266 



ANIMAL BIOLOGY 



easily seen in such places as the back of the hand. If the 

 wrist is grasped tightly so as to check the return flow of the 

 blood the veins of the hand may be seen to swell. 



Most of the blood sent to the stomach and intestines 

 is not returned at once toward the heart, but is collected 

 by the portal vein which distributes it to the liver. The 

 portal circulation is therefore peculiar in that it both begins 

 and ends in capillaries. Much of the food material ab- 

 sorbed from the stomach and intestines passes into the capil- 

 laries that lead to the portal 

 vein. Sugar in the form of 

 dextrose passes into the 

 portal circulation and is con- 

 verted into glycogen in the 

 cells of the liver. 



Besides the blood, the 

 body has a similar but 

 colorless fluid, the lymph. 

 The lymph contains leuco- 

 cytes, but no red corpuscles. 

 It flows in numerous ves- 

 sels, the lymphatics, which 

 pour their fluid into the 

 blood. Lymph vessels are 

 abundantly supplied to the 

 intestines where they are 

 called lacteals on account 

 of the milky appearance (lac, milk) of the lymph in this 

 region caused by the presence of fatty substances absorbed 

 from the intestine. The intestinal lymphatics converge 

 into the thoracic duct which empties into a vein near the 

 left side of the neck. 



The lymph does not have a complete circulation like the 

 blood. It carries material from the surface of the body 



FIG. 198. Lymphatic system, 

 lacteals; T, thoracic duct. 



L, 



