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 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 parietes. 

 According to the measurements of Mr. Rainey 

 the arcolce 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 

 __i__ of an inch. An epithelial cell taken 

 from the lining membrane of the bronchi in 

 a pigeon measures in length -g^, and in breadth 

 g^oo. 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 

 eVo f an hich. 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 csecal exh .rallies 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 ! 



Resfiiratory Organs of Reptiles. 



Temporary branchics 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 larvne 

 of the frog retain the external branchiae 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 familiar 

 tritons of our pools, carry the external bran- 

 chiae for a much longer period, affording thus, 

 an opportunity for the study of the structure 



Fig. 227. 



Head and branchial appendages of the larva of the 

 Water -newt (Triton aquaticus vulgaris). (Or/</.) 

 The branchiae are enveloped in a prolongation of 

 the general cuticle of the body. The cells of the 

 epithelium covering the gills are, however, reduced 

 to a state of great attenuation, compressed into 

 scales, and polygonal in outline. The nearer the 

 period of transition from the larva to the perfect 

 reptilian type, the more intimate the resemblance 

 between the epithelium of the branching and that of 

 the general body. For some time before their obli- 

 teration, the branchiae cease to be distinguished 

 by ciliary vibration. This results from the change, 

 which gradual!}' occurs in the anatomical characters 

 of the component cells. The branchiae in the larva 

 of the newt consist of a trilobed extension of the 

 cuticle at either side of the head, the two posterior 

 lobes, which, in figure, resemble compressed finger- 

 like processes, presenting on either side secondary 

 projections, by which the respiratory area is mul- 

 tiplied. In relation to the size of the body, they 

 are larger than those of the larva of the frog. 



and function of these appendages. The ge- 

 nera syren, protcus, and menobranchus are 



