200 The Alligator and Its Allies 
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.* 
The arterial branch, carrying venous blood to 
the lungs, develops a capillary network close to 
the alveolar walls, which leads away over the low 
alveolar septa, while over the tops of the higher 
septa and on the inner surface of the tube-like 
bronchial processes it forms a wide-meshed net- 
work of capillaries that are apparently chiefly 
nutrient. 
All the respiratory capillaries are attached by 
only one side to the alveolar wall; the free side 
that projects into the air space of the alveolus is 
covered by a continuous pavement epithelium. 
While the respiratory surfaces are covered with 
an alveolar epithelium of large polygonal cells, 
the free borders of all high septa and ridges, as 
well as the inner surfaces of the bronchial processes, 
are covered with ciliated cylindrical cells. 
1 Miller (45c) says: ‘‘In the crocodile and alligator the bronchus enters the lung 
near its center, and passes somewhat obliquely into the lung until it reaches the 
junction of the lower middle third; here it breaks up into eight to fifteen tubular 
passages. These tubular passages are studded with a great many air-sacs. In 
these animals the lung for the first time gives the structure as it is found in 
mammals. There are many air-sacs which communicate with a common cavity, 
or atrium, all of which in turn communicate with a single terminal bronchus. A 
single lobule of the mammalian lung is simply enlarged to form the lung of the 
crocodile; the lung of the former is only a conglomerate of that of the latter.” 
