120 ADVANCED PHYSIOLOGY 



blood vessels) and then pass up through the mesentery. 

 Thus there are three sets of vessels in the mesentery: 

 arteries bringing blood, veins carrying food-laden blood 

 away, and lacteals carrying off the fat; Fig. 65. The 

 lacteals finally empty into a duct which runs past the 

 liver and stomach, through the diaphragm, up through 

 the thorax into the neck region, a little above the heart. 

 This tube, which is called the thoracic duct, finally empties 

 into one of the large veins which bring blood bafck to the 

 heart; Fig. 65. 



The contents of the thoracic duct are white and milky, 

 due to the fat in emulsion; in fact, the contained 

 materials are much the same as the chyle in the intestine. 

 The lacteals, then, and the ducts connected with them act as 

 a temporary storehouse for fats. The flow from the thoracic 

 duct into the large vein at the base of the neck is slow and 

 interrupted and it is only after several hours from the 

 time fat enters the lacteals, that it passes into the main 

 blood system. 



The passage of the absorbed fats through the thoracic 

 duct is not produced by any heart-like organ. The simple 

 pressure of the surrounding organs, the peristaltic move- 

 ments of the intestine, the constant displacement of organs 

 by breathing muscles these and other lesser influences 

 produce a slow flow. This movement can take place in 

 but one direction 'on account of valves which open only one 

 way* and are located at very frequent intervals throughout 

 the ducts. 



Even the fat in time gets into the blood; but it seems as if 

 the thoracic duct and the lacteals were designed to switch 

 the fat around the liver and bring it to the blood without 

 flowing through that organ. The liver can readily store 

 sugars and is not injured by the proteids passing through it, 

 but apparently it is necessary for the fats to reach the blood 

 system by some other course. 



