^ 



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. luciiis, 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. 



' Miller (4Sc) 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. 

 Of 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." 



