RESPIRATION BY LUNGS. 319 



muscles that perform, deglutition, and which 

 then closes accurately the entrance into the air-> 

 tube. It is an exceedingly beautiful contriv- 

 ance, both as to the simplicity of the mechanism, 

 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 

 interposition of a series of firm cartilaginous 

 ringlets, interposed between their inner and 

 outer coats, and placed at small and equal dis- 

 tances 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 analogous 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, proceed- 

 ing from the bifurcation of the trachea, open at 

 once, and without further subdivision. Cells are 

 formed within the sides of this great cavity, by 

 fine membranous partitions, as thin and delicate 

 as soap bubbles. The lungs of serpents have 

 scarcely any of these partitions, but consist of 

 one simple pulmonary sac, situated on the right 

 side, having the slender elongated form of all 

 the other viscera, and extending nearly the 



