DIGESTION, ASSIMILATION, NUTRITION 221 



of the muscular walls, grinds the feed. The feed is whirled 

 around and around in the gizzard until it becomes a soft 

 mass, when it passes into the intestines. 



Other Digestive Organs. Digestion is carried on, more or 

 less, in all parts of the digestive tract, the chief divisions of 

 which, named in the order in which the feed passes through 

 them, are as follows: The mouth, oesophagus, crop, stomach, 

 gizzard, and intestines. In practically all of these organs the 

 changes of digestion are brought about by the solvent action 

 of fluids, or chemical secretions, which ooze from the numer- 

 ous membranes that line the walls of the organs. The 

 secretions act upon the feed, and convert it into more easily 

 digested forms. Some of the fluids are acid in reaction, 

 others are alkaline. Some act upon one nutrient, some on 

 another. 



Assimilation. Assimilation takes place by diffusion and 

 in the lower portion of the intestines. By diffusion is meant 

 the passing of a liquid through a porous membrane. The 

 intestinal walls are covered with little hairs, called villi. 

 The walls of the intestine and the so-called villi are full of 

 small capillaries. The only partition lying between the 

 blood and the digested mass is the thin wall of these 

 blood vessels, and the digested feed has now become so 

 fine that it can pass through this membrane into the blood. 



Nutrition. When assimilated by the blood it is carried 

 to all parts of the body for the process, of nutrition. 

 After it is digested, most of the nutriment is transported 

 from the intestines to the arteries just outside of the heart, 

 by means of a large duct, or tube, called the thoracic duct. 

 This thoracic duct pours into the arteries large quantities 

 of digested food material. The heart is the center of the 

 circulating system, being neither more nor less than a 

 pumping engine. It receives from the veins the impure 

 blood which comes from all parts of the body, and this, after 

 passing through the right side of the heart, is sent to the 



