424 CHYLIFEROUS SYSTEM. 



union of their trunks form a large elongated irregular reser- 

 voir, from which proceed two or sometimes more anasto- 

 mosing thoracic ducts, which form almost a continued plexus 

 by their frequent unions, as they proceed forwards along the 

 left branch of the aorta, to the anterior part of the trunk, 

 where they terminate in the jugular or the subclavian vein, or 

 in the angle of union between these vessels. The frequent 

 unions, and the great multiplicity of chyliferous vessels in all 

 vertebrated animals compared with the arteries or the veins, 

 are rendered necessary by their imperfect means of propelling 

 their contents, and by the variable pressures to which they are 

 subjected. From the approximation of the crocodilian reptiles 

 to the carnivorous mammalia, in structure, food, and habits, 

 they already exhibit a larger proportion of fibririous globules, 

 and a more sanguineous character of their chyle, than have 

 been observed in other reptiles. During the active feeding sea- 

 son of the chelonian reptiles, their lacteals are found turgid 

 with chyle, which can be pressed forwards, in large quantity 

 and repeatedly, from capillaries to trunks, and from trunks 

 backwards to capillaries, in the opened bodies of these ani- 

 mals, so remarkable for their tenacity of life. So frequent 

 are the anastomoses of the several thoracic ducts, which form 

 a plexus round the aorta in the tortoise, that when inflated 

 with air, they entirely cover and conceal that vessel. Before 

 entering the veins, these ducts receive the lymphatic trunks 

 from the head and arms, but no anterior lymphatic hearts 

 are seen, although they are seen on the posterior lymphatic 

 trunks of many reptiles. 



The coats of the chyliferous vessels are still very thin and 

 distensible in birds, and their valves, which are more abun- 

 dant on the trunks and branches than in reptiles, are still so 

 incomplete as to aUow injections to pass freely against their 

 course, from trunks to capillaries ; and although conglomerate 

 glands are already perceived on the lymphatics, especially 

 in the neck, no similar glands are yet developed on the lac- 

 teals of the mesentery. The chyle is still limpid and colour- 

 less, as in the cold-blooded vertebrata, and the more conglo- 

 merate glands of quadrupeds are still represented by simple 

 plexuses of lacteals between the folds of the mesentery, in 

 this warm-blooded oviparous class. The lacteal vessels are 

 now more crowded in layers below the serous and above the 



