DIGESTION 139 



are specially adapted to receive; at other times they are 

 hardly discernible. The lacteals all unite to form one tube, 

 the thoracic duct, which passes upward through the thorax, 

 or chest, and empties into a large vein, situated just beneath 

 the left collar-bone. 



43. The Absorbents. The lacteals belong to a class of ves- 

 sels known as absorbents, or lymphatics, which exist in nearly 

 all parts of the body, except the brain and spinal chord. The 

 fluid which circulates through the lymphatics of the limbs, 

 and all the organs not concerned in digestion, is called lymph. 

 This fluid is clear and colorless, like water, and thus differs 

 from the milky chyle which the lacteals carry after digestion : 

 it consists chiefly of the watery part of the blood, which was 

 not required by the tissues, and is returned to the blood by 

 the absorbents or lymphatics. - (Read Note 9.) 



44. Osmosis is the name given to that process which enables 

 two fluids, capable of being mixed, to pass through animal 

 membrane, which separates them. This may be readily illus- 

 trated by filling a glass tube half full with a strong salt solu- 

 tion, and closing its open end by a piece of bladder or intestine, 

 and then standing the tube, with the membrane downward, in 

 a glass of water. After a while it will be noticed that the sur- 

 rounding fresh water has become salty, and also that the fluid 

 in the glass tube has increased in volume. Upon analyzing the 

 two fluids now, we will find that more water has passed into 

 the salt solution than salt into the water, showing that the 

 lighter fluid (water) passes more readily into the denser fluid 

 (salt solution). The more abundant passage of the water 

 through the membrane to the salt is called endosmosis; and the 



48. To what class of vessels do the lacteals belong ? 



In what parts of the body are absorbents not found ? 



What fluid circulates through the lymphatics of the limbs ? 



In what respect does it differ from the chyle carried by the lacteals after digestion ? 



Of what does it consist? 



How is it returned to the blood ? 

 44. Describe the process of osmosis. 



How may it be illustrated ? 



Describe the experiment. 



What is endosmosis ? Exosmosis ? 



