The Respiratory Organs 199 
divisions of the trachea does not increase with age 
except, perhaps, in C. acutus and biporcatus. 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 
