432 ELEMENTARY ANATOMY. [LESS. 



be both more complex and that it may be the seat of more 

 activity. 



The right lymphatic duct may be so developed that there 

 may properly be said to be two thoracic ducts, side by side, 

 as in Birds. 



The thoracic duct, even in man's own class, may be double, 

 and may bifurcate at a higher or lower situation. 



Again, the receptaculum may be in the form of a plexus, 

 as in the Kangaroo. 



The lymphatics of the right side of the head and neck 

 may open directly into the jugular vein, into which vein the 

 thoracic ducts also empty themselves, as in Birds ; and in 

 the same class the lymphatics about the kidney open also into 

 the renal and sacral veins. The thoracic duct may unite 

 with the azygos vein, as, at least sometimes, in the Hog. 



The lymphatics may open directly into the coccygeal vein, 

 as in Fishes. 



Lymphatic glands may be absent, as in Reptiles below the 

 Crocodiles. They may be few, and confined to the region of 

 the neck, as in Birds. 



The lymphatics generally may take on an exaggerated 

 form of that condition which they have in the human brain ; 

 that is, they may generally appear as large reservoirs 

 (sinuses) surrounding the true blood-vessels. Such is the 

 case in the lower Vertebrata, especially in Batrachians, where 

 also they may form great sinuses between the skin and the 

 flesh, or between the muscles. 



The lymphatics may be devoid of valves (which exist only 

 at their junction with the veins), as in the lower classes of 

 the Vertebrate sub-kingdom ; or the valves may be few in 

 number, as in Birds. 



The walls of the lymphatics, in certain localities, may 

 become muscular and rhythmically contractile. Such pulsatile 

 structures are called lymphatic hearts. There may be four 

 of these structures, as in the Frog, where two such organs 

 pump the contained fluid into small veins communicating 

 with the subclavian veins at the shoulder, while two others, 

 placed at the coccyx, send their contents into the crural vein. 



Two dilated lymphatic structures, answering to the hinder 

 lymphatic hearts of the Frog, may exist, as in some Reptiles, 

 and also in Birds, e.%. the Goose, Ostrich, and others. In 

 these classes, however, they are not rhythmically contrac- 

 tile pulsating structures, though even in Birds they contain 

 striated muscular fibres. 



