In the boa and python the length of the left lung is generally less by a third or half than that of the opposite side ; but in caliber, crotalus and others, it is much smaller and quite rudimentary, appearing as little more than an obliterated appendage. The genera Ceacilia and Anzphisbwna have the left pulmo nary organ developed, and the right short ened : this arrangement probably varies ac cording to the species. Vipers and other serpents possess only a single lung, which on that account is very long. The lungs of the saurian reptiles conform in character to those of the ranidee and salamandridce. They are elongated sacs, cellulated internally. They extend far back along the roof of the abdominal cavity. Like those of the ophi dians, they are divisible into a cellulated and smooth or non-cellulated portion. The former is limited to the upper and anterior half of the organ ; the latter to the inferior wall and posterior half. There may be a mechanical reason in this peculiar. distribu tion of parts. The cells in the lungs of the saurians exhibit none of the regularity so characteristic of those of the ophidians. They are larger and more irregular. The partitions of the cells are more slender and more de licately membranous.
The whole interior of the lungs in the higher saurians is multiplied into cells. An axis without definite walls, like a trachea, runs from one end of the organ to the other, as is the case in the lung of the turtle. From either side of this axis, large orifices lead to the more subdivided portions, or secondary and tertiary air-chambers. On the contrary, each lung in Scincus officinalis forms a single continuous cavity ; but the entire surface of the parietes is cellulated by small projecting reticulate septa. The internal dorsal and anterior half of the lung of the chameleon is, as usual in the sauria, minutely cellulated. Further back the cells become larger, and the septa smaller, until at the posterior part the walls consist only of plane membrane both less vascular and less can cellated than the anterior. The ccecal ex tremity of the organ is drawn out into an appendage-like process, which reaches the farthest boundary along the roof of the abdo minal cavity. These appendages may be aptly compared to the abdominal air-cells of birds which communicate with open ex tremities of the bronchial tubes. The " dia phragm " . in the mammals precludes this interblending of the thoracic and abdominal organs, or the diffusion of air into any of the cavities of the body.
With reference to the minute structure of the lungs in the. saurians, it coincides pre cisely with the account given of those of the ranicke. Each septum consists of a central basis or framework of elastic fibrous tissue lined on either side by a reticulate layer of vessels. This plexus is again overspread by
a " hyaline pavement epithelium." Rich tracts of ciliary epithelium niay be discovered along the margins of cells, the course of vessels, and the lines of condensed structures. The double layer of vessels borne by each septum may be noted as a point of structure dis tinctive of the reptilian lung. The lungs of the chelonian reptiles are very voluminous. They extend over the whole dorsal part of the trunk as far as the pelvis. They are fixed by the pleura to the ribs, which also separate them from the cavity containing the digestive and generative organs. They are symmetrically developed on the two sides. Through the centre of each lung longitudi nally an unwalled axis extends from the an terior to the posterior extremities. This is the main road for the air-currents. From this axis, secondary passages, parietally cel lulated, radiate towards every point of the circumferences of the organ. The ultimate cells are very capacious. They- communicate little with each other. Each group has its common outlet, thus resembling a lobule. In the reptilian lung, however, there exist no lobules ; an anatomical particular in which they are distinguished from that of all mam malia. It is a criterion of lower organisation. The vibratile cilia which line the nasal and buccal pm,sages, the pharynx and cesophagus, the larynx and trachea of all reptiles are most remarkable for tenacity of life in the lungs of the chelonia. In the trachea of the turtle, along certain tracts of the lungs, the motion of cilia may be detected several months after death. The physiological value of the breathing process in any given animal cor responds, not with the volume of air inspired per any unit of time, but with the measure of the blood-surface exposed to its agency, the rate at which the blood-current moves, the numerical proportion of its red corpuscles,and the frequency of the respiratory movements. The small, but minutely, subdivided lung of the mammal presents a much more extensive surface for the outspreading of the rete mira bile than the very voluininous, but spacious chambered lung, of the chelonian. The total volume of air inhaled by the mammal is less than that which the lung of the turtle is capable of containing ; but in the former case it is more minutely distributed and divided ; it is more effectually employed ; the contact betvveen it and the blood-web is far more extensive and .intimate ; while it acquires a higher temperature than in the latter. In these several particulars, cold differ from warm-blooded animals.