NUTRITION 375 



materials from the digestive tract to the blood. However, absorp- 

 tion in a living animal is not merely a mechanical " soaking up " 

 of prepared foods, but other changes take place, as the products of 

 digestion enter the circulation. 



Absorption may take place (1) directly into the blood capillaries 

 which richly supply the walls of the stomach and intestine or 

 (2) indirectly, by way of the lymph capillaries of the villi (lacteals) 

 which eventually empty into the blood circulation also. 



The capillaries of the gastric vein in the stomach walls absorb 

 some water, a little digested proteid, and still less sugar, but the 

 principal region of absorption is in the villi of the small intestine. 

 Here the thin walls and enormous surface bring the digested foods 

 close to the blood and lymph capillaries. Peptones, sugars and 

 fatty acids, salts and water are passed into the blood stream, while 

 the fats that have been emulsified are taken up by the lymph capil- 

 laries (lacteals) and carried by the lymphatic circulation to the 

 thoracic duct and finally emptied into the general circulation, 

 near the left jugular vein. 



Assimilation. All the steps of digestion and absorption lead to 

 the final process of assimilation, which either builds up the cells 

 or supplies them with energy. For this purpose the blood carries 

 the absorbed foods to the tissues. These foods pass as lymph 

 (by osmosis) from the capillaries to the lymph spaces which sur- 

 round every living cell, and there the assimilation occurs. Every 

 cell of the body is practically an island, bathed on every side by 

 lymph, which brings from the blood the digested food stuffs (and 

 oxygen as well) and removes to the blood stream the waste matters 

 produced by the cells' activities. 



Nutrition. All these processes by which food is obtained, pre- 

 pared, and built into tissues, are grouped together as nutrition 

 and include: 



1. Food-getting, selection, and preparation. 



2. Digestion .which mainly goes on in mouth, stomach, and 

 intestines. 



3. Absorption which occurs principally in the small intestine 

 and stomach, by means of the blood capillaries and lacteals. 



