

 vertebra, each heart connecting with 

 a vertebral vein, which in turn joins 

 an internal jugular vein. The two 

 posterior hearts lie one on either side 

 of the tip of the urostyle, each heart 

 connecting with the transverse iliac 

 vein, which is a vein joining the sci- 

 atic and femoral veins near the hip 

 joint. Each heart is provided with a pair of semilunar valves 

 at its venous opening. The lymph hearts pulsate rhythmically 

 and slowly pump the lymph into the blood. There is no relation 

 between the beating of the heart and the pulsations of the 

 lymph hearts. The action of the lymph hearts on opposite sides 

 of the body is not correlated. Another connection between the 

 lymphatic system and the blood stream is through ciliated open- 

 ings (nephrostomes) from the body cavity into some of the 

 tubules opening on the ventral surface of the kidney. The rela- 

 tion of these tubules to the blood system will be discussed in 

 connection with the kidneys in the following chapter. The 

 lymph entering the nephrostomes comes from the body cavity. 



Human Lymphatic System. — In the human body there are 

 well-developed lymphatic vessels, which originate in lymph 



Fig. 83. — Diagram to show the 

 position of the cup-like valves in 

 an opened vein spread out flat. 

 The arrow shows the direction of 

 blood flow. 



