THE SPINAL CORD AND ITS NERVES 131 



spinal cord and enter the cerebellum by way of its superior peduncle (bra- 

 chium conjunctivum) . 



The spinal lemniscus. Under this name are included several tracts to 

 the midbrain and thalamus. These fibers arise from neurons of the dorsal 

 gray column, cross in the ventral commissure, and ascend in the lateral and 

 ventral funiculi of the opposite side, partly superficially mingled with those 

 of the ventral spino-cerebellar tract and partly deeper in the fasciculus 

 proprius. This system of fibers includes a tractus spino-tectalis to the roof 

 (tectum) of the midbrain and a tractus spino-thalamicus to the ventral and 

 lateral nuclei of the thalamus. The deeper fibers of the latter tract are 

 arranged in two groups, the tractus spino-thalamicus lateralis for sensory 

 impulses of temperature and pain, and the tractus spino-thalamicus ven- 

 tralis for sensory impulses of touch and pressure (see p. 138, 173). 



Tractus spino-olivaris, fibers arising from the entire length of the spinal 

 cord and terminating in the inferior olive (Goldstein). 



DESCENDING TRACTS 



Tractus cortico-spinalis (fasciculus cerebro-spinalis, pyramidal tract). 

 This system of fibers conducts voluntary motor impulses from the precentral 

 gyrus of the cerebral cortex to the motor centers of the spinal cord. It di- 

 vides at the upper end of the spinal cord into two tracts, the larger division 

 immediately crossing through the decussation of the pyramids to the oppo- 

 site side of the spinal cord, where it becomes the tractus cortico-spinalis 

 lateralis (fasciculus cerebro-spinalis laieralis, lateral or crossed pyramidal 

 tract). A smaller number of these fibers pass downward into the spinal 

 cord from the medulla oblongata without decussation to form the tractus 

 cortico-spinalis ventralis (fasciculus cerebro-spinalis anterior, direct pyra- 

 midal tract, column of Tiirck). These fibers cross in the ventral commis- 

 sure a few at a time throughout the upper levels of the cord, and finally ter- 

 minate in relation with the motor neurons of the opposite side. Both parts 

 of the pyramidal tract, therefore, decussate before their fibers terminate. 



Tractus rubro-spinalis (tract of Monakow), from the nucleus ruber of the 

 midbrain to the spinal cord, for thalamic and cerebellar reflexes. 



Tractus olivo-spinalis (Helwig's bundle, tractus triangularis), fibers de- 

 scending from the inferior olive of the medulla oblongata to the lower cer- 

 vical or upper thoracic segments of the spinal cord. 



Tractus tecto-spinalis (predorsal bundle, tract of Lowenthal), from the 

 roof (tectum) of the midbrain to the spinal cord, chiefly for optic reflexes. 



Tractus vestibulo-spinalis, from the primary centers of the vestibular 

 nerve in the medulla oblongata to the spinal cord, for equilibratory reflexes. 



The two tracts last mentioned, together with several others, compose the 

 .fasciculus marginalis ventrahs. 



THE FASCICULUS PROPRIUS 



The fasciculus proprius system of fibers (also called ground bundles, basis 

 bundles, and fundamental bundles) comprises chiefly short ascending and 

 descending fibers arising from neurons of the spinal gray matter, for intrinsic 

 spinal reflexes. In general, these fibers border the gray pattern, but in the 

 dorsal funiculus some are aggregated in the tractus septo-marginalis and the 

 fasciculus interfascicularis (comma tract, tract of Schultze), these two tracts 

 also containing descending branches of the dorsal root fibers. Some fibers 

 of the fasciculus proprius ventralis lie adjacent to the ventral fissure and are 

 termed the fasciculus sulco-marginalis, these fibers forming the direct 

 continuation into the cord of the fasciculus longitudinalis medialis (posterior 

 longitudinal bundle) of the brain (see pp. 185, 211). 



