The Respiratory Organs 199 



divisions of the trachea does not increase with age 

 except, perhaps, in C. acutus and hiporcatus. The 

 lateral bend that the tracheal stem of so many 

 Crocodilia exhibits is not due to the greater number 

 of rings because in some species (gavials) where 

 the bend is present the number of rings is smaller 

 than in the Crocodilia where the bend is absent. 



According to Rathke and others most of the 

 tracheal rings are closed, but a varying, though 

 at most small, number are open on the dorsal side. 

 These openings become wider as the larynx is 

 approached. The transverse muscle fibers which 

 are found in the most anterior and largest of these 

 breaks in the tracheal rings were found, says 

 Rathke, in embryos after the middle period of 

 incubation. 



The cartilaginous rings of the bronchi, b, are 

 also apparently open for a time after their for- 

 mation, but soon close. Not infrequently in em- 

 bryos and in young animals are found rings that are 

 split like a fork, with one or both branches fused 

 with neighboring branches. 



The Lungs. The lungs. Fig. 57, 1, are more highly 

 developed among the Crocodilia than among any 

 other Saurian or Hydrosaurian group. The histo- 

 logical groundwork of the whole lung tissue is a con- 

 nective tissue of fine elastic fibers. In the lungs, on 

 the canal that appears as the elongation of the 

 bronchus, cartilage appears, according to Rathke, as 

 bands lying one behind the other; some of these 



