186 PHYSIOLOGY 



and these they gain very readily from the substances 

 which they eat. The canal is definitely divided by 

 enlargements and constrictions into the oesophagus, the 

 stomach, the large and small intestine. Time and space 

 for the digestion and absorption of food are afforded 

 by the length of the intestine, which is coiled in the abdo- 

 men. The pancreas and liver are large glands con- 

 nected with the canal which aid in forming digestive 

 juices. The kidneys excrete the nitrogenous waste. 



Respiration. — In higher forms the body cavity is 

 divided by the diaphragm into two parts. In the upper 

 part the lungs are situated. The lungs correspond to the 

 gills of aquatic forms. They are composed of a thin 

 membrane arranged to form air sacs, in which blood ves- 

 sels are situated. Air from outside enters the air sacs 

 and comes in contact with the moist membrane of the 

 lungs; oxygen passes into the blood and from the blood 

 into the cells of the body. Carbon dioxide passes from 

 the cells into the blood and from the blood to the air in 

 the lungs, thence to the outside. The entire process 

 depends upon the law by which gases move from 

 a place of high pressure to a place of low pressure. 

 The air enters the lungs because the pressure is reduced 

 there by the sudden enlargement of the chest cavity 

 caused by the contraction of the diaphragm and the 

 elevation of the ribs. Oxygen enters the cells because 

 its pressure is reduced in the cells through its use in 

 the manufacture of protoplasm. 



Reproduction. — The reproductive organs are very 

 prominent, especially in the breeding season. In verte- 

 brates reproduction is sexual and depends on the union 



