II. Isolated lateral branch; a, its opening into the cavity of the air-bladder.
Air-bladders with a pneumatic duct are found in Ganoids and Physostomes, the duct entering the dorsal side of the intestinal tract, with the exception of Polypterus and the Dipnoi, in which it enters on the ventral side of the œsophagus. In the majority the orifice is in the œsophagus, but in some, as in Acipenser, in the cardiac portion of the stomach, or in its blind sac, as in many Clupeoids. The air-bladder may be single, or consist of two divisions situated one behind the other (Fig. [52]); its inner surface may be perfectly smooth, or form manifold pouches and cells. If two divisions are present the anterior possesses a middle elastic membrane which is absent in the posterior; each division has a muscular layer, by which it can be separately compressed, so that part of the contents of the posterior may be driven into the elastic anterior division, and vice versa. The posterior division being provided with the ductus pneumaticus does not require the elasticity of the anterior.
Fig. 64.—Air-bladder of Pogonias chromis.
Some Siluroids possess a peculiar apparatus for voluntarily exercising a pressure upon the air-bladder. From the first vertebra a process takes its origin on each side, expanding at its end into a large round plate; this is applied to the side of the air-bladder, and by pressing upon it expels the air through the duct; the small muscle moving the plate rises from the skull.
The connection of the air-bladder with the organ of hearing in some Physostomes has been described above, p. 117.
In the modifications of the air-bladder, hitherto mentioned, the chief and most general function is a mechanical one; this organ serves to regulate the specific gravity of the fish, to aid it in maintaining a particular level in the water, in rising or sinking, in raising or depressing the front part of its body as occasion may serve. Yet a secretion of gas from the blood into its cavity must take place; and if this be so, it is not at all impossible that also an exchange of gases between the two kinds of blood is effected by means of the extraordinary development of retia mirabilia in many air-bladders.
In all fishes the arteries of the air-bladder take their origin from the aorta or the system of the aorta, and its veins return either to the portal, or vertebral, or hepatic veins; like the other organs of the abdominal cavity it receives arterial blood and returns venous blood. However, in many fishes the arteries as well as veins break up below the inner membrane into retia mirabilia in various ways. The terminal ramifications of the arteries may dissolve into fan-like tufts of capillaries over almost every part of the inner surface, as in Cyprinoids. Or these tufts of radiating capillaries are more localised at various places, as in Esocidæ; or the tufts are so aggregated as to form gland-like, red bodies, the capillaries reuniting into larger vessels, which again ramify freely round the border of the red body; the red bodies are formed not only by minute arteries but also by minute veins, both freely anastomosing with its kind, and being inextricably interwoven. The rest of the inner surface of the air-bladder receives its blood, not from the red bodies, but from normally ramifying vessels. This kind of rete mirabile or “vaso-ganglion” is found in the Perch and Gadoids; it is generally distributed in closed air-bladders, but also sometimes observed in air-bladders’ with pneumatic duct. In Anguilla and Conger two similar vaso-ganglia are situated at the sides of the opening of the pneumatic duct.
Fig. 65.—Lung of Ceratodus opened in its lower half to show its cellular pouches. a, Right half; b, Left half; c, Cellular pouches; e, Vena pulmonalis; f, Arterial blood-vessel; oe, Œsophagus opened, to show glottis (gl.)