NUTRITION OF ANIMALS 259 



are separated by a thin but strong partition. Between the 

 auricle and the ventricle on each side of the heart are valves 

 which at times are closed and prevent the passage of blood. 

 Small valves in the vessels leading to the auricle help to pre- 

 vent the backward flow of blood. When the blood returns in 

 the larger veins, it enters the right auricle and passes from it 

 into the right ventricle. As the ventricle fills, its valves are 

 closed, so that the blood cannot flow back into the right au- 

 ricle. There is a large vessel leading from the right ventricle 

 to the lungs. The right ventricle contracts, and when it does 

 so the blood within it is driven through the vessel to the 

 lungs, where it passes through many capillaries. These small 

 vessels gradually merge into the larger ones, until four large 

 vessels lead the blood back to the heart. Upon its return the 

 blood enters the left auricle and then passes into the left ven- 

 tricle. It is held momentarily in the left ventricle, and cannot 

 flow backward into the left auricle because of the closure of 

 the valves. The left ventricle is the most muscular of all the 

 four chambers of the heart. Upon its contraction the blood is 

 driven through the large artery which leads away from the 

 left ventricle. From this artery the blood passes to smaller 

 and smaller arteries over the entire body, and finally into the 

 capillaries. From the capillaries the blood passes to the veins, 

 and through them it returns to the right auricle of the heart 

 and again begins its round of circulation. 



284. Transportation and assimilation of food. The digested 

 food is absorbed and enters the blood. A new supply of 

 oxygen is taken up as the blood passes through the lungs. 

 This blood, laden with food and oxygen, passes through the 

 heart and is driven through the arteries into the capillaries. 

 From the capillaries the protoplasm takes up needed materials, 

 and the waste products pass into the blood. The process by 

 means of which living protoplasm takes up food material and 

 makes it a part of itself is known as assimilation and was 

 described in the discussion of plant nutrition in section 273. 



