64: ON THE niYSIOGNOMY OF SERPENTS. 



dcr or sac Avithout cells, simply formed by a prolongation 

 of the serous membrane enveloping the lung ; and, second^ 

 because the most considerable brandies of the air-tubes 

 open into this cavity of the lung. By this first arrange- 

 ment, the lung of the Ophidians becomes a reservoir so 

 spacious as to contain a volume of air, equivalent to a great 

 number of inspirations ; by the second, the air contained 

 in this reservoir can be forced into the cells by the con- 

 tractions of the thorax, ^vithout the necessity of a fresh in- 

 spiration. 



The figure of the lung undergoes numerous modifications 

 in the different species of 0})hidians. The form of that or- 

 gan is generally that of a simple sac, conical, and extend- 

 ing from tlie heart towards the lower region of the stomach, 

 where it terminates in a membranous pouch. The trachea^ 

 consisting of a great number of semi-annular cartilages 

 united anteriorly by a membrane, ends in the upper part of 

 the limg by an obli(iue opening ; this organ divides into two 

 branches in the Boa, in most of the genus Tortrix, the 

 Dipsas, and several other Ophidians, where we find a vestige 

 of a second pulmonary lobule, sometimes about half the size 

 of that on the other side. The air-cells of the pulmonary cavity 

 of other Ophidians extend on the membrane which unites the 

 rings of the trachea, so that they sometimes occupy this mem- 

 brane through all its length. There are other species in which 

 that membrane is much dilated, and incloses a number of 

 air-cells, as considerable as the lung itself. In the Xenodon 

 severus, in the true poisonous snakes, and in some other 

 Ophidians, this membrane enlarges into a capacious sac, 

 which alone contains all the air-cells ; so that from this 

 peculiar disposition, Ave find the lung displaced completely 

 by this organ, which, contrary to what we observe in other 

 Ophidians j is situated in the fore-part of the neck, between 

 the glottis and the heart. The position of the organs of 

 respiration in sea-serpents is still more extraordinary. In the 

 Hydrophis colubrina the trachea is prolonged to the hypo- 

 chondria, where it terminates in a membranous sac, wdiich 

 extends to within two inches of the anus ; but, instead of 

 a membrane uniting the tracheal rings, it is the lung which 

 invests it through its whole length. In the Hydrophis pe- 

 lamis, the trachea, dilated at its origin and contracted to- 



