364 HISTORY OF THE HUMAN BODY 



dally erythrocytes (red blood corpuscles). Although lym- 

 phatic vessels secondarily invade these organs, and are hence 

 found in the adult in close association with them, they are 

 not to be considered parts of the lymphatic system any more 

 than one would consider the kidneys a part of the circulatory 

 system because their tissues are invaded by special forms of 

 blood-vessels. 



In their function as formative nidi for the cellular ele- 

 ments of the blood and lymph these organs form physiologi- 

 cally important auxiliaries to the vascular system as a whole, 

 but belong elsewhere in their anatomical and developmental 

 affinities. 



