790 THE POPULAR SCIENCE MONTHLY. 



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 branchise. In the Ascidise 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 eventu- 

 ally 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 por- 

 tion 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 ni;tritive 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 neai'est 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 floAV directly 

 favored by gravity.* 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 circula- 

 tion I'eceives 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 



