Descending pathways from the brain stem to the spinal cord in some reptiles. II. Course and site of termination.

The course and termination of the pathways descending from the brain stem to the spinal cord have been studied by tracing the ensuing anterograde fiber degeneration, following appropriate lesions in the reptiles Testudo hermanni, Tupinambis nigropunctatus and Python reticulatus. In these reptiles the presence of interstitiospinal, vestibulospinal and reticulospinal pathways has been demonstrated. A crossed rubrospinal tract has been shown in the turtle and lizard, but could not be demonstrated in the Python. The presence of a tectospinal pathway of any importance could not be shown. However, the tectum mesencephali has been found to project profusely to the brain stem reticular formation. The interstitiospinal tract projects predominantly to the ipsilateral side of the spinal cord. The vestibulospinal projection, arising from the large-celled nucleus vestibularis ventrolateralis, comprises a large uncrossed and a small decussating component. The rubrospinal pathway terminates in a particular area of the spinal gray, i.e., the intermediate zone, whereas the interstitiospainal, reticulospinal and vestibulospinal tracts all terminate in the medial part of the ventral horn. It appeared that the classification of descending pathways as advocated in mammals by Kuypers ('64) into lateral and medial systems can be readily applied to reptiles. The lateral system terminates in the dorsal and lateral parts of the intermediate zone, the medial system predominantely in the dorsomedial part of the ventral horn. This classification renders it likely that the absence of a lateral focus of termination as well as the absence of a rubrospinal tract in the Python, is correlated to the absence of limbs. A comparison of experimental data concerning the systems descending from the brain stem to the spinal cord in amphibians, reptiles, birds and mammals suggests that these systems with regard to origin, course and termination have a basic pattern in common.