3^ THE FUNCTIONS OF ANIMALS 



insects, these substances are split up not by secreted 

 ferments but by the action of symbiotic microbes within the 

 food-canal, or in close relation to it. 



Absorption. — But the food must not only be rendered 

 soluble and diffusible, it must be carried to the different 

 parts of the body, and there incorporated into the hungry 

 cells. It is carried by the blood stream, and in part also 

 by what are called lymph vessels, which contain a clear 

 fluid resembling blood minus red blood corpuscles. 



Absorption begins in the stomach by direct osmosis into the capillaries 

 or fine branches of blood vessels in its walls, and a similar absorption, 

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

 But lining the intestine there are delicate 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 food-stuffs; 

 the greater part certainly passes into the capillaries of the portal 

 system, which are contained in the villi. The digested protein, 

 chiefly in the form of amino-acids, passes into the blood of the portal 

 vein, either directly or through the intermediary of leucocytes, which 

 flock to the intestine when protein food is being digested. 



Functions of the liver. — The absorbed products of the 

 digestion of proteins and carbohydrates are carried from 

 the intestine to the liver by the portal vein, which splits 

 up into fine channels (sinusoids) in close connection with 

 the liver cells. In digestion the more complex carbo- 

 hydrates, such as starch, glycogen, and cane sugar, are 

 split into molecules of simple sugars such as glucose. 

 A large part of the glucose absorbed after a meal is stored 

 in the liver in the form of glycogen ; the muscles of the 

 body also contain glycogen, from which lactic acid is pro- 

 duced, and as their glycogen stores are depleted they 

 draw upon the glucose of the blood. One of the functions 

 of the liver is to maintain the concentration of glucose in 

 the blood at a constant level, by mobilising its glycogen 

 stores as required. These equilibria are partly controlled 

 by a substance (insulin) formed in the pancreas. 



