STRUCTURE OF THE VERTEBRATES 245 



ance of a definite group of vessels leading from the inter-cellular 

 spaces and digestive tract to the veins. 



The mammalian lymphatic system shows the following 

 changes: (1) the terminal capillaries ramify more fully through 

 the tissues; (2) there are no definite sinuses with the exception 

 of minute ones in several glands; (3) the posterior openings into 

 the veins are lost, the entire body lymph entering the veins 

 through the two anterior connections with the subclavians; (4) 

 the area drained by the right lymphatic duct is limited to the 

 head and thoracic region, while the left duct drains the left 

 anterior half and the entire posterior part of the body; and (5) 

 lymph nodes develop. 



The lymph nodes of the mammal are ovoid masses of glandu- 

 lar connective tissue, each divided into a number of independent 

 lobes or follicles. The lobe has a center of blood capillaries sur- 

 rounded by small lymphatics. The connective tissue is character- 

 ized by rather close-packed small cells with relatively large 

 nuclei, without any definite arrangement. The largest lymph 

 nodes are in the neck, the axilla (arm pit), and groin. The lymph 

 nodes have a double function, (1) the formation of certain white 

 blood cells and (2) filtration of the blood. In many types of in- 

 fection the lymph nodes swell due to the concentration of bacteria 

 at these points. Similar masses of lymphatic tissue appear in 

 other parts of the body: in the mesenteries; the intestine where 

 they are called Peyer's patches; the thymus; and the adenoidal 

 tissues of the naso-pharynx. 



Developmext. The mammalian lymphatics begin develop- 

 ment as seA^eral outpocketings from the veins at the juncture of 

 the jugulars and subclavians. As these grow anteriorly and 

 posteriorly lymph hearts appear, and from the latter many 

 branches grow out into the tissues. The branches continue divid- 

 ing until the entire system of vessels is formed, each h'mphatic 

 capillary ending blindly in the tissues. There is little evidence 

 to support the theory that the lymphatic vessels have a double 

 origin, with the capillaries arising as enlarged inter-cellular 

 spaces independent of the outpocketings from the veins. The 

 terminations of the lymphatic capillaries are closed, the lining 

 cells of all the vessels are similar and continuous, and the em- 



