THE LYMPHATIC SYSTEM. 319 



It tli us appears that both the fibrin and the albumen of the blood 

 actually transude to a certain extent from the bloodvessels, even in 

 the ordinary condition of the circulatory system. But this transuda- 

 tion takes place in so small a quantity that the albuminous matters 

 are all taken up again by the lymphatic vessels, and do not appear 

 in the excreted fluids. 



The first important peculiarity which is noticed in regard to the 

 fluid of the lymphatic system, especially in the carnivorous animals, 

 is that it varies very much, both in appearance and constitution, at 

 different times. In the ruminating and graminivorous animals, 

 such as the sheep, ox, goat, horse, &c., it is either opalescent in 

 appearance, with a slight amber tinge, or nearly transparent and 

 colorless. In the carnivorous animals, such as the dog and cat, it 

 is also opaline and amber colored, in the intervals of digestion, but 

 soon after feeding becomes of a dense, opaque, milky white, and con- 

 tinues to present that appearance until the processes of digestion 

 and intestinal absorption are complete. It then regains its original 

 aspect, and remains opaline or semi-transparent until digestion is 

 again in progress. 



The cause of this variable constitution of the fluid discharged 

 by the thoracic duct is the absorption of fatty substances from the 

 intestine during digestion. Whenever fatty substances exist in con- 

 siderable quantity in the food, they are reduced, by the process of 

 digestion, to a white, creamy mixture of molecular fat, suspended 

 in an albuminous menstruum. The mixture is then absorbed by 

 the lymphatics of the mesentery, and transported by them through 

 the thoracic duct to the subclavian vein. While this absorption is 

 going on, therefore, the fluid of the thoracic duct alters its appear- 

 ance, becomes white and opaque, and is then called chyle; so that 

 there are two different conditions, in which the contents of the great 

 lymphatic trunks present different appearances. In the fasting 

 condition, these vessels contain a semi-transparent, or opaline and 

 nearly colorless lymph; and during digestion, an opaque, milky 

 chyle. It is on this account that the lymphatics of the mesentery 

 are called " lacteals." 



The chyle, accordingly, is nothing more than the lymph which 

 is constantly absorbed by the lymphatic system everywhere, with 

 the addition of more or less fatty ingredients taken up from the 

 intestine during the digestion of food. 



The results of analysis show positively that the varying appear- 

 ance of the lymphatic fluids is really due to this cause ; for though 



