Following lesions of various areas of the cerebral cortex in 15 cats, degenerating corticospinal fibres have been looked for in the spinal cord in sections stained with the Nauta silver impregnation technique. In previous studies corticospinal fibres have been demonstrated from the primary sensorimotor area and the 'supplementary motor area' and the anterior cingulate area. In the present study corticospinal fibres have been shown to arise from the gyrus proreus (areas 8-12, prefrontal cortex of primates), the second somatic area in the anterior ectosylvian gyrus and from areas 5 and 7 of the posterior parietal cortex. From these areas corticospinal fibres descend even to sacral levels of the cord and terminate in laminae IV-VII of the spinal grey matter as do those from the primary sensorimotor area and those from the medial aspect of the hemisphere. The largest contribution of corticospinal fibres demonstrated in the present account comes from the gyrus proreus. Only few fibres originate in areas 5 and 7. The findings are discussed in relation to relevant previous studies on the origin of the pyramidal tract, and the conclusion is reached that the primary sensorimotor cortex from a quantitative point of view is the most important contributor to the corticospinal projection in the cat. It appears from the literature that the same is the case in the monkey. However, relatively more corticospinal fibres seem to originate from areas 5 and 7 in the monkey than in the cat, while the evidence for corticospinal fibres from the prefrontal cortex is less in the monkey than in the cat. © 1969.
