426 CIIYLIFEROUS SYSTEM. 



chyli is frequently formed, here and in lower classes, by a 

 simple plexus of chyliferous and lymphatic trunks, and that 

 plexus is sometimes extended forwards along the aorta, dimi- 

 nishing the frequency of the anastomoses, to constitute one or 

 two principal thoracic ducts which enter the subclavian veins. 

 Sometimes the branches of the thoracic duct open directly 

 into the vena azygos, as in the hog, and a similar communi- 

 cation has been seen as an abnormal structure in man. 



The chyle is conveyed so abundantly through the thoracic 

 duct, that six ounces per hour have been computed to flow 

 through that canal in the dog, and on tying the thoracic duct, 

 the chyle has been found still to pass forwards to the veins by 

 the enlargement of the lateral lymphatic trunks. There are 

 always distinct valves at the entrance of the thoracic ducts 

 into the veins*, to check the return of blood into these dila- 

 table canals, as in all the lower vertebrated classes. The 

 influence of the mesenteric glands on the chyle which passes 

 through their highly vascular and complicated network, 

 is not determined; but, by their grouping and uniting 

 together, they often imitate the lobu'lated and conglomerate 

 form of other less ambiguous glands. They are of great 

 size in the cetacea, they are more detached in the solidun- 

 gulous pachyderma and in the ruminantia, and they are 

 often collected into a conglomerate mass, termed pancreas 

 Asellii, in the carnivorous quadrupeds. There are forty of 

 these small round lenticular glands in the bradypus ; they are 

 approximated into a group of about thirty in the hog tribe. 

 In the myrmecophaga, those of the small intestine are col- 

 lected into a mass, and about twenty small glands are spread 

 apart on the mesocolon ; they are also grouped into a 

 mass in the armadillo, the mole, and the nasua. In the 

 hyrax they are a little separate, and in the manis they are 

 at a distance from each other ; but in the roderitia, so infe- 

 rior in most of their characters, they are the smallest in 

 number and the least developed. 



Besides the pancreas Asellii, which has always its efferent 

 as well as its inferent ducts, there are generally a few de- 

 tached mesenteric glands in the carnivora, as in the otter, 

 the seal, the badyer, the dog, the cat, and other species, 

 and the same is seen in the hedgehog ; but in the bear they 

 form two masses, the one belonging to the mesentery, and 



