The infant's body is pure yang. Being pure yang it has marvelous ch'i. When the ch'i is marvelous, then the blood is full. When the blood is full, then the vessels are supple. Supple blood vessels are unique to infants. A person who is close to death but still hopes to return to being a child can accomplish this only by developing the ch'i to become supple. To save ourselves in the midst of crashing waves and to regain life requires "Water and Fire After Completion.
Cheng Tzu's Thirteen Treatises on T'ai Chi Ch'uan ( Teatrise Three: Developing the Ch'i to Attain Suppleness. pg.32-33 )