[7] 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.” 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 network 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. CHAPTER VIII THE VASCULAR SYSTEM The account given by Bronn in his _Thierreich_ is apparently the only published description of the circulatory organs in the Crocodilia. This account, even when translated, is not very satisfactory, especially because it contains no diagrams of the circulation. It was, therefore, deemed worth while to work out the circulation in the Florida alligator in order that we might have not only a written description, but also a series of more or less accurate diagrams of the veins and arteries. A number of departures from the description of Bronn were found, some of which are noted below. Most of the work was done upon animals of about thirty inches length, which were obtained alive from the Arkansas Alligator Farm at Hot Springs, Ark. The arteries were injected with a colored starch mass by inserting a two-way cannula into the dorsal aorta. With the blood thus forced into them from the arteries, the veins could, in most cases, be traced without difficulty. In the diagrams the outlines of the more important organs are accurately shown by dotted lines, and the relative diameters of the blood-vessels are shown as accurately as possible by the solid black lines. THE HEART In the Crocodilia, as is well known, the heart is four-chambered and has about the same general shape as in the higher vertebrates, Fig. 58.