Circulation Of The Blood
( Originally Published 1921 )
THERE is nothing that creates more surprise in the world's history than the late period at which the circulation of the blood was discovered. It is hard to conceive of a time when the fact that the blood surged back and forth from one part of the body to another was not noticed. What was really discovered in the seventeenth century was that the blood made a complete circuit in the body and came back to the place where it started. This was such an astonishing addition to human knowledge that it took its place as a fact in the human mind, and became much more definite than was justified by the actual conditions.
The truth is that, in addition to the circulation of the blood, there is a surging to and fro, as is evident when a person becomes pale or flushed. It is an error to suppose that the circulation of the blood is easy to understand. The whole subject is one upon which there is great difference of opinion and belief, as there are many factors that control it. The only method that I know of by which a clear understanding of the circulation can be obtained is by studying it in the light of the development of the circulatory organs in the animal kingdom.
'Whether we believe in evolution or not, it is a fact that there is a gradual increase in the complexity of the structure of animals, when we go from the very lowest forms of life to the highest, and often by studying the lower and simpler form of animal we are able to understand the structure and action of higher animals. This is no more true of animal development than it is of the circulation of the blood. Our own hearts and blood vessels are an extraordinary elaboration of the circulation in the low and simple forms of life.
The lowest form of life is simply a mass of jelly, containing within itself a certain amount of moisture. In the next higher order, we find the fluids of the body contained in separate channels something like a sponge. A little higher in the animal scale, we find the circulation commencing, in that the body fluids are contained in channels and tubes, but with no particular arrangement to make the fluid move. Still higher in the scale we find that these tubes are connected together in circuits, and occasionally one part of the tube will have the power of contraction, so that the fluid is moved. In a yet higher order we find that the tubes have developed little flaps of tissue that keep the fluids moving in one direction. A little higher in the scale we find the tubes with definite muscular developments in several places, which contract and cause the fluids to move forward. Later on we find in certain animals a definite number of these contractile bunches of muscle, and we commence to call them hearts ; so that some of the lower animals have several hearts, in different parts of the body, which pump the blood forward. Skipping a lot of intervening changes, we come to the fishes, where there is a single chambered heart; and finally to the animals of our own class, where there are two hearts bound together to make one one circulating the blood through the lungs, and the other through the body.
When we think of the human heart and the human blood vessels as having come by gradual changes from a lower form, and having been finally developed to meet the demands of the body, it becomes easy to understand what the heart really is, and to know that it is simply two muscular pumps which have been developed into a single organ and arranged to fit into a small space.