More universal than this looped structure there is found another peculiar structure in the crocodilian trachea. It is a short vertical partition in the stem just before its division into the two branches. This partition is partly membranous and possesses one or more stiffening cartilaginous strands which are outgrowths of so many cartilaginous rings of the trachea. The number of the stiffening fibers varies in the different species. [Illustration: FIG. 57. RESPIRATORY ORGANS. _b_, bronchus; _e_, œsophagus; _g_, glottis; _l_, lung; _t_, tongue; _tr_, trachea.] The number of the tracheal rings varies not only in the different species but also in different individuals of the same species. There are between fifty and sixty in _A. mississippiensis_. According to Rathke the number of rings in the individual animal almost certainly does not increase with age. The number of rings is smallest in the gavials and greatest in the crocodiles (genus _Crocodilus_). The number of rings in the two 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 formation, but soon close. Not infrequently in embryos 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, l, are more highly developed among the Crocodilia than among any other Saurian or Hydrosaurian group. The histological groundwork of the whole lung tissue is a connective 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 bands form complete, others partial rings; some of the latter are forked. The hindermost appear to be the broadest and most irregular. Their number is different in different species and varies in different individuals of the same species. They range in number, according to Rathke, from nine, in _A. lucius_, to twenty-five, in _C. acutus_.[7]