The intrinsic connections of the cortex of area 4 of the monkey.

The intrinsic connections of area 4 of the monkey have been investigated with axonal degeneration methods after the placement of microelectrode lesions within the cortex. The fibre degeneration is restricted to within a few millimetres of the damage and is asymmetrically distributed in the form of an ellipse with its long axis anteroposteriorly. The same pattern is found in all topographic subdivisions of the motor cortex. There are two distinct zones of degeneration, dense fine terminal degeneration for 200 to 300 micrometer all around the lesion, and a moderate degree of fibre terminal degeneration for a further 2 to 3 mm. The intrinsic connections are disposed predominantly in a horizontal or oblique direction and within the laminae of origin, but there are fibres passing between adjoining laminae and between layers III and V and VI. Two horizontal plexuses of degenerating fibres are present, at the boundary layers II and III and at the level of the Betz cells, and these fibres arise within the cortex. The afferent and efferent fibres of the cortex are arranged strictly perpendicular to the surface. The extent and pattern of the intrinsic connections of area 4 are very similar to those of area 17 of the visual cortex.