♦ PHYSIOLOGY 177 



In these larger organisms, the respiration process is divided into 

 external respiration, the gas exchange occurring between the circula- 

 tory medium and the environmental medium (air, water, etc.), and 

 internal respiration, the exchange of gases occurring between the cir- 

 culatory medium (blood, etc.) and the protoplasm of the organism. 



The gills of most aquatic forms are richly supplied with a capil- 

 lary supply of blood and then membranous surfaces are directly 

 exposed to surrounding water from which the dissolved oxygen is 

 absorbed. In many aquatic worms the gill filaments are outgrowths 

 of the sides of the body wall. Likewise, the more or less plumelike 

 gills of crayfish are pocketlike outpushings of the body wall. In 

 a number of aquatic insects, worms, fishes, and turtles, the rectum 

 serves as an accessory respiratory organ. 



Aerial respiration is accomplished in terrestrial animals through 

 special internal surfaces which must be kept moist. In insects a 

 system of branched tubes called trachea, which open through 

 spiracles along the sides of the body, distribute oxygen to and re- 

 ceive carbon dioxide from all of the cells of the body. In pulmonate 

 snails the **lung" is simply an invagination of the skin, as are also 

 the tracheae of insects. The real lung is a development found in the 

 terrestrial vertebrate, and it is a specialized surface derived from 

 the anterior or pharyngeal portion of the digestive tube. In higher 

 vertebrates, such as birds and mammals, they are extensively lobed, 

 and made spongy by the innumerable small air sacs which provide 

 the enormous respiratory surface necessary. It has been estimated 

 that if all of these pitlike alveoli of the internal lining of the lungs 

 of the average human being were spread out in an even surface, the 

 area of it would be more than 100 square yards. 



The muscles which control the breathing actions in vertebrates 

 are automatically stimulated through the nervous system to contract 

 when the carbon dioxide level of the blood reaches a certain point. 

 A respiratory center, located in the medulla oblongata, is affected 

 by the carbon dioxide and determines the rate of respiratory move- 

 ments. There are also nerves from the lungs themselves which 

 extend to this center and contribute to the maintenance of the 

 proper rhythm. Abundance of venous blood stimulates an increase 

 of the respiratory action. In addition to exchanging gases the lungs 

 also discharge moisture and give off a certain amount of heat. 



