72 FARM ANIMALS 
thrown out of the system. At the same time oxygen is 
taken in with great greediness by the cells of the blood, 
which distribute it where needed in all parts of the body. 
When plants are growing, oxygen is released and thrown 
into the air. At the same time, by means of leaves, the 
DISTRIBUTION OF CAPILLARIES 
IN POSTERIOR EXTREMITIES. 
HEAD AND ANTERIOR 
EXTREMITIES, 
: 
Right \ || {Ler CAPIUARIES IG 
VENTRICLE’ VENTRICLE ABDOMINAL ORGANS 
How THE BLoop CiRCULATES THROUGH THE Bopy 
Blood is collected from all parts of the body and delivered into the right auricle, 
which on contracting, forces the blood into the right ventricle; this in turn contracts 
and forces the blood into the lungs, where oxygen is taken on and carbonic acid gas 
and other impurities are thrown off. From the lungs the blood is returned to the 
left side of the heart and distributed through arteries and capillaries to all parts 
of the body. 
carbonic acid gas is drawn in and used in the construction 
of the plant compounds. This was got from the air. An 
animal, in performing its functions and in building tissue, 
inhales oxygen from, and exhales carbon dioxide into, the 
air. Animals in this manne: use what is waste to the 
plant, and the plants what is poison to the animal. 
15. Excretion.—The products resulting from the oxida- 
tion of animal tissues, or of the food consumed, are re- 
moved from the body by the lungs, kidneys and skin. ‘The 
chief products of oxidation in the body are carbonic acid, 
water, urea, and salts. Carbonic acid is removed through 
the lungs, and to a smaller extent by the skin; urea and 
