We shall now describe such cortical fields as easily leap to the eye, even to a superficial observer. We notice, however, that there are two different situations that may make a delimited region of cortex clearly stand out between its neighbours. An area may be defined by a boundary where the appearance of the layering abruptly changes. The classical example in human anatomy is the striate area (area 17, Brodmann). There are other areas, however, which equally leap to the eye, because of prominent characteristics, even if they may blend gradually into the appearance of surrounding areas. A well-known example for this is the giganto-pyramidal area, area 4, of the human cortex, which is well characterized by the enormous cell bodies of the Betz cells, but much less by its boundaries. We find examples for both in the mouse cortex. In the numbering of areas we shall follow Caviness (1975).
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