320 



MANUAL OF PHYSIOLOGY. 



FTO. 144. 



that of exposing their tissues to a medium containing oxygen, and 

 of removing the carbonic acid gas. 



In some of the invertebrate animals it suffices to distribute the 

 medium containing oxygen throughout the tissues of the animal 

 by means of tubes. Thus in the Echinodermata a water vascular 

 system exists which seems to carry on the function of respiration. 

 A similar distribution of oxygen takes place in arthropoda, deli- 

 cately branching open tubes (tracheae) distribute air to the tissues 

 of the animal's body. 



When more active changes occur in the tissues there is always 

 a perfect blood vascular system, and the blood is invariably used 

 as the distributing and collecting agent of the gases in the tissues, 

 and by flowing through some special organ exposed to the sur- 

 rounding medium it insures the gas-interchange between the body 



and the outer world. These organs 

 are formed on two general types : (1) 

 external vascular fringes ; and (2) 

 internal vascular sacs. 



Animals living in water have com- 

 monly the external fringe arrange- 

 ment (gills), whilst those living in 

 air have sacs (lungs). Some animals 

 (frogs, toads, etc.) have gills in the 

 early stages of their life and lungs 

 when they are more fully developed. 

 In frogs and serpents the lungs are 

 simple sacs, with the inner surface 

 increased by folds of the lining mem- 

 brane, which gives it a honey-comb 

 appearance ; into each sac opens one 

 of the divisions of the air-tube. In 

 crocodiles the air-tubes divide into 

 several branches which open into a 

 series of anfractuous vascular re- 

 cesses which communicate one with 

 another. 



In birds wide bronchial tubes 

 course through the lung tissue to reach large air cavities, and 



Diagram of the Kespiratory 

 Organs. The windpipe leading 

 down from the larynx is seen 

 to branch into two large bron- 

 chi, which subdivide after they 

 enter their respective lungs. 



