RESPIRATION BY LUNGS. 287 



overspread with a beautiful network 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 intervals to come to 

 the surface of the water in order to breathe atmos- 

 pheric air : this is the case with the Onchidiiim, the 

 Planorbis, the Lymncea, &c. 



The structure of the pulmonary organs becomes 

 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 

 muscles that perform deglutition, and which then 

 closes accurately the entrance into the air-tube. It 

 is an exceedingly beautiful contrivance, both as to 

 the simplicity of the mechanism, and the accuracy 

 with which it accomplishes the purpose of its for- 

 mation. At the upper part of the chest the trachea 

 divides into two branches, called the bronchia, 

 passing to the lungs on either side. Both the wind- 

 pipe and the bronchia are prevented from closing 

 by the interposition of a series of firm cartilaginous 

 ringlets, interposed between their inner and outer 

 coats, and placed at small and equal distances from 

 one another. The natural elasticity of these ring- 

 lets tends to keep the sides of the tube stretched, 



