(1940). This important difference between Fishes and Reptiles as relates to their mode of respiration would seem, at first sight, to draw such a distinct line of demarcation between these two great classes of Vertebrata that it would be impossible for the most superficial zoologist to confound one with the other, or to be for a single moment at a loss in attempting to assign to any creature belonging to either of these divisions of the animal world its proper position; indeed, to mistake an air-breathing reptile for a fish properly so called, would appear to be an error which the most ignorant naturalist could hardly be in danger of committing.

* Vide Berlin Annals for 1832; and also Panizza, sopra il Sistema Linfatico dei Rettili. fol. Pav. 1833.

(1941). We have, however, again and again had opportunities of observing how nearly animals of neighbouring classes approximate each other, not only in their outward form, but in their anatomical construction, and in considering this portion of our subject we shall have another most striking illustration of this great law in zoology.

(1942). The perfect and typical Reptile, as the Lizard, the Tortoise, and the Serpent, breathes air, and air only, and is therefore only provided with lungs adapted to this kind of respiration; but the Perenni-branchiate Amphibia, possessing both lungs and gills, participate to a greater or less degree in the characters of Fishes; so that in some, as, for example, in the Lepidosiren (fig. 328), so near is the approximation, that it becomes almost impracticable for the most accomplished anatomist precisely to determine whether the animal ought rather to be called a reptile or a fish; and lastly, in the Batrachian Amphibia, as we have already seen, we have the same animal gradually changed from a fish into a complete and perfect reptile.

(1943). In considering the apparatus provided for circulation and respiration in the animals comprised in the class before us, we shall therefore first describe the organization of these viscera in Reptiles furnished with lungs only; secondly, of those having permanent gills as well as lungs; and thirdly, the metamorphoses that take place in the construction of the breathing organs during the development of the lungs, and the obliteration of the branchiae in those forms in which the branchiae are not persistent.

Fig. 340. Anatomy of Proteus anguin us.

(1944). The lungs of Reptiles are two capacious membranous sacs occupying a considerable portion of the visceral cavity, which, as there is no diaphragm as yet developed, cannot properly be divided into thorax and abdomen, as it is in Mammalia. From the internal surface of the walls of each lung membranous septa project inwards, so as partially to divide the interior of the organ into numerous polygonal cells, which are themselves subdivided into smaller compartments in a similar manner. This structure is well seen in the lung of the Tortoise (fig. 341).

Fig. 341. Lung of the Tortoise.

(1945). The pulmonary cells are most numerous and complete towards the anterior extremity of the lung, and it is here that the pulmonary vessels principally ramify: towards the hinder part of the viscus the cells become larger, and the breathing-surface proportionately less extensive, until in some cases, as in Serpents, the cells being quite obliterated, the lung terminates posteriorly in a simple membranous bladder.

(1946). The air is brought into the lungs through a long trachea, composed, as in other Vertebrata, of a series of cartilaginous rings; but there is this peculiarity in the construction of the Reptile lung - the trachea never divides into bronchial ramifications, but terminates abruptly by one or more orifices which open at once into the general pulmonary cavity.

(1947). It must be evident, from the whole construction of a lung of this description, that, owing to the comparatively limited surface that it presents internally, it is far less adapted efficiently to expose the circulating fluid to the influence of the atmosphere than the more complex apparatus of Birds and Mammalia: the respiration of Reptiles is consequently proportionately imperfect; and hence that coldness of their blood and feebleness of muscular movement which are so characteristic of the entire class.

(1948). The air required for purifying the blood is, of course, continually changed, being alternately taken into the lungs, and again expelled in a deteriorated condition, by a mechanism which will be found to vary in different reptiles in accordance with the peculiarities of their organization. No Reptile possesses a diaphragm; and being destitute of this important muscle, the movements whereby inspiration and expiration are accomplished are, in such genera as are furnished with moveable ribs, entirely dependent upon the mobility of the framework of the chest - the dilatations and contractions of the thorax, consequent upon the alternate elevation and depression of the ribs, being sufficient to ensure the inhalation and expulsion of air: such is the case in the Serpent and the Lizard.

(1949). In the Amphibia, however, there are not even ribs developed, or, if they exist at all, they are such mere rudiments as to be quite useless as instruments of respiration; and on the other hand, in the Chelonian Reptiles, the large and expanded bones of the thorax are so consolidated together, and so immoveably fixed to the broad and osseous sternum, that respiration in the ordinary manner would be altogether impracticable. Under these circumstances, as a compensation for the want of mobility in the chest, the os hyoides and the muscles of the throat are converted into a kind of bellows, by which the air is forced mechanically into the lungs, and they are thus distended at pleasure.