CHORDATA. 335 



347. Lungs arise as outpocketings of the ventral wall of 

 the pharynx. These may persist as relatively simple sacs, as 

 in the frog, or by great growth and folding they may become 

 very complicated, and thus increase their surface to a wonder- 

 ful degree. They are lined throughout with the entodermal 

 epithelium. The blood capillaries are in contact with this 

 layer and through these thin walls the gases are exchanged. 

 The outer surface o.f the lung is covered by the pleura^ the 

 lining of the body cavity. The tube connecting the pharynx 

 with the body of the lung is known as the trachea. The upper 

 or anterior end of the trachea is modified into a chamber 

 known as the larynx in the air-breathing vertebrates. The 

 epiglottis closes the opening (glottis) from the pharynx 

 into the larynx, whenever food is passing from the mouth 

 through the pharynx into the gullet. On account of the 

 presence of currents of air passing in and out and capable of 

 producing vibration, certain portions of the tract are used in 

 making definite sounds whereby the animals are put into com- 

 munication with their kind. The parts so used are the lips, 

 teeth and vocal cords. The latter are membranous folds in 

 the mucous lining of the larynx, which may be brought into 

 such a position as to close that organ, in part, to the escaping 

 current of air. The tense edges of the membrane are put into 

 vibration. The resulting sound, reinforced or otherwise modi- 

 fied by the other organs, is voice. 



348. Supplementary Exercises for Library. Where does 

 the " swim-bladder " in fishes occur? Is any thing known of 

 its function ? Is any thing like a lung known among the fishes ? 



What are the most important differences between the " voice- 

 box " of mammals and that of birds? Have all the vertebrate 

 groups vocal organs? Do they all have voice? What is the 

 difference between voice and speech? What are the uses of 

 voice to animals possessing it ? 



349. Circulation. The blood in vertebrates contains both 

 colorless and colored (red) corpuscles. The red coloring 



