202 FUNCTIONS OF NUTRITION. 



these vessels were discovered in 1622 by Asellius, who first saw them 

 on opening the abdomen of a dog, a few hours after the ingestion 

 of food. The discovery of the general lymphatic system was made 

 subsequently by Rudbeck and Bartholin, in 1651 and 1653, and- 

 was consequent upon the previous observations on the lacteals of the 

 abdomen. 



That the white color of the chyle during digestion is really due to 

 the presence of fatty substances absorbed from the intestine, is shown 

 by the fact that its intensity is in proportion to the quantity of fat 

 in the food. It is generally less marked in herbivorous than in carn- 

 ivorous animals. According to the observations of Tiedemann and 

 Gmelin, in a dog fed with fatty matters the lacteals are abundantly 

 filled with an opaque white fluid, while in the same animal fed with 

 starchy matters alone, the chyle is pale and but slightly opaline ; and 

 Bernard has shown that if, in a dog after several days' fasting, a little 

 ether, containing fat in solution, be injected into the stomach, without 

 the introduction of other food, at the end of a few hours the lacteals 

 are fully distended with chyle, similar in appearance to that seen during 

 ordinary digestion. 



Passage of Absorbed Materials into the General Circulation. 



The products of digestion, taken up by the blood-vessels and lym- 

 phatics of the intestine, pass by two different routes into the general 

 circulation. The blood of the portal vein, containing peptone, glucose, 

 and molecular fat, is carried to the liver, where it traverses the capil- 

 lary vessels of this organ before reaching the vena cava and the 

 right side of the heart. The chyle, on the other hand, containing 

 a large proportion of fatty ingredients, passes by the thoracic duct to 

 the left subclavian vein, and there mingles with the returning current 

 of the venous blood. But all these substances, after entering the cir- 

 culation and coming in contact with the blood, are so modified as no 

 longer to be recognizable in their original form. This change takes 

 place very rapidly with peptone and glucose. Peptone passes, in all 

 probability, into the condition of albumen ; while the glucose is for the 

 most part deposited in the liver in an insoluble form, those portions 

 which reach the general circulation being decomposed or transformed, 

 and thus losing their characteristic properties. The fatty matters 

 also undergo a transformation while passing through the lungs by 

 which their distinctive characters are destroyed, and they are no longer 

 visible as oleaginous particles. This alteration is so complete, during 

 the early part of digestion, or when the proportion of fat in the food 

 is small, that all the oleaginous master disappears in the lungs, and 

 none is to be detected in the general circulation. 



But as digestion proceeds, especially with food abundant in oleaginous 

 substances, an increasing quantity of fat finds its way into the blood, 

 and a time arrives when the whole of it is not destroyed during its 

 passage through the lungs. Its absorption then taking place more 



