RESPIRATORY MECHANISM. 



325 



FIG. 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 tis- 

 sues 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 sacks. 



Animals living in water have com- 

 monly the external fringe arrange- 

 ment (gills), whilst those living in 

 air have sacks (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 sacks, with the inner surface 

 increased by folds of the lining mem- 

 brane, which gives it a honey-comb 

 appearance ; into each sack opens one 

 of the divisions of the air tube. In 

 crocodiles the air tubes divide into 



Diagram of the Respiratory Or- 

 Several branches which Open into a gans. The windpipe leading down 



series of anfractuous vascular recesses frora the lar y nx is 8een to branch 



. into two large bronchi, which sub- 



which communicate one with another. divide after they enter their respec- 



In birds wide bronchial tubes tive lungs, 

 course through the lung tissue to 



