Freiburger historische Bestände - digital

278

Cunningham Memoirs.

Herv£ * claims for the sulcus frontalis medius a precedence over the
other frontal furrows which it does not deserve. He states that it
appears in the sixth month, and asserts that it is " le sillon primitif," which
divides "the frontal lobe into two stages, or lobules, previous to its
decomposition into convolutions." It is true that in a few instances I
have observed the sulcus frontalis medius in the cerebrum of a sixth-month
foetus, and I have figured what seems to me to be four examples of this
(PL ii., figs. 22, 23, 25, and 26), but in each of these cases, with one
exception, it is associated with the other frontal sulci. This early
development of the sulcus medius is quite exceptional. As a general rule
it does not appear until about the middle of the seventh month, The
anterior sagittal stem is first formed. In figure 63, p. 276, it will be seen
as a short continuous furrow ; but in figure 62, p. 274, it is merely represented
by two pit-like depressions. Its two posterior parts come into
existence later, and may either remain distinct as two separate transverse
furrows, or establish a superficial connexion with each other and the main
stem (fig. 54, p. 250, and fig. 60, p. 268).

Traces of the fronto-marginal sulcus, or in other words of the terminal
cross-piece of the sulcus frontalis medius, are sometimes seen before the
main portion of the furrow makes its appearance. This is not common.
Often it is developed simultaneously with, but apart from, the sagittal stem.
Another mode of formation is that in which the termination of the sagittal
stem turns inwards to form the inner piece of the transverse furrow, whilst
the outer portion is developed as an independent depression, which ultimately
joins its fellow. The reverse condition may also be met with, but
much more rarely.

In a few cases the series of shallow furrows which we have grouped
under the name of sulcus frontalis mesialis may be seen in the brain of
the full-time foetus. As a rule, however, they do not appear until the first
month after birth.

Although the variability which is exhibited by the development of the
frontal sulci is so great that it is necessary to proceed with the greatest
caution in deducing any general laws from the process, I believe that we

* La circonvolution de Broca, p. 84,