392 TEXTBOOK OF ZOOLOGY 



Aerial respiration is accomplished in terrestrial animals through 

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

 system of branched tubes called tracheae, 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 mechanism 

 for the accomplishment of breathing in the cat and other mammals 

 by the use of the diaphragm and thoracic wall is described in the 

 chapter on mammals. 



The muscles which control these actions are automatically stimu- 

 lated 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 movements. 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 respira- 

 tory action. In addition to exchanging gases the lungs also discharge 

 moisture and give off a certain amount of heat. 



The Circulatory System. — The circulatory system is a closed sys- 

 tem of vessels supplying all parts of the body with blood and a 

 system of spaces, sinuses, and vessels collecting lymph from the vari- 

 ous organs to return it to the blood vessels. The blood circulatory 

 system centers in a contractile heart from which tubular arteries 

 lead out to various organs of the body where they branch into min- 

 ute vessels or capillaries. The capillaries converge as they carry the 

 blood away from the organs to form the veins which carry the blood 

 back to the heart. This is a closed system of vessels. The blood is 

 composed of the clear fluid, plasma, and the Uood corpuscles. The 

 red corpiiscles contain the red pigment matter, hemoglobin, which 



