512 ABSORPTION OF FAT. [B OK 



The Course taken by the Several Products of Digestion. 



306. 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 dif- 

 fusible 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 capillary, passes 

 on to the deep-seated lacteal radicle, and so finds its way into 

 the lymphatic system. And the results of observation and experi- 

 ment, 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 faeces 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 undergoes a destructive 

 decomposition in the alimentary canal. Collecting 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, shewing 

 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, during 

 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 appears in the 

 portal blood during digestion is, for the most part at least, not fat 

 absorbed by the capillaries of the alimentary canal but fat absorbed 

 by the lacteals. Moreover, when the chyle of the thoracic duct is 

 diverted through a cannula, and not allowed to flow into the blood, 

 the quantity of fat in the portal blood as in the blood at large is 

 very small indeed. Lastly, when a villus of an intestine in full 



