The course and location of vestibulospinal, reticulospinal and descending propriospinal fibres in man are reported. The investigation was carried out on three patients with supraspinal lesions, four with transection of the spinal cord and 33 with anterolateral cordotomies. The lateral vestibulospinal tract at the medullospinal junction and in the first three cervical segments lies on the periphery of the spinal cord lateral to the anterior roots. It moves to the sulcomarginal angle in the remaining cervical segments. In the thoracic cord, it moves laterally, being traversed by the most lateral of the anterior roots. Reticulospinal fibres descend bilaterally in the spinal cord with a preponderance of ipsilateral fibres. Reticulospinal fibres in general do not form well-defined tracts, but are scattered throughout the anterior and lateral columns. They are intermingled with propriospinal fibres and with ascending and descending fibres of other systems. Most reticulospinal fibres move posterolaterally as they descend. It follows that fibres from the brainstem that enter the cord in the anterior column may be in the lateral column anterior to the lateral corticospinal tract at lower levels. Reticulospinal fibres within the lateral column lie anterior to the lateral corticospinal tract. They consist of scattered fibres between the lateral horn and the periphery, most of them in the medial two-thirds of the column. In addition, they are present in a more compact group, forming a triangle on transverse section on the periphery of the lateral column, immediately anterior to the lateral corticospinal tract. On the periphery of the anterior and anterolateral columns reticulospinal fibres descend as small groups or as a continuous band of fibres. The most medial of these reaches the sacral segments and is included in the sulcomarginal fasciculus. A compact group of fibres, shown previously to be central sympathetic fibres ending in the intermediolateral and intermediomedial cell columns, surrounds the lateral horn. They do not extend throughout the thoracic cord in all cases. Anterior to this group is another group of fibres lying on the anterolateral surface of the anterior horn. As these fibres were degenerating following a pontine lesion, they must be reticulospinal fibres. The fibres were not seen in all cases and they did not always reach the lowest thoracic segments. Reticulospinal fibres enter the grey matter in the zona intermedia and along the anterolateral and anterior surfaces of the anterior horns. Caudal to the cervical enlargement, the number of reticulospinal fibres decreases, and their place is taken by propriospinal fibres. But they are not totally replaced by the propriospinal fibres, for reticulospinal fibres continue down into the lowest sacral segments. Of the propriospinal fibres, the majority are short: descending fibres within the juxtagriseal layer are one to two segments long or less.

The course, location and relations of the corticospinal tracts within the spinal cord of man are demonstrated on the basis of cases with lesions above the spinal cord restricted to the corticospinal tracts, of motor neuron disease, and of anterolateral cordotomies; control cases were of normal spinal cords. The following features of the lateral corticospinal tract are emphasized in the cervical cord: (1) the large extent of the white matter of the cord covered by the tract, and the anterior extent of the tract, the border being anterior to the central canal; (2) in the lower cervical cord, the separation of fibres from the main mass of the tract, which reach the periphery of the cord in the anterolateral sector; (3) the presence in many cords of the ventral crossed bundle; and (4) the relationship of the denticulate ligament to the tracts in the cervical segments. The following features of the anterior corticospinal tracts are emphasized: (1) their location, caudal extent and asymmetry; and (2) the changes in location in relation to the median fissure as the tract descends and its relationship to other tracts of the anterior column. Three-quarters of spinal cords are asymmetric and in three-quarters of asymmetric cords the right side is the larger. The asymmetry is due to a greater number of corticospinal fibres crossing to the right side. As more fibres have crossed in the decussation, the anterior tract opposite the large lateral tract is smaller than the ipsilateral anterior tract: that accounts for the asymmetry of the two halves of the cord. The greater number of corticospinal fibres in the right side of the cord is unrelated to handedness, but correlates with the fact that in three-quarters of corticospinal decussations, the crossing from left to right occurs at a more cranial level than the opposite crossing. A group of short peripheral ascending fibres is described running along the sides of the median fissure in the thoracic cord.

Nine cases are presented which illustrate the segmental anatomy of the posterior columns with respect to the long ascending fibres. It is concluded that the fasciculus gracilis (FG) and the fasciculus cuneatus (FC) should be considered as separate anatomical entities. It is shown that the shape of each fasciculus is different in, and characteristic of, each of the upper thoracic and cervical segments. A certain degree of segmental lamination is present in the FG, but with extensive overlapping of fibres from different segments. The orientation of the laminae is not the same in all segments, being very approximately parallel to the medial border of the posterior horn in most caudal segments, approximately parallel to the median septum in intermediate segments, and oblique in an anteromedial posterolateral direction in cranial sections. The pattern of lamination in the FC and the degree of overlapping of fibres resembles that in the caudal FG. There is no, or minimal, overlapping of fibres of the FC with those of the FG. The most medial fibres of the FC, lying along the lateral border of the FG, are in proximity with fibres, in that fasciculus, from many different segments.

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