2O8 ANIMALS 



further changes before it is admitted to the general circulation. 

 The carbo-hydrates and peptones are collected by the hepatic 

 portal vein and carried to the liver, where certain substances 

 are absorbed and ultimately pass back into the intestine. This 

 occurs in the case of some substances which would be deleterious 

 if permitted to pass into the general circulation. Excess car- 

 bo-hydrates are also stored temporarily in the liver and other 

 organs in the form of glycogen. The fats are broken up into 

 fatty acids and glycerine, and then, after absorption, resyn- 

 thesized as fats of a different kind -in the cells of the mucous 

 epithelium. They finally appear as globules in the lacteal 

 capillaries of the villi and thus come into the blood through the 

 thoracic duct. 



473. Fats are also stored, sometimes in large quantities, 

 and represent a large reserve of energy. They are usually 

 found in the connective tissues, under the skin, among the 

 muscles, covering the visceral organs and elsewhere. From the 

 liver the absorbed food materials get into the circulation through 

 the inferior vena cava, while the lacteals pour their contents 

 into the thoracic duct and thus into the left sub-clavian vein. 



474. So long as the nourishing fluids remain in the blood 

 vessels they can be of no service to the tissues. But the walls 

 of the capillaries are so thin that the fluid portion of the blood 

 can seep through. In this way the lymph arises which is found 

 in all the living tissues of the body, filling the minute spaces 

 between the cells. Fresh supplies of lymph are continually 

 escaping from the capillaries and the impoverished lymph drains 

 off out of the lymph spaces into the lymph vessels, which finally 

 empty into the thoracic duct. Thus the lymph enters the cir- 

 culation agin. 



RESPIRATION 



475. When the fuel is consumed in the firebox, to return to 

 the analogy of the steam engine, there must be free access of 



