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318

Cunningham Memoirs.

in front of the lower end of the sulcus praecentralis inferior, and that there
is a horizontal limb running straight forward from the same seat of origin,
cutting the fronto-orbital operculum. These two anterior limbs enclose
the promontory (i. e. " Cap " [cape] of Broca).

Where these two anterior limbs are present their topographical relation
to the exterior appears to be, in the adult, as follows :—

The ascending anterior limb starts from the angle of the ptero-squamous
junction, i. e. the posterior extremity of the horizontal portion of the pterion,
and from thence runs upwards under or just in front of the coronal suture,
crossing that line a short distance above the pterion. Sometimes, when
the prsecentral sulcus is further back than usual, the anterior ascending
limb has been seen to run upwards just behind the coronal suture.

The horizontal anterior limb extends along the horizontal part of the
pterion; in other words, along the upper border of the great wing of the
snhenoid.

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These relations of the anterior limbs are notably altered in their vertical
level by age. Thus before the fifth year, when the squamous borders of the
basal cranial bones are not yet fully developed, the whole of the Sylvian
fissure is nearer the temporal ridges than the sphenoid and squamous
sutures; and further, owing to the late development and growth of this
region—when the line of the coronal suture moves forward (see Cunningham
, p. 156)—it overtakes the ascending limb of the fissure, which in the
full-term foetus lies in front of the suture.

In the infant and young child, as just stated, the position of the
anterior limb is further forwards and higher up when projected on the
cranial vault. This has to be noted in connexion with the commencement
of the posterior limb of the fissure.

The point at which separation of the limbs occurs is, owing to the
non-descent of the upper opercula, far above the pterion, and may be
considered to descend close in front of the coronal suture. At birth it is
just above the anterior extremity of the pterion line, i.e. at the ptero-frontal
junction. From this point it very slowly recedes posteriorly along the
pterion line, till the common adult position is arrived at. In a noteworthy
proportion of cases the point lies, however, not at the ptero-temporal