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336

Cunningham Memoirs.

but usually below this level, and consequently has not much value from this
point of view. Further it is a curve of much larger radius than the inferior
genu, and for this reason is less easy of determination. In general, it is
just below the level of the main axis of the superior frontal sulcus, the
denotation of which affords sufficient guidance to it.

(C) External Parieto-Occipital Fissure. — The external parietooccipital
fissure is generally taken (Broca,* Bischoff, Hamy, Hefftler,
Seeligmuller), as coinciding at its upper extremity with the apex—i e. the
highest and most anterior point of the lambdoid suture, or as it is termed
by anatomists the lambda.

But it is abundantly clear from the observation of the position of the
fissure in the lower apes, as well as the study of its relative position during
development, that the upper end of the fissure is well in front of the lambda
in earlier stages, and that it is only in adult man, and to a less degree the
adult anthropoid that it approaches, becomes coincident with, or even in
rare cases lies a little behind the lambda.

For present purposes it would therefore be advisable if possible to arrange
the facts according to a sliding scale of age, as well as to take the average
position in the adult.

Unfortunately the only evidence on the first point is founded on comparatively
few observations which bear a relative value to the growth of the
skull, viz. those made by Cunningham. (See Chap. I., pp. 62 et seq., of this
Memoir). His observations, however, are not only perfectly definite on the
main issue, but are abundantly confirmed by the less valuable (because not
relative to the growth of the skull) facts collected by Fdre\ Foulhouze,f
Hamy, Symington. The summary of the evidence adduced by these observers
, collated with Cunningham's, permits of the following conclusions:

1. From foetal life to the fifth year the index of the relative distance of
the external parieto-occipital fissure in front of the lambda is 4 to 4*9.

* In his first Paper describing his employment of the peg method, Broca states (1861) that
in 11 males the parieto-occipital fissure coincided with the lambda, and was rarely in front of it.

f Having had the opportunity of examining and measuring the specimens deposited in the
Musee Broca, it is gratifying to bear witness to the accuracy with which the measurements of
Dr. Foulhouze were made.