PROJECTION OF THE SENSORY MOTOR CORTEX TO THE THALAMUS, THE DORSAL COLUMN NUCLEUS, THE TRIGEMINAL NUCLEUS AND THE SPINAL CORD IN THE CAT.*

S ummary In the sensory motor cortex small lesions are unilaterally produced by suction in 5 cats and bilaterally in 14 cats. The brains and spinal cords are studied using the Nauta‐Gygax method. We have incidentally observed that in myelin sheath stained preparations the posterior ventral thalamic nucleus is divided into three clear divisions, that is to say, the VPLI, VPLm, and VPM, these divisions being made by the fiber bundles which are usually quite distinct (see Figs. 5, 6, 7, and 8) and are important in defining accurately the projections from the cortical areas. The VPLI and VPLm in our study, appear to correspond mostly to the VPL of Jasper and Ajmone Marsan's map, and the VPM in our study appears to correspond mostly to the VPM caudal to the section about Fr. 8.5 of Jasper and Ajmone‐Marsan's map. The lateral ventral nucleus, in our study, appears to correspond to the VL and to the most part of the VPM cranial to the section about Fr. 9.0 of Jasper and Ajmone‐Marsan's map. The results of the study of projections are summarized as follows. 1) Projections from the sigmoid and coronal gyri to the thalamus are ipsilateral except for those to the inferior part of the lateral central nucleus and to the nucleus centrum medianum, which receive a very small number of contralateral fibers in addition to the many ipsilateral ones. Projections to the trigeminal nuclei, the dorsal column nuclei, and the spinal cord are predominantly contralateral. Ipsilateral projections to the main sensory trigeminal nucleus and to the oral nucleus of the spinal trigeminal nucleus are very small, but definite. Ipsilateral projections to the interposed and the caudal nuclei of the spinal trigeminal nucleus are very few, if any. 2) The anterior part of the ASl and the anterior part of the Co, the motor facial cortical area, send projections to the medial part of the lateral ventral thalamic nucleus and to the reticular formation just medial to the trigeminal nucleus. From the middle and posterior parts of the Co occur only a few fibers to the reticular formation just medial to the trigeminal nucleus. The ventrolateral part of the lateral ventral thalamic nucleus, and the intermediate zone of the upper spinal cord, receive fibers principally from the ASl and additionally from the anterior part of the PSl. These cortical fields are the motor arm area. Projections to the intermediate zone of the lower spinal cord are observed in only 2 cases in which the lesions in the ASm extend deeply into the white matter under the cruciate sulcus. This finding is interpreted as showing that the motor leg area is not located on the free surface of the sigmoid or coronal gyrus. 3) Projection fibers from the PSm, the cortical sensory leg area, are demonstrated in the VPLl and in the dorsolateral corners of the VPLm and VPM, in the gracile nucleus, and in the central part of the posterior horn of the lower spinal cord. The VPLl and the dorsolateral corners of the VPLm and VPM show the regional differences between the projections from the anterior part of the PSm and those from its posterior part. The gracile nucleus and the central part of the posterior horn of the lower spinal cord do not show such a topical organization, but only a quantitative difference; its anterior part projects many fibers to the gracile nucleus and the spinal cord, but its posterior part projects sparsely. The principal projectional area to the VPLm apart from its dorsolateral corner, the cuneate nucleus, and the central part of the posterior horn of the upper spinal cord is the PSl. An additional one is the posterior part of the ASl and the posterior part of the Co. This cortical area is the sensory arm area. The topical organization within the projections from the sensory arm area is shown in the VPLm and the cuneate nucleus, but not in the central part of the posterior horn of the upper spinal cord. The middle part of the Co sends fibers to the central part of the VPM and to the larger ventromedial part of the main sensory and oral and interposed nuclei of the spinal trigeminal nuclei. On the other hand, the posterior part of the Co projects fibers to the dorsolateral part of the VPM, apart from its dorsolateral corner, and to the ventrolateral parts of the trigeminal nuclei. Many fibers to the caudal nucleus of the spinal trigeminal nucleus do not show topical organization. 4) If the lesion extends into the gyrus proreus, projection fibers can be followed through the loose fiber bundles in the lateral ventral thalamic nucleus to the dorsal medial thalamic nucleus. 5) Following lesions in the posterior sigmoid gyrus or in the posterior part of the coronal gyms, degenerating fibers are observed in the posterior lateral nucleus of the thalamus. 6) The inferior part of the lateral central nucleus and the nucleus centrum medianum receive fibers from all parts of the sigmoid and coronal gyri. 7) Projections to the basal part of the posterior horn and the nucleus cornu‐commissuralis posterior are discussed.