4 COMPENDIUM OF THE VJF.TEIUNARY ART, 



curious kind of valve, called epiglottis, which 

 is always open, except in the act of swallowing: 

 it is then forced down upon the larynx, so as to 

 prevent food, or any thing which may he passing 

 over the throat, from falling into the wind- 

 pipe. Where the trachea joins the chest, it 

 divides into numerous branches, which gradually 

 becomino- smaller, at lenoth terminate in minute 

 cells : the lungs, indeed, are made up of the 

 ramifications of the trachea and blood-vessels; 

 the interstices being hlled with a cellular mem- 

 brane, which serves not only to unite them, but 

 likewise to sjive a uniform and homoo'eneous 

 appearance to the whole mass. The lungs are 

 covered with a fine delicate membrane called the 

 pleura, which also covers the internal surface 

 of the ribs and diaphragm, and, by stretching 

 across the chest from the spine to the breast- 

 bone, divides the thorax into two cavities ; this 

 part of the pleura is therefore named viedias- 

 tiinim. On every part of the pleura an aqueous 

 fluid is secreted for the purpose of preventing a 

 cohesion of the parts ; and when this is produced 

 too abundantly, it constitutes the disease termed 

 hydrothorax, or dropsy of the chest. The 

 pleura, though so fine a membrane, is im- 

 pervious to air; which may be proved on the 



