24 ORGANIZATIONOFREPTILES. 



made up of polygonal cells. Serpents are remarkable in having a single lung; a 

 small blind depression, first observed by Nitzsch,* must be regarded as the rudmient 

 of another. The lung is here long and very dilatable, and is analogous to the air 

 bladder of some Fishes; it is conical in shape, with its parietes vascular, having 

 numerous small and short folds on the inner surface. The Batrachia form the 

 connecting link between the Reptiles and Fishes, in their mode of respiration. In 

 Frogs, the lungs are large, with distinct cells, fillmg up a great portion of the cavity 

 of the abdomen when distended; in the Salamander, they are simple sacs, in the 

 walls of which may be seen cells analogous to those observed in Serpents. The 

 Sirens have branchial arches joined to the lingual bone, to which are attached 

 gills, and extensive puhnonary air sacs, reaching almost to the posterior part of 

 the abdomen. 



The trachea presents considerable variety in its structure and mode of subdivision; 

 in some, it is composed of complete cartilaginous rings; in others, the cartilages 

 are incomplete. In the Testudo (Gopher) it is long, subdividing in the thoracico- 

 abdominal cavity into primitive bronchia; in the Emys, the subdivision is still lower 

 down, and the bronchial tubes remain for a short distance attached to each other. 

 In the Sauria, the cartilaginous rings are complete, and the trachea only subdivides 

 when it has reached the lungs. In the Serpents, where we meet with but one 

 lung, of course no subdivision of the tubes can take place; the trachea commences 

 by a longitudinal fissure, behind the sheath of the tongue, and the cartilages that 

 enter into its formation are only complete rings at its upper portion; the inferior 

 rings, surroimding only the anterior part. In the Batrachia, the bronchia are 

 extremely short, as the limgs begin just below the larynx, which is also short, but 

 of great breadth. Much variety is also observed in the modes of termination in 

 the bronchial tubes; in the Chelonia they terminate by small lateral communications 

 with the puhnonary cells, while in Serpents they end in large orifices. 



Mechanism of Respiration. — This differs greatly in Reptiles from the Mam- 



* Nitzsch. Comment, de Respirat, &c., p. 13. 



