The " spaces" between the vessels forming the walls of the intercellular passages lead to no definitely bounded cells or chambers. They lead only to the interval which divides the contiguous bronchi from each other (c,c). This interval is filled densely with the ultimate pulmonary vessels. (B,fig. 225.) It was first determined by Mr. Rainey that these vessels, in the bird's lung, are arranged in a peculiar manner. They do not form plane reticular definitively bounded air chambers. Each ultimate capillary crosses an air-space of its own. It is thus surrounded by air. The ultimate vessels interlace and interloop in every direction, forming a cubic mass of capillaries permeated everywhere by the air. The apparently naked loops of the ultimate vessels may be seen projecting into the areas of the intercellular passages. No thing can be conceived more mechanically perfect than this arrangement of the vessels for the exposure of the blood to the opera tion of the air. The latter is in immediate contact with each individual vessel (n, fig. 226.) It surrounds the blood-current borne ing clenzent. 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 bronchial 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 branchim of fishes and the saccular lungs of amphibia, are invested by a separate epithelium, the conclusion first stated appears at present to be most reasonable and most in accordance with analogy. In these examina tions it is important not to mistake the out line of the red corpuscles in the vessels for that of the epithelial scales on their patietes.
According to the measurements of Mr. Rainey the areolce between the capillary vessels, which in the bird's lung are the real air spaces, — equivalent to air-cells, — are gene rally smaller in diameter than the capillaries themselves, and average in diameter about of an inch. An epithelial cell taken from the lining membrane of the bronchi in a pigeon measures in leng,th -ffire, and in breadth . It is therefore certain that, as Mr. Rainey contends, epithelium of such magni tude could not, by physical possibility, line spaces the diameter of which did not exceed of an inch. The error here committed consists in overlooking the difference between the dimensions of the cylinder epithelium which lines the bronchi, and those of the won drously attenuated hyaline epithelium which belongs to the true respiratory, capillary, areas of the lungs of birds, reptiles, and mammalia. A similar distinction between the epithelium which lines the cmcal extz .mities of glandular ducts, and that covering the merely convective or excretory stages of the same ducts, obtains in nearly all the simple and compound glands of the animal body. How singular if a prin ciple so wide-spread should be violated in the instance of the lungs I .Respiratozy Organs of Reptiles.
Temporary branchice of Amphibia.— In the life of all batrachian reptiles, the period which immediately follows the emergence of the young from the ovum is remarkable for the existence of organs capacitating the animal to live in water. In different genera these or gans vary in duration of existence. The larvm of the frog retain the external branchim only for a few days, after which these organs be come internal. Those of the toad remain in the egg state for a longer, and in that of the fish condition for a shorter, time than those of the frog. The tadpoles of the terrestrial sala manders of this country retain the external gills only for a brief interval, early assuming an exclusively atmospheric life. Those of the aquatic species, exemplified in the fitmiliar tritons of our pools, carry the external bran chim for a much longer period, affording thus, an opportunity for the study of the structure and function of these appendages. The nera syren, proteus, and menoGranchus are those only in which the external gills are per sistent throughout the whole term of adult life. Whether temporary, as in the caduci branchiate, or persistent, as in the perenni branchiate genera, the branchial organs of amphibia are supported by no skeletal frame work analogous to that which sustains the soft parts of the breathing apparatus of fishes. They are, essentially, only " productions," un der a modified form, of cutaneous structures. Contemplated only as a rnechanical contri vance, whether provisional or permanent, up on which devolves the most important func tion in the animal economy, it demands a more minute investigation than it has hitherto received at the hands of anatomists.