347. After the venous blood of the system has been thus exposed to the action of the respiratory medium, it is taken up by the vessels called the branchial veins (fig. [CXXIX]. 6), which for the reason assigned ([372]) are functionally arteries, as the branchial artery (fig. [CXXIX]. 4) is functionally a vein. The branchial veins uniting together form the great arterial trunk of the system, (fig. [CXXIX]. 7) by which the aërated blood is carried out to every part of the body.
348. But as if even this extent of apparatus were insufficient to afford the amount of respiration required by the system of the fish, the entire surface of its body, which in general is naked and highly vascular, respires like the branchiæ. Moreover, many fishes swallow large draughts of air, by which they aërate the mucous surface of their alimentary canal, which also is highly vascular; and still further, numerous tribes of these animals are provided with a distinct additional organ, a bag placed along the middle of the back filled with air. Commonly this air bag communicates with some part of the alimentary canal near the stomach, by means of a short wide canal termed the ductus pneumaticus, but sometimes it forms a simple shut sac without any manifest opening; at other times it is divided and subdivided in a perfectly regular manner, forming extended ramified tubes; while at other times its ramifications present the appearance of so many pulmonary cells. It is the rudiment of the complex lung of the higher vertebrata, and it assists respiration; although since in some tribes it contains not atmospheric air but azote, it is without doubt subservient to other uses in the economy of the animal.
349. In water-breathing animals, from the lowest to the highest, it is then manifest that a special apparatus is provided for, constantly renewing the streams of water that are brought into contact with their respiratory surface.
1. Integument or skin of the body. 2. Spiracula opening on the external surface of the skin. 3. Tracheæ, or air tubes, proceeding in form of radii from the spiracles to 4. the alimentary canal.
350. It is the same in aërial respiration. In the simplest form of aërial respiration the apparatus consists of minute bags or sacs, placed commonly in pairs along the back, which open for the admission of the air on the external surface, by small orifices called spiracula or spiracles (fig. [CXXX]. 2), at the sides of the body. In the common earth-worm there are no less than one hundred and twenty of these minute air vesicles, each of which is provided with an external opening placed between the segments of the body. In the leech, the number is reduced to sixteen on each side, which open externally by the same number of minute orifices. Over the internal surface of these air vesicles the blood of the system is distributed in minute and delicate capillaries; and is capable of being aërated by whichever medium may pass through the external orifices, whether water or air.
351. In this simple apparatus is apparent the rudiment of the more perfect aërial respiration by the organs termed tracheæ, minute air tubes which ramify like blood-vessels through the body (fig. [CXXX]. 3). These air tubes open on the external surface by distinct apertures termed spiracula or spiracles (fig. [CXXX]. 2), which are commonly placed in rows on each side of the body (fig. [CXXX]. 2), with distinct prominent edges (fig. [CXXX]. 2), often surrounded with hairs; sometimes guarded by valves to prevent the entrance of extraneous bodies, and capable of being opened and closed by muscles specially provided for that purpose. These tubes, as they proceed from the spiracles to be distributed to the different organs of the body, often present the appearance of radii (fig. [CXXX]. 3), and when traced to their terminations are found to end in vesicles of various sizes and figures, but commonly of an elongated and oblong form. These minute vesicles, when examined by the microscope, are seen to afford still minuter ramifications, which are ultimately lost in the tissues of the body.
352. The tracheæ are composed of three tunics, the external dense, white and shining; the internal soft and mucous, between which is placed a middle tunic, dense, firm, elastic, and coiled into a spiral. By this arrangement the tube is constantly kept in a state of expansion, and is therefore always open to the access of air. A great part of the blood of the body, in the extensive class of creatures provided with this form of respiratory apparatus, including the almost countless tribes of insects, is not contained in distinct vessels, but is diffused by transudation through the several organs and tissues of the body. All the creatures of this class live in air, and possess great activity; they therefore require a high degree of respiration; yet they are commonly small in size, and often some portions of their body consist of exceedingly dense and firm textures; hence to have localized the function of respiration, by placing the seat of it in a single organ, would have been impossible, on account of the disproportionate magnitude which such an organ must have possessed; in this case it was easier to carry the air to the blood, than the blood to the air, and accordingly the air is carried to the blood, and, like the blood in creatures of higher organization, is diffused through every part of the system.