The Visual Cortex, Its Localization, Histological Structure, and Physiological Function

In the study of the human cortex it seems well to follow the same primary divisions for the entire cortex, while in certain areas some layers may assume certain modifications; viz., in the area striata it seems clearer to retain the sixth layer ground type, of which the fourth is subdivided into three secondary layers. The visual cortex, on account of its relation to the act of seeing, is an especially interesting field for study. About the calcarine fissure one can readily see with the naked eye a stripe of white fibers lying in the gray substance, called stria of Gennari or of Vicq-d' Azyr. This layer is considered by Brodmann in the second subdivision of the fourth primary layer. The two most characteristic types of the cortex are the giant pyramid type, called area gigantopyramidalis, and the calcarine type, called area striata. The higher types of mammals afford excellent material for comparison. Minkowski's experimental study of the dog indicates that the clinical, anatomical visual sphere is covered with the cytoarchitectonic sphere; for lasting visual trouble can be caused only by extirpation of the area striata. The subcortical visual center in mammals undergoes secondary degeneration through destruction of the area striata, and therefore depends on the cortex. Also certain parts of the corpus geniculatum externum have an exact projection over certain areas of the cortex.