e formed by the
interlacement with them of large numbers of fine medullated fibres
running tangentially to the surface. These are derived chiefly from
the collaterals of the pyramidal cells and of the centripetal fibres.
They form two specially marked bundles, one within the layer of the
polymorphous cells known as the _inner band of Baillarger_, and
another in the layer of large pyramidal cells called the _outer band
of Baillarger_. This latter is very thick in the calcarine region, and
forms the _white stria of Gennin_, while the inner band is best seen
in the precentral gyrus. As both these strands cross the already
mentioned radial bundles at right angles, they are regarded as
specialized parts of an _interradial reticulum_ of fibres, but, nearer
the surface than the radial bundles penetrate, tangential fibres are
found, and here they are called the _supraradial reticulum_. In
certain parts of the brain the fibres of this reticulum are more
closely set, and form the _band of Bechterew_ in the superficial part
of the small pyramidal cell zone.

[Illustration: From _The Museum Catalogue of the Royal College of
Surgeons of England_.

Fig. 16.--Brain of _Petromyzon marinus_ (dorsal view). A, Brain; B,
choroid plexus removed.]

For further information on the structure of the cerebral cortex, see
A.W. Campbell, _Proc. R. Soc._ vols. lxxii. and lxxiv.

_Comparative Anatomy._

A useful introduction to the study of the vertebrate brain is that of
the Amphioxus, one of the lowest of the Chordata or animals having a
notochord. Here the brain is a very slightly modified part of the
dorsal tubular nerve-cord, and, on the surface, shows no distinction
from the rest of that cord. When a section is made the central canal
is seen to be enlarged into a cavity, the neurocoele, which, in the
young animal, communicates by an opening, the neuropore, with the
bottom of the olfactory pit, and so with the exterior. More ventrally
another slight diverticulum probably represents the infundibulum. The
only trace of an eye is a patch of pigment at the anterior end of the
brain, and there are no signs of any auditory apparatus. There are
only two pairs of cerebral nerves, both of which are sensory (Willey,
_Amphioxus_, 1894). In the Cyclostomata, of which the lamprey
(Petromyzon) is an example, the minute brain is much more complex,
though it is still only a very