THE SPINAL CORD 813 



The rubrospinal, cerebellospinal, lateral yestibulospinal, and olivospinal tracts 

 consist of descending axones which are intermingled so that their mutual topo- 

 graphical relations cannot at present be described. Collectively they constitute 

 the fasciculus intermedius of Lowenthal and Bechterew (inter mediolateral tract 

 of Bruce and Campbell) and they lie ventrad of the crossed pyramidal tract and 

 mesad of the combined spinothalamic and spinomesencephalic tracts. 



The rubrospinal tract (Monakow's tract; prepuramidal tract) originates in the 

 red nucleus in the tegmentum of the mid-brain of the opposite side and its axones 

 terminate in relation with ventral horn cells. In their course these fibres are seen 

 to invade the area of the crossed pyramidal tract. 



The cerebellospinal tract (Marchi's tract] is supposed to arise in the cortex of 

 the ce rebel la r hemispheres, to become distributed to the motor centres in the 

 ventral horn. 



The lateral vestibulospinal tract arises in the lateral nucleus of the vestibular 

 nerve (Deiters' nucleus), and by its relations with spinal centres establishes a 

 connection with the equilibratory apparatus. 



The olivospinal tract of Helweg (Helweg's Dreikantenbahn; Bechterew 's Oliven- 

 biiiulel) is found only in the higher segments of the cord, at its periphery and just 

 laterad of the emergence of the ventral nerve roots. Its connections and functional 

 direction are uncertain; so'me investigators have traced its fibres between the olive 

 and certain ventral horn cells; Obersteiner suggests a relationship with the pyram- 

 idal tract. The coincidence, in point of time, of the myelinization of both tracts 

 is significant in this connection. 



Several other descending tracts ending in the spinal cord and arising in higher 

 centres like the corpora quadrigemina, central gray substance of the mesencephalon, 

 and the cerebellum have been recently described by Held, Boyce, and Bechterew. 



Ground Bundle of the Lateral Column (fasciculus lateralis proprius). This lies 

 in the concavity of the lateral aspect of the gray column and consists of axones 

 of neurones having a purely commissural function. In the regions where the 

 reticula is best marked it is subdivided into a group of smaller bundles by numerous 

 glial septa. 



In the ventral column are described the following tracts: 



Descending: 



1. Direct pyramidal tract. 



2. Sulcomarginal tract. 



3. Ventral vestibulospinal tract. 

 Associating: 



1. Association axones between spinal centres and several cranial nerve nuclei. 



2. Fasciculus ventralis proprius. 



The direct pyramidal tract (fasciculus cerebrospinalis ventralis; fasciculus of 

 Tti rck) is the uncrossed portion of the pyramidal tract below the decussation in 

 .the medulla oblongata, and constituting only 10 to 15 per cent, of the fibre system 

 arising in the motor cortex of the same side. It is a small, oblong bundle, as seen 

 on trans-section, lying parallel with the ventral fissure, from which it is separated 

 in the higher segments by the relatively narrow sulcomarginal tract. The tract 

 diminishes in bulk as the cord is descended, to disappear in the thoracic portion 

 )f the cord; though, in rare instances, it has been observed to extend throughout 

 he lumbar portion as well. This diminution and eventual disappearance of 

 ,he tract is due to the successive decussations of its fibres throughout its course. 

 ; or, with a few exceptions, these cross in the ventral white commissure to come 

 nto relation with the ventral horn cells (motor cells) of the opposite side. This 

 aartial longitudinal extension of the pyramidal decussation and consequent for- 

 mation of an uncrossed, ventrally situated pyramidal tract is peculiar to the 

 arimate order of vertebrates. 



