i8 THE FUNCTIONS OF ANIAiWZS. 



duct, being part of the lymphatic system, contains some lymph(which may 

 be roughly described as blood mintis its red corpuscles), while the chyle 

 which forms the greater part of the content of the duct is lymph //«« 

 the finely divided fat absorbed from the intestine. Every movement ot 

 the body aids the fiirther progress of the chyle by causing alterations ot 

 pressure, while flow is only permitted in one direction owing to thp 

 great number of valves. The thoracic duct, finally, opens into the 

 'unction of the left jugular and left subclavian veins at the root of the 

 neck. 



The contents of the duct of a fasting animal are clear ; after 

 a meal they become milky ; the change is due to the matters 

 discharged into it by the lacteals. It is probable that nearly 

 all the fat of a meal is absorbed from the intestines by the 

 lacteals, but it is not certain in what measure, if at all, this 

 is true of the other dissolved food-stuffs ; the greater part 

 certainly passes into the capillaries of the portal system, 

 which are contained in each villus. The peptone or 

 digested proteid, as it passes through the cells of the villi, 

 is changed into other proteids nearly related to those of the 

 blood, for no peptone is found in the portal vein. 



We now know the fate of the fats, and of the proteids of 

 the food, and the manner in which they pass into the blood ; 

 but we must follow the starchy material, or carbohydrates, 

 Va little further^ The starch, we know, is converted into sugar, 

 andTliis,~wifh the sugar of the food, passes into the capil- 

 laries of the villi, and is carried to the liver. During diges- 

 tion there is an increase of sugar in the blood vessel going 

 to the liver from the intestine, that is, in the portal vein, but 

 no increase in the vessel leaving the liver. The increase 

 must therefore be retained in that organ, and we recognise 

 as one of the functions of the liver, the regulation of the 

 amount of sugar in the blood. There is no special organ 

 for the regulation of the amount of fat ; the drops pass 

 through the capillary walls, and are retained in the connective 

 tissues. 



We must remember that all the products of digestion, 

 except the fat, pass through the liver which receives every- 

 thing before it is allowed to pass into the blood of the 

 system. Thus many poisons, especially metals, are arrested 

 by the liver, and many substances which result from digestive 

 l)rocesses and would be harmful, are altered into harmless 

 compounds by it. The excess of sugar, we have already 



