2l6 APPLIED PHYSIOLOGY 



The air bags are expanded with air during flight, and 

 thus the body is made lighter in proportion to its size, 

 in order that the bird may fly more easily. 



371. Respiration in water animals. Some water ani- 

 mals, such as the porpoise and the whale, possess lungs like 



land animals, and are compelled 

 to come to the surface of the 

 water in order to breathe, but 

 fish have a special apparatus so 

 that they can use the oxygen 



which is dissolved in water. On 



Gills of a fish. each gide of a fish > s head is a slit> 



like opening reaching from the interior of the mouth to 

 the surface of the body. In each opening are four half 

 circles of limber bone. From the back of each circle a 

 row of thin fingerlike plumes projects, so that it looks 

 like a red feather with plumes only on one side. These 

 half circles are the gills. Each plume contains a blood 

 tube which is separated from the water by a very thin 

 wall. The fish forces the water through his mouth and 

 out between the gills, and the oxygen contained in it 

 readily passes through the thin wall of the blood vessel 

 into the red blood cells. 



372. Respiration in a frog. A frog in the tadpole 

 form is provided with gills which project into the water 

 from its neck, but when it becomes a perfect frog the 

 gills disappear and lungs are formed. But the frog's skin 

 is able to absorb oxygen and to give off carbonic acid 

 gas about one eighth as rapidly as the lungs. 



373. Respiration in insects. In insects from three to 

 nine tubes extend into ea.ch side of the abdomen and 

 divide into small branches, but do not communicate with 

 any cavity. The fluid which answers for the insect's 



