CHIEF FUNCTIONS OF THE ANIMAL BODi. 27 



especially of water, takes place along the whole of the digestive tract. 

 But lining the intestines there are special hair-like projections called 

 villi ; they contain capillaries belonging to the portal system (blood 

 vessels going to the liver), and small vessels known as lacteals connected 

 with lymph spaces in the wall of the intestine. The lacteals lead into 

 a longitudinal lymph vessel or thoracic duct, which opens into the 

 junction of the left jugular and left subclavian veins at the root of the 

 neck. The contents of the duct in 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 foodstuffs ; 

 the greater part certainly passes into the capillaries of the portal 

 system, which are contained in the villi. 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. 



Function of the liver. — 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, a little further. The 

 starch, we know, is converted into sugar, and this, with the 

 sugar of the food, passes into the capillaries of the villi, 

 and is carried to the liver. During digestion 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 hepatic veins, the vessels leaving the liver. The 

 increase must therefore be retained in that organ, and we 

 recognise as one of the functions of the liver, the regula- 

 tion 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 walls of the capillaries, and are 

 stored in connective tissue cells. 



All the products of digestion, except the fat, pass through 

 the liver, which receives everything before it is allowed to 

 pass into the general circulation. Thus many poisons, 

 such as metals, are arrested by the liveT, and various harm- 

 ful substances which are formed in the course of digestion 

 are changed by the liver into harmless compounds. The 

 excess of sugar, we have already noted, is stored in the 

 liver. It is converted there into a substance called glycogen, 

 which can be readily retransformed into sugar according to 

 the needs of the system. Glycogen is stored in the muscles 

 also, and forms an important part of the fuel for the 



