CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 525 



the thoracic duct vary according as this or that region contributes 

 more or less largely to the total amount, and further according to 

 what is going on in this or that region. The difference between 

 the lymph of a fasting animal and the chyle of a fed animal is only 

 one marked difference among many others. 



298. We shewed in 289 that the large serous cavities of 

 the peritoneum, pericardium &c. were to be considered as parts of 

 the lymphatic system, and that the ' serous fluid ' in these cavities 

 was continually joining the lymph stream ; indeed pericardial or 

 other serous fluid has all the general characters of lymph. We 

 have already said, 20, that these fluids when taken fresh from 

 the body, clot (this is, at least, the case in most animals); the clot 

 when examined microscopically is found to consist of colourless 

 corpuscles like those of lymph or of blood entangled in the meshes 

 of fibrin. Both in their proteid and other chemical constituents 

 these serous fluids resemble lymph. Analyses of the accumulations 

 of fluid occasionally occurring in these cavities shew that they 

 contain sometimes less and sometimes more solid matter than 

 ordinary lymph. The aqueous humour of the eye and the cerebro- 

 spinal fluid, though both part of the lymphatic system, are so 

 peculiar that each had better be considered by itself in its proper 

 place. 



299. Chyle. In fasting animals the fluid flowing along the 

 lacteals, as may be seen by inspection of the mesentery, is usually 

 clear and transparent ; it is lymph, differing, as we have said, in no 

 essential respects from the lymph flowing along other lymphatic 

 vessels. Shortly after a meal containing fat (and every meal does 

 contain some fat), the lymph becomes white and opaque like milk, 

 the more so the richer the meal is in fat ; it is then called chyle. 

 Owing to the relatively large quantity of this milky fluid which 

 for some time after a meal continues to be poured into the 

 thoracic duct, the contents of that duct also become milky, and are 

 also called chyle. In the thoracic duct the chyle of the lacteals is 

 more or less mixed with lymph from other lymphatic vessels, but 

 the former is so preponderating that the contents of the duct may 

 be taken as illustrating the nature of chyle. 



Chyle differs from lymph in one important respect, and one 

 only : whereas lymph ordinarily contains a small quantity only of 

 fat, chyle contains a very large amount. The actual amount of 

 fat present in the chyle of the thoracic duct varies, as may be 

 expected, very considerably, according to the nature of the meal, 

 the stage of digestion, and various circumstances. Five per cent, 

 is a very common amount ; in the dog it has been found to vary 

 from 2 to 15 per cent. The increase in fat is chiefly if not ex- 

 clusively due to an increase in the neutral fats ; though whether 

 the small quantity of soaps and of lecithin present is greater 

 than in lymph has not been distinctly ascertained. Cholesterin 

 is probably present in greater amount than in lymph, since it 



