844 THE NERVE SYSTEM 



The sulcomarginal tract (tractus tectospinalis) is a thin bundle whose axones 

 arise in the corpora quadrigemina of the opposite side, immediately decussating 

 and descending through the medulla oblongata, to be distributed to various spinal 

 centres in a manner not yet accurately ascertained. This tract, which is most 

 developed in the cervical region of the cord, is assumed because of its proximity 

 to the lower optic, ocular muscle, and acoustic nuclei. 



The ventral vestibulospinal tract (Lowenthal's tract; anterior marginal fascic- 

 ulus; ventral cerebellospinal tract) lies at the periphery of the ventral column, 

 extending, as seen on trans-section, from the ventral root zone to the ventral 

 fissure. Its axones arise from (1) the lateral (Deiters') and (2) superior (Bech- 

 terew's) nuclei of the vestibular nerve; and (3) from the nucleus fastigii of the 

 cerebellum. Their termination about the ventral horn cells has been traced as 

 far as the sacral region of the cord. 



As in the lateral column, and continuous with the like formation, there is in the 

 ventral column an intermediate zone of mixed systems of axones which serve to 

 associate various levels of the cord with ganglionic masses in the medulla oblongata. 

 as well as with the cerebellum and corpora ([uadrigemina. The nuclei of thp tri- 

 geminus. facial, auditory, glossopharynffeal. and vagus nerves, together with the 

 olive and the cerebellum, seem most intimately associated with thp spinal cpntrp* 

 for movements of the head and neck. 



Ground Bundle of the Ventral Column. The white substance of the ventral column 

 contiguous with the central gray is made up of intersegmental axones of associa- 

 tion connecting different levels of the cord. 



The ventral (white) commissure (commissura ventralis alba) is composed of 

 myelinic fibres which decussate with or cross each other and, on trans-section, are 

 seen to form a narrow band connecting the ventral columns of the two sides. The 

 axones composing it are chiefly (1) those arising from ventral horn cells, which 

 after crossing the midline, course horizontally or cephalad and caudad to come 

 into relation with neurones at the same or at different levels of the gray substance; 

 (2} the decussating axones of the direct pyramidal tract; (3) numerous collaterals 

 from the ventral and lateral column axones. The white commissure is most 

 massive in the enlargements where the associations of the limb centres are neces- 

 sarily greater in number. 



Myelinization of the Axones of the Cord. The acquisition of the myelin sheath 

 is not cotemporary for all axones in the cord, but is characterized by a regular 

 progression in the myelinization of separate fasciculi. As a rule, those axone 

 systems which are concerned with simpler or intrinsically spinal reflexes become 

 myelinic or "mature" at an earlier stage of fetal development than do those con- 

 cerned in the more elaborate connections of the cord with the brain. 



According to Flechsig and Trepinski the order of myelinization is as follows: 



1 . Ventral and dorsal roots during fifth month. 



2. Ground bundles during sixth month. 



3. Dorsal columns during fifth to seventh month. 



4. Direct spinocerebellar tract, seventh month. 



5. Ventral spinocerebellar (Gowers') tract, seventh to eighth month. 



6. Olivospinal (Helweg's) tract, ninth month or later. 



7. Pyramidal (crossed and direct) tracts, at or soon after birth. 



The order of myelinization of the separate fasciculi is indicated in Fig. 614. 



Summary. The Gray Substance. The gray substance consists, aside from its. 

 .supporting tissues, of sentient and reacting nerve cells, with their dendrites and 

 axones. and of the terminals of axones entering from without. These nerve 

 cells may be classified as follows: 



(a) Nerve cells whose axones pass directly out of the cord. These lie in the 

 ventral horn, are "motor" in function, and their axones form the ventral nerve_ 



