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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
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