OF THE DIFFERENT MEASUREMENTS OF BOYS AND MEN.
There is a great difference in the length between the joints in men and boys for, in man, from the top of the shoulder [by the neck] to the elbow, and from the elbow to the tip of the thumb and from one shoulder to the other, is in each instance two heads, while in a boy it is but one because Nature constructs in us the mass which is the home of the intellect, before forming that which contains the vital elements.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.