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190 HISTORY OF DENTAL SUEGEEY

In the first of these he describes quite fully the structural elements and
characteristics, and the physical properties of enamel ; its developmental lines,
the lines of cleavage, and the direction of the enamel rods in the different parts
of the tooth surface ; also the physical phenomena of caries of the enamel and
the manner in which caries spreads in the dentine and undermines the enamel
after it is penetrated. In closing the first chapter he says: ''To so form the
enamel margins that they shall have the greatest strength and give the best
form for the adaptation of the filling material, requires an accurate knowledge
of the direction of the enaniel-rods on all parts of the margins of each cavity."
In the second of these chapters Dr. Black formulated the doctrine of "e.x-
tension for prevention, or position of enamel margins considered in relation
to recurrence of caries after filling." Extension for prevention is extension
of the outline of the cavity, otherwise the enamel margin, "from a line of
greater liability to caries to a line of lesser liability, or to change the phrase,
it is to cut the enamel margin from the lines that are not self cleansing to
lines that are self cleansing." He sums up the discussion of these forms and
lines in three cardinal rules:
'"1. Cut the enamel to such a point tliat the surface of the iilliiig may
be so formed that the enamel margin will be self cleansing, or Ije jirotected
by the gum margin.
"2. Do not form an enamel margin in such a position as to leave a small
portion of enamel between it and one of the developmental grooves.
"3. So form the immediate margin of the enamel that it will present no
short ends of enamel rods on its outer edge, without sloping it outward so
much that the filling material will not have good edge strength."
The last of the papers is devoted to a detailed description of the methods of
packing gold in the various parts of cavities and over the enamel margins. The
whole series of papers covers sixty or seventy pages of the "Cosmos," and any
extended account or summary of them would be foreign to the purpose of this
writing. The phrase, "extension for prevention," caught the ear of the pro-
fession, the character and re]iutation of Dr. Black commanded attention, and
he was asked to explain and illustrate these doctrines and methods before
various societies in different parts of the country. Considerable dissent and
opposition was developed, some of it genuine and much of it through misap-
prehension of the proper applications and limitations of the principles set
forth in the "Cosmos" papers. The widespread and long continued discussion
of these and kindred topics has resulted in a very material improvement in the
average excellence of operations by the more skillful and intelligent men of