270 PHYSIOLOGY CHAP. 



done in Ludwig's laboratory by his pupils Rb'hrig (1874), Zawilski 

 (1876), von Mering (1877), Schmidt-Mulheini (1877), and con- 

 firmed by I. Munk and Koseustein (1890), for man in a case of 

 lymphatic listula, corrected this erroneous assumption, and showed 

 by a great number of observations that it is only the fats (and 

 again only a part of these) which pass by way of the thoracic duct, 

 while all the other foods absorbed travel, for the most part at least, 

 by the blood capillaries of the villi, and pass directly to the liver 

 by the venous portal system. 



On introducing a cannula into the thoracic duct of a dog near 

 its opening into the left subclavian vein, it is seen that the flow 

 from the cannula in the time unit is not sensibly augmented 

 during the period of digestion and absorption ; but the lymph, 

 which is semi-transparent previous to digestion, turns into chyle, 

 i.e. becomes opaque and milky, during digestion, particularly after 

 a meal rich in fatty substances. In fact, comparative analysis of 

 the chemical composition of the lymph and chyle from the thoracic 

 duct shows that the sole difference between the two fluids lies in 

 the preponderance of fat in the chyle, which varies in character 

 according to the nature and amount of the fats ingested. Most of 

 it is present as emulsified neutral fats, in the form of the finest 

 droplets visible under the microscope, which dissolve in ether, 

 and stain black with osnaic acid. A small part (about ^V) is 

 present in the form of saponified fatty acids. After a meal rich in 

 fatty substances the chyle collected from man may contain 4'7 per 

 cent fat (I. Munk), and that from the dog 14 - 6 per cent (Zawilski) ; 

 while the lymph that flows from the fistula in the fasting state 

 contains only O'OG-0'26 per cent. 



This naturally leads to the question whether the whole of the 

 alimentary fat absorbed by the gastro-intestinal mucous membrane 

 passes by the thoracic duct to reach the blood stream, or whether 

 part of it may pass along the portal system or by other paths. 

 On giving a measured quantity of fat to a dog with fistula of the 

 thoracic duct, and subtracting the small amount of fat which 

 leaves by the faeces, we obtain approximately the quantity of fat 

 absorbed, which may be compared with the total amount that 

 escapes by the fistula during the whole time in which the lymph 

 preserves the milky appearance of chyle. On estimating this 

 quantity, it is found to be about 40-50 per cent of the total fat 

 absorbed. There is thus a deficit of 50-60 parts of the fat, which 

 must pass into the blood, not by the thoracic duct, but by some 

 other way. This fat might conceivably pass by the paths that feed 

 the portal system, since some authors have found that the serum 

 separated from, the portal blood was also milky during digestion. 

 According to Foster, however, this argument is fallacious, because 

 during the digestion of a meal rich in fats the whole of the blood, 

 including that of the carotid, contains much fat, and the serum of the 



