80 PHYSIOLOGY, 



to live an aquatic life, and only dead cells, as of hair, 

 epidermis, etc., live in air. We might also say that not 

 only the human body, but all animal life is aquatic. 



Importance of Lymph. We can see that the move- 

 ment and renewal of lymph are as necessary as the circu- 

 lation of the blood itself ; is, in fact, the most important 

 part of it. 



Lymph Cavities or Serous Cavities. We have noticed 

 the pericardial liquid. There is also a small quantity of 

 similar liquid around the lungs in the pleural cavities, and 

 in the abdominal or peritoneal cavity, around the digestive 

 organs ; also in the cavities of the brain. The liquid in 

 each case is lymph, and these cavities, often called serous 

 cavities, are lymph cavities. They communicate with the 

 lymph tubes. 



Dropsy. In health the amount of the liquid in these 

 cavities is small, but in certain disorders it may accumu- 

 late. In general, such affections are called "dropsy." 

 The lymph may also accumulate in the tissues of the 

 extremities, causing swelling of the limbs. 



Variation in the Composition of Lymph. It is evi- 

 dent that the materials needed by the cells of the different 

 tissues are not the same. So, as one tissue takes certain 

 materials and another tissue others, it is clear that the 

 lymph will not be of quite the same composition in the 

 different parts of the body. This difference is further 

 due to the difference in the waste products thrown out 

 by the different cells. Hence the composition of the 

 blood varies considerably in different regions. But the 

 lymph from all the tissues unites with the blood from all 

 the tissues in the right heart, and on their way to it in the 



