362 HISTORY OF THE HUMAN BODY 



left duct thus comes to furnish two long lateral vessels, one 

 on each side of the aorta, which extend posteriorly until they 

 enter the posterior system near the lymph-hearts, thus uniting 

 the two systems ; and the severance of the original connection 

 between the posterior lymph-hearts and the posterior cardinals, 

 now the postcava, renders the posterior system dependent upon 

 the anterior connections. During later development several 

 important changes take place. The lymphatics gradually 

 spread from the four primary centers over the entire body, 

 the original anterior and posterior systems communicating 

 at every point until all distinction between the two becomes 

 lost; the lymph-hearts lose their identity; two posterior en- 

 largements, the cisterns [receptacula] chyli, appear in the 

 posterior parts of the lateral thoracic ducts ; and the growing 

 intestine becomes supplied with lymphatics from the left lateral 

 thoracic duct. During the growth of the lymphatic vessels 

 numerous secondary centers are formed, from which several 

 vessels radiate in various directions,, and about these, through 

 participation of the connective tissue and the blood-vessels, 

 there develop the characteristic lymphatic nodes or " glands." 

 Similar centers, developed in the course of the left lymphatic 

 duct as it branches within the mesentery, form the mesenteric 

 glands. In these glands the physiological unit seems to be 

 a tuft of blood capillaries surrounded by lymphatic vessels, 

 the whole packed in a loose connective tissue, the lymphoid or 

 adenoid tissue. Such a structure is called a lymph follicle, and 

 a node may consist of a large number of such structures. 

 Within the interstices of the lymphoid tissue occur large quan- 

 tities of lymphocytes, or wandering cells that appear to be 

 identical with the leucocytes or white blood corpuscles, and 

 although proof is thus far wanting, it is probable that the 



^ lymphatic nodes form one of the localities in which these cells 

 are formed. 



In the walls of the colon of mammals appear aggregations 

 of nodules, similar to those connected with the lymphatic 

 system, and forming large areas known as noduli lymphatici 



y dggvcgati \Peyer' s patches']. Many attempts have been made I 



