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Me. Victoe Hoesley—Cranio-Cerebral Topography.

331

behind the arc) is consequently opposite a number equal to J total
distance + 1 cm. + 6 cm.

The Rolandic arm of the instrument is now turned until its arrow-pointed
end is exactly opposite the degree which the fissure may be supposed to
make according to the cephalic index previously ascertained. It is then
secured firmly with a turn of the screw. The Rolandic arm has a long
slot in its centre, so that all that is now required is to draw a brush carrying
some aniline dye down the slot almost as far (2-5 mm. Hefftler) as the line
marking the fissure of Sylvius.

It only remains to state to what use this instrument may be put in cases
of different cephalic index.

Theoretically what was sought for as an ideal was a sliding scale of
parallel columns, the one giving the units of the cephalic indices, and the
other giving the corresponding variations in the angle. Unfortunately,
the material at my disposal was too small, and at present I believe we are
not justified in making more than the following rough rule:—

Accepting 69° as the average* angle which the fissure makes with the
middle line in the adult British head, then the angle may be expected to
increase or diminish by one degree for a corresponding increase or diminution
of two units in the cephalic index. In the absence of exact facts such
a conclusion, of course, does not pretend to be more than an estimate, but
as such I have found it to be practically useful.

A further assistance in this matter is derived from a knowledge of the
length of the fissure. The correct degree to which the line in this method
should be drawn has just been given, viz. as far as within a few millimetres
of the line which denotes the posterior limb of the fissure of Sylvius; but
an extra aid to control of the estimate is the average relative length of the
fissure. As Cunningham truly says (p. 190), the absolute length is of no
value in determining this point. Measuring the whole sinuous line of the
fissure, and, in addition, the length of the hemisphere, Cunningham's
figures afford the only evidence on this point. To avail ourselves of the
indices he has thus found, the procedure must be as follows :—Having found

* The mean between Hare's 67° and Cunningham's 71°.

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