" Throughout its circumference, area 17 is surrounded by a zone of transi- tional, though cytoarchitectonically simi- lar, cortex. The medial sector only (but including area 29d - see section on Me- dial cortical region) was called field 18 by Krieg after the precedent of Brodmann.....It is recognized, however, that 18a is the VII representation (Drager, '75) and, therefore, the homolog of area 18 of the primate. For this reason the medial field, 18 of Krieg, has been renamed 18b. "
" The cortex of field 17 is wider than the other two occipital fields. The width and cellular density of its layer IV, the distinctness of its hypocellular zones, Va and Vc, and the width of its sublamina VIa are all relatively greater than in 18a and 18b. The widths of layers I11 and V, on the other hand, are less in field 17 than in the other two fields.
Fields 18a and 18b are virtually indistinguishable in cell stains (also, Schneider, '69). The division between the two fields rests entirely upon the moderately heavy radial and tangential fiber pattern which continues through the more rostral and lateral part of field 17 into 18a. The more medial aspect of field 17 and adjacent 18b, by conhast, contains a much reduced complement of radial and tangential fibers. "