The source of Ki
According to the old type theory of Ki, it comes from Dan-jeon, a lower stomach. It says that everybody has three Dan-jeon in his body; one is the lower stomach, the second is the center of between chest and stomach, and the third is between your eyebrows.
The theory says it is first step to accumulate Ki in the first Dan-jeon(the lower stomach) and you should learn how to control its flow or energy. If you have been much interested in Ki for a long time, you've heard this sort of explanation already. But it is difficult to understand this theory on the basis of the anotomy.
According to my understanding, this theory describes the subjective tips of how to grow and control Ki, like almost other theories in oriental area. You can confirm this understanding, by way of describing how to breathe. An oriental theory(many of old type martial art theoris, also) emphasizes you should breathe deep down to the lower stomach. But your lung cannot expanded down to your lower stomach. Then try to breath deep, however. You can feel something like a flow down to the lower stomach from your throat. Thus I think the lower stomach can be described to be a powerplant of Ki, from the subjective view. But anotomically, there is nothing we can find that can be a center of Ki. Then we should ask what is Ki? According to my present understanding, Ki can be understood as a flow of man's vitality. It is not material but nothing beyond material.(Refer to the previous discussion about Ki.) Ki is something like a flow of nervous energy, which can be controled by man's mind and which controls or leads the harmony of man's energy, I think. Then it should exist all over the body always objectively, yet it can be observed to exist on a particular part or moves from a part to another part subjectively. Considering this thought positively, we can conclude that Ki comes from the whole body of a man objectively. Then we should grow and develope our whole body and spirit to grow the Ki in us. I say like this, since some people train themselves of only breathing or of meditation without physical training. I think this would not be good method. Even though training of breathing were a direct method to grow Ki, the training of whole mind and body are necessary indirectly. This is my answer to the above question. Thank you.