RESPIRATION BY LUNGS. 227 



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 Onchidium, the 

 Planorbis, the Lymnsea, 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 vesi- 

 cles, 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 ac- 

 curately the entrance into the air-tube. It is an exceedingly 

 beautiful contrivance, both as to the simplicity of the me- 

 chanism, and the accuracy with which it accomplishes the 

 purpose of its formation. 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 inter- 

 position 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 ringlets tends to keep the sides of the tube stretched, 

 and causes it to remain open: it is a structure very analo- 

 gous to that of the trachea of insects, or of the vessels of the 

 same name in plants. 



The lungs of Reptiles consist of large sacs, into the cavity 

 of which the bronchia, proceeding from the bifurcation of 

 the trachea, open at once, and without farther subdivision. 

 Cells are formed within the sides of this great cavity, by 

 fine membranous partitions, as thin and delicate as soap 



