ously performed in animals. In primitive life it acts through the outer skin. In higher forms a portion of this skin becomes adapted to that purpose, as exterior branchiæ. In the Ascidiæ it is performed by the anterior portion of the intestine. In all vertebrates it is an intestinal function, the forward portion of the intestinal tube becoming the gill of the fish. A sac-like ingrowth from the intestine—the swimming bladder of the fish—becomes the lung of the air-breather. It eventually separates into two portions, and adapts itself more perfectly to a function which it may have partly performed in the fish.
Thus the respiratory function has gradually moved inward to a position of safety, which is only fully attained in the air-breathers. Its position in the anterior, instead of in the mid or the posterior portion of the intestine, has another reason besides that of inheritance. This is its connection with the pulmonary circulation.
In considering the division of labor in the circulation we did not speak of its separation into two distinct portions, the nutritive and the pulmonary portion. This division of function is partly attained in fishes and reptiles, fully only in birds and mammals. And the heart, which serves as a force-pump to drive the blood through the body, becomes here a double pump, one half driving the blood through the arteries, the other half driving it to the lungs, there to become aerated.
The former circulation needs to penetrate the whole body. The latter can be fully performed with little extension of its blood-vessels. And in the labor-saving principle of organs, which hinders any excess of material or of effort, the tendency is to a curtailing of the length of this pulmonary circulation. Natural selection, therefore, acts to bring the heart and the lungs into close contiguity.
But there is another reason for the position of the heart in the anterior portion of the body. Its action as a force-pump renders it an advantage that it should be placed nearest the point where it has most work to perform. Now, the brain receives a much larger percentage of the blood than any similar portion of the body. More force, then, must be exerted by the heart in the direction of the head. If it be so placed that its labor in every direction may be equalized, it should occupy an anterior position.
In quadrupeds this need becomes still stronger, for the blood going to the head has to overcome gravity, that going to the body and limbs is largely aided by gravity. This need is, of course, strongest in man, in whom the requirements of the brain are the greatest, and in whom the upward flow is directly against gravity, the downward flow directly favored by gravity.[1] But in all the higher animals the heart, and therefore the lungs, necessarily occupy an anterior position.
- ↑ Very probably, however, the aid which the arterial downflow of the body circulation receives from gravity is balanced by the resistance of gravity to the venous upflow. And, likewise, in the head circulation the gravitative resistance to the arterial current
