RESPIRATION AND THE VOICE. 125 



have an opening, in which are a number of flat plates 

 of membranes, called gills, with net-works of blood-ves- 

 sels in them. The water is swallowed, and 

 passes out between the plates through these 

 openings. The oxygen which it contains 

 enters through the thin membranes into 

 the vessels. 



5. Other animals take in oxygen in a 

 different way. But, though the way is dif- 

 ferent, the principle is the same. In every GlLL f f; EBL> 

 case, the capillary vessels are spread over 



a thin membrane; and the air is on the other side of 

 that membrane. 



6, In some of the lower animals, the lung is a single 

 sac, like a bladder, with the capillaries on the outside. 

 In man, instead of being a single sac, each lung is a vast 

 number of very small sacs bound together in a mass, with 

 fine blood-vessels and air-tubes surrounding and connect- 

 ing them. 



Imagine a bunch of grapes with the contents of each 

 grape taken out, leaving only the skins to represent the 

 air-cells. Suppose the stems to be hollow, and they will 

 represent the air-tubes. Now, put several such bunches 

 together, so that the main stems all join in one large 

 stem, and you have something which represents the air- 

 cells and air-tubes of a lung. To make it complete, sup- 

 pose all the grapes to be joined by fine threads, like a 

 spider's web. This represents the fibrous tissue, which is 

 quite elastic. Suppose a blue tube to run along the main 

 stem, and divide every time the stem does, until its small 

 branches finally reach every grape, and form a net-work 

 on it. From that net-work suppose red tubes to run 



