140 DIGESTION 



more scanty passage of the salt outward into the water is 

 called exosmosis. (Read Note 9.) 



45. Dialysis. Different substances will penetrate animal 

 membrane with different degrees of rapidity. As a rule, crys- 

 tallizable substances, such as mineral salts, glucose, or urea, 

 pass with facility ; while non-crystallizable, such as albumen, 

 starch, or gum, pass through not at all, or with difficulty. The 

 former substances are called diffusible, and the latter non- 

 diffusible. This distinction is not absolute, however, for by 

 increasing the pressure on one side of the membrane, nearly 

 all soluble substances can be made to pass through to some 

 degree. The separation of substances of different diffusibility 

 from each other is called dialysis. To illustrate : If a mixture 

 of sugar and gum, dissolved in water, be placed on one side of 

 the membrane, and pure water * on the other, it will be found, 

 that the sugar will pass through the membrane, while the gum 

 will be left behind. It is by this process of dialysis that the 

 products of digestion are separated from the waste and indi- 

 gestible parts of food, and absorbed into the circulation. 



46. The absorption of the food, when it reaches the intes- 

 tine in liquefied form, is as follows : all of it passes through the 

 epithelial cells of the villi ^(see Fig. 31), and thence directly 

 into both blood-vessels and lacteals ; peptones, water, salts, and 

 glucose pass directly into the vessels and lacteals, and emulsi- 

 fied fats after passing through the epithelial cells pass into 

 the lacteals. Peptones, salts, water, and fats are taken from 

 the blood by the tissues directly, while the glucose is carried 

 through the portal ^circulation into the liver. This organ 

 changes the glucose to a new substance, called glycogen, and 

 stores it up in its interior, to remain here until the system 

 needs it; when the tissues require glucose, this glycogen is 

 re-transformed into glucose, and passes into the general 



45. What substances easily pass through animal membrane ? What are these substances 

 called ? What substances pass through with great difficulty, if at all ? What are they 

 called ? Why is this distinction not absolute ? What is dialysis ? Illustrate the process. 



46. Describe the process of the absorption of food, after it has reached the intestines in 

 liquefied form. What substances pass directly into the vessels and lacteals ? How is glu- 

 cose modified by the action of the liver ? What is this modified substance called ? 



