. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 278 RESPIRATION. ing element. In the bird's lung there exist, therefore, no air-cells. It is argued by Mr. Rainey that the ultimate vessels in the bird's lung, as in the mammal's, are literally naked ; that is, that they have no other covering whatever than their own proper coats, of which at irregular intervals the cell-nuclei may be distinguished. In other words, that the epithelium, so percep- tible on the bronchi, is not under any shape continued beyond the termination of these tubes. To this view it has already been
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 278 RESPIRATION. ing element. In the bird's lung there exist, therefore, no air-cells. It is argued by Mr. Rainey that the ultimate vessels in the bird's lung, as in the mammal's, are literally naked ; that is, that they have no other covering whatever than their own proper coats, of which at irregular intervals the cell-nuclei may be distinguished. In other words, that the epithelium, so percep- tible on the bronchi, is not under any shape continued beyond the termination of these tubes. To this view it has already been objected that it is at variance with all ana- logy ; the branchial and pulmonary vessels of fishes and amphibia are provided, as will be subsequently shown, with pavement epithe- lium, the scales of which may be seen to be continuous with those of the ciliated divi- sion of the membrane; that a law of ana- tomical structure applying to the respiratory organs of the lower vertebrata must also govern that of the higher. It is impos- sible to demonstrate on the injected vessels of the bird's lung the presence of a separate investment of epithelium. The vessels do appear to be literally naked. But in the recent structure, in their sections through the bronchi and intercellular passages, it is perfectly easy to the practised eye to trace the epithelium of the bronchi over the larger vessels amid the intercellular passages just before the former break into the mass of the ultimate capillaries. The continuity of the pavement epithelium of the larger vessels with the cylindrical of the bronchi may be un- doubtedly traced by the eye. Now, what is true of the larger vessels is very probably true also of the smaller. Although, therefore, it cannot be directly proved at present that in the bird's lung the ultimate capillaries, as in the branchiae of fishes and the saccular lungs of amphibia, are invested by a separate epithelium, the conclusion first stated appears at present to be most
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