NUTRITION 109 



aminoacids, are reconverted into albumins, globulins, etc., and 

 the fatty acids into fats. It is uncertain whether these changes 

 take place to a large extent within the walls of the intestines 

 or subsequently. However this may be, the absorbed materials 

 soon pass into the blood. 



The small veins with which the intestines are abundantly 

 supplied unite to form larger ones, and these in turn unite to 

 form still larger vessels which lead ultimately to the heart. 

 The great arteries leading from the heart branch again and 

 again, gradually diminishing in size, and end finally in a close 

 meshwork of fine capillaries which are intimately associated 

 with all the tissues throughout the body. The walls of these 

 capillary vessels are necessarily very thin and some of the 

 blood plasma exudes through them. This fluid is then called 

 lymph. It fills up all the interstices between the cells and 

 thoroughly bathes the tissues which are restored and nourished 

 by the substances present in the lymph and which are 

 derived from the food. 



The lymph is drained off by a special set of vessels known 

 as the lymphatics. The smaller lymphatics, which are of 

 capillary size, unite to form larger vessels, and ultimately a 

 junction is formed with one of the larger veins by which the 

 blood returns to the heart. From the heart the blood goes 

 to the lungs, then back to the heart, and out again through the 

 arteries as before. 



In the lungs, the blood is exposed to the oxidising action 

 of the air which is inhaled by them ; and when sent out again 

 from the heart, it carries the oxygen in the form of a compound 

 oxyhaemoglobin to all parts of the body. The tissues are 

 thus oxidised, and the carbon dioxide and water which are 

 formed are given up when the blood again returns to the lungs. 

 The nitrogenous matter is not completely oxidised. No free 

 nitrogen is liberated. This element appears in the form of 

 comparatively simple compounds chiefly urea, uric acid and 

 hippuric acid (pp. 53, 54, 55). These pass into solution in 

 the blood, from which they are eliminated by the kidneys, 

 and are discharged in the urine. 



It appears, therefore, that two opposite kinds of change 



