102 THE SPLEEN. 



is as necessary as the circulation of the blood itself ; is, in 

 fact, the most important part of it. 



We have noticed the pericardia! liquid. There is also a 

 small quantity of similar liquid around the lungs in the pleu- 

 ral 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. In health the amount of the liquid in these 

 cavities is small, but in certain disorders it may accumulate. 

 In general, such affections are called " dropsy." The lymph 

 may also accumulate in the tissues of the extremities, causing 

 swelling of the limbs. 



It is evident 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 n-ot 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 differ- 

 ent cells. Hence the composition of the blood varies con- 

 siderably 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 larger veins. So the con- 

 stant slight differences in composition of the blood and lymph 

 in the various tissues are counterbalanced by the mingling of 

 the currents from these various parts in the large streams to 

 and from the heart that we call the arteries and veins. 



The Spleen. The function, or functions, of the spleen 

 are not well understood. It is believed to have something to 

 do with the renovation of the blood, perhaps forming colorless 

 corpuscles and destroying colored corpuscles. At any rate, 

 the physiologists generally call it a blood gland. It is unlike 

 true glands in that it has no duct, and forms no secretion to 



