138 



THE NERVOUS SYSTEM 



fourth ventricle the nuclei are displaced laterally and gradually come to an end 

 as the restiform body becomes clearly defined (Fig. 101). 



As one would expect from the fact that there is no sharp line of separation between the 

 spinal cord and medulla oblongata, some of the fibers of the cuneate fasciculus end in the 

 substantia gelatinosa (here known as the nucleus of the spinal tract of the trigeminal nerve) 

 and in the remnant of the head of the posterior gray column (Fig. 100). There are three 

 smaller gray masses within the funiculus cuneatus: (1) the external round nucleus, an iso- 

 lated portion of the substantia gelatinosa, near which it is situated; (2) the internal round 

 nucleus, more variable in position; and (3) the accessory or lateral cuneate nucleus superficial 

 to the main nuclear mass. 



Funiculus gracilis 



Nucleus gracilis 



Spinal tract of trigeminal 

 nerve 



Nucleus of spinal tract 

 of N. V 



Dorsal motor nucleus of. 

 vagus 



Nucleus of hypoglossal 

 nerve 



Decussation of medial 

 lemniscus 



Lateral reticular nucleus 



Medial accessory olivary 

 nucleus 



Ventral external arcuate 

 fibers 



Funiculus cuneatus 

 Nucleus cuneatus 

 Central canal 



Internal arcuate fibers 

 Reticular substance 



Dorsal spinocerebellar 

 tract 



Ventral spinocerebellar 

 tract 



Ventral fasciculus 

 proprius 



Hypoglossal nerve 



Pyramid, corticospinal 

 tract 



Fig. 99. Section through the medulla oblongata of a child at the level of the decussation of the 

 medial lemniscus. (Pal-Weigert method.) (X 6.) 



The Medial Lemniscus and its Decussation. The great majority of fibers 

 which arise from the cells in the nucleus gracilis and nucleus cuneatus sweep 

 ventromedially in broad concentric curves around the central gray substance 

 toward the median raphe (Fig. 99). As has been stated on a preceding page, 

 these are known as internal arcuate fibers, and as they cross those from the 

 opposite side in the raphe they form the decussation of the lemniscus (decussatio 

 lemniscorum, sensory decussation). After crossing the median plane they turn 

 rostrally in the medial lemniscus (fillet), and end in the thalamus (Fig. 235). 

 These longitudinal fibers constitute a broad band which lies close to the median 

 raphe, medial to the inferior olivary nucleus, and dorsal to the pyramids (Figs. 

 96, 97). By the accession of additional internal arcuate fibers this band in- 

 creases in size and spreads out dorsally until at the level of the middle of the 

 olive it is separated from the gray matter of the ventricular floor only by the 



