318 THE VITAL FUNCTIONS. 



respiratory organs, essentially land animals. The 

 air is received by a round aperture near the 

 head, guarded by a sphincter muscle, which is 

 seen to dilate or contract as occasion may re- 

 quire, but which is sometimes completely con- 

 cealed from view by the mouth folding over it. 

 The cavity, to which this opening leads, is lined 

 with a membrane delicately folded, and over- 

 spread with a beautiful net-work of pulmonary 

 vessels. Other mollusca of the same order, 

 which are more aquatic in their habits, have 

 yet a similar structure, and are obliged at in- 

 tervals to come to the surface of the water in 

 order to breathe atmospheric air : this is the 

 case with the Oncliidiwn, the Planorbis, the 

 Lymncea, &c. 



The structure of the pulmonary organs be- 

 comes gradually more refined and complicated 

 as we ascend to the higher classes of animals. 

 In all vertebrated terrestrial animals they are 

 called lungs, and consist of an assemblage of 

 vesicles, into which the air is admitted by a 

 tube, called the trachea, or wind-pipe, extending 

 downwards from the back of the mouth, parallel 

 to the oesophagus. Great care is taken to guard 

 the beginning of this passage from the intrusion 

 of any solid or liquid that may be swallowed. A 

 cartilaginous valve, termed the epiglottis, is 

 generally provided for this purpose, which is 

 made to descend by the action of the same 



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