CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 413 



culation without having passed through the liver. When this 

 is done very grave troubles result. We may therefore consider 

 first which of the two paths is, as a matter of fact, taken by the 

 several products, and subsequently study the mechanism of ab- 

 sorption in the two cases. 



The Course taken by the Several Products of Digestion. 



247. From what has already been said we have been led 

 to regard the villi as the most active organs of absorption, and 

 the structure of a villus leads us further to conclude that the 

 diffusible peptones and sugar pass, together with the water in 

 which they are dissolved, into the superficially placed capillary 

 network of the villus and so into the portal system, while the 

 merely emulsified fat, unable to traverse the wall of the capil- 

 lary, passes on to the deap-seated lacteal radicle, and so finds 

 its way into the lymphatic system. And the results of obser- 

 vation and experiment, as far as they go, support this view. 



Fats. After a meal containing fat the lymph of the lacteals 

 contains fat, and is now called chyle ; and the richer the meal 

 in fat the more conspicuous is the fat in the lymph-vessels. 

 We cannot however prove that all the fat of a meal absorbed 

 from the alimentary is poured by the thoracic duct into the 

 venous system. If a meal containing a known quantity of fat 

 be given to a dog and the small quantity of fat present in the 

 f geces corresponding to the meal be subtracted from that amount, 

 we can determine the amount of fat absorbed, for we have no 

 evidence whatever that any appreciable amount of fat under- 

 goes a destructive decomposition in the alimentary canal. Col- 

 lecting by means of a cannula inserted into the thoracic duct 

 the whole of the chyle during and after the meal so long as it 

 remains milky, showing that fat is being absorbed, we can 

 ascertain the quantity of absorbed fat, which would, but for 

 the operation, have passed into the venous system. When this 

 has been done, a very remarkable deficit, amounting it may be 

 to 40 or 50 p.c. has been observed ; that is to say, of every 100 

 parts of fat which disappear from the alimentary canal only 

 about 60 parts find their way through the thoracic duct into 

 the venous system. 



Are we then to conclude that the missing quantity finds its 

 way into the portal system ? Now the portal blood does, dur- 

 ing digestion, contain a certain quantity of fat; indeed the 

 serum is said at times to appear milky from the presence of fat. 

 But the whole circulating blood during the digestion of a fatty 

 meal contains, for a while, the fat poured into it by the thoracic 

 duct ; and it has been ascertained in the dog that the blood of 

 the portal vein during digestion contains not more but less fat 

 than the blood of the carotid artery, so that the fat which 



