THE SUPPLY OF BLOOD TO THE LUNG The necessity for some definite system of transport arises partly from the greater size of animals as compared with the amoeba, and partly from the greater intensity of their oxidative processes. The inhabitants of an island a mile square would need no special transport system for the carriage of their fish, but the popula tion of a continent does, and not only the mere machinery for moving the fish but all the accessory apparatus of cold storage and the rest for moving it in good condition. Moreover, if the con tinent be inhabited by persons with an insatiable craving for fish the capacity of the transport system must be correspondingly increased.
In the more lowly organized aquatic animals the system of transport is as f ollows: A circulation is maintained throughout the animal of fluid which differs little from seawater. At some point, known as the gill, this circulation comes very near to the surface, being only separated from the seawater by the wall of the vessel in which it is coursing. That wall is no thicker than the body of the amoeba, and so the gases, oxygen and carbon dioxide, have no difficulty in diffusing into and out of the circu lating fluid. The amount of gas which can be dealt with depends principally on the extent of the surface of circulating fluid that can be exposed at any one time to the water. Therefore, for the purpose of creating the maximal surface, the gills of some crea tures take on curious and feathery forms. Such are those of the lobster, which may be seen by breaking away the shell at the side. Indeed, the surface is so great that the water around the gill would be completely denuded of oxygen were there not a special mechanism for ensuring a constant circulation of fresh water, carrying a continuous supply of oxygen-charged water over the surface of the gill feathers.
The gill system of the lobster exhibits the principles on which the respiratory systems of almost all the higher animals are based, i.e., the exposure of a large surface of fluid which circulates in the animal (the blood) to a corresponding large surface of either air or water which is constantly being replenished. The oxygen
containing medium and the circulating blood are not in actual contact, but are separated from one another by a membrane through which the oxygen (and carbon dioxide) must diffuse. One section of the animal kingdom has attempted a respiratory sys tem on different lines, namely, the insects. In them the air is piped all over the interior body to, or almost to, the actual functioning cells. There is no intermediary circulating fluid. The whole tis sues of the insects are therefore permeated by an elaborate sys tem of tubes, the tracheae, with walls stiff enough to prevent their collapsing. The tracheal tubes are often extremely narrow in bore. This system has grave limitations. The rate at which gases can diffuse along very fine tubes is very slow, and sufficient oxygen can only penetrate, therefore, for a short length. No portion of the insect, therefore, can be far removed from the external air, and for that reason all insects are small ; the largest development of which they seem capable is that of the dragon fly, which has a relatively long but extremely attenuated body. Developmentally the insects are side-tracked.
To return to the normal line of development, the principles of respiration are simply portrayed in such an air breathing animal as the newt (fig. ia). Imagine a grape with a tubular stalk and with air inside the skin instead of fruit, and you have something like the lung of the newt. In the substance of the wall the blood circulates, a large surface being exposed in a close network of capillaries to the air in the lung, which air is intermittently forced in and out of the lung by swallowing movements on the part of the newt, the stalk of the lung (or trachea) being an outgrowth of the gullet.
Leaving for the moment the consideration of the circulating fluid, we may follow two other lines of development: (I) the mechanism for increasing the amount of surface of fluid exposed to air and (2) the mechanism for perfecting the ventilation of the lung.