162 Life and Letters of Francis Galton
the great toes, it will be seen that the thumb is inward and the little finger outward. [Finger-prints taken in this order from left to right are in "natural order." j Inward and outward are respectively thumb-side and little finger-side, but these terms are awkward when we have to use them for the thumb and little finger themselves, and the same criticism applies to the anatomical terms radial and ulnar when applied to those bones themselves *.
Next Galton gives for the first time his explanation of the manner in which the "core" of a pattern originates ; he held that it is due to the nail; the ridges, instead of going straight across the bulb of the finger, are distorted to cover the top of it; the space between the originally adjacent parallel ridges Galton terms the "core." This core or interspace is filled up
Primary or Arch ; formation of Interspace.
Fig. 20. Cores in Interspace, showing "deltas."
with an additional scroll work of ridges, which in themselves form the pattern on which classification depends. When the scroll work of the core consists of a series of ridges separated on the central portion of the bulb by wider intervals than at the sides of the bulb, Galton in this memoir terms it a "primary," but later he uses the term "arch." Next the "deltas" are defined. These are the small "islands" at the points where the adjacent parallel ridges begin to diverge to form the core. In a primary there are no deltas, in a loop one and in a whorl usually two are discoverable. When there are two deltas, the line joining them and its perpendicular bisector serve as axes of reference; when there is only a single delta, in a loop, Galton
* Calton unfortunately transposed "inner" and "outer" in this memoir, calling the inside of the thumb that "nearest to the rest of the hand" (p. 4). He corrected this error in his Finger Prints (p. 70).