THE NERVOUS SYSTEM 493 



and are known, respectively, as the nucleus of the column of Goll or the nucleus 

 gracilis, and the nucleus of the cohimn of Burdach or the nucleus cuneatus. In the 

 higher sensory decussation levels there is usually an accessory cuneate nucleus. 



These nuclei serve as nuclei of termination for the fibres of the posterior 

 funiculi. Their termination in these nuclei is the ending of that system of fibres 

 which has been traced upward from their origin in the cells of the spinal ganglia; 

 the completion of the course of the spinal peripheral afferent neurones. As the 

 fibres of the posterior columns are constantly terminating in these nuclei, there 

 is, in passing from below upward, a constant increase in the size of the nuclei 

 and a corresponding decrease in the size of the posterior columns, until, just below 

 the olive, the whole of the column of Goll and most of the column of Burdach 

 are replaced by their respective nuclei, (pp. 467, 468.) 



Study the plexus of fine fibres in these nuclei, formed by the terminals of the 

 column fibres, also the coarser fibres (a.xoncs of the cells of the nuclei) gathered 

 in the ventral part of the nuclei, whence they emerge and curve^ around the cen- 

 tral gray, cross to the opposite side ventral to it and dorsal to the pyramids, and 

 then turn brainward forming the bundle of fibres known as the medial lemniscus 

 or medial fillet. 



The spinal V has increased and also its terminal nucleus, the dorsal horn. 

 The spino-cerebellar and spino-thalamic tracts occupy about the same positions. 



In the central gray dorsal to the central canal is a nucleus representing a 

 union of the caudal ends of the terminal nuclei of the fasciculi solitarii (see next 

 section") — the nucleus commissuralis. Fibres of the fasciculi solitarii also de- 

 cussate here. 



Intersegmental Nevirones. — The rubro-spinal and Deitero-spinal tracts and 

 the tract from the nucleus of the medial longitudinal fasciculus occupy about the 

 same positions. The reticular formation has increased, the whole of the ventral 

 horn and intermediate gray containing bundles of longitudinal fibres. The 

 formation is also traversed by transverse fibres, representing the beginnings or 

 terminations of various longitudinal fibres. 



Efferent Suprasegmental Neurones. — The decussation of the pyramids has 

 now nearly or entirely ceased. The lateral pyramidal tracts are no longer in the 

 lateral columns but are parts of the anterior pyramidal tracts which form two 

 large masses of fibres one on each side of the ventral sulcus. The coUiculo- 

 spinal tract occupies the same position. 



3. Transverse Section of the Medulla through the Lower Part of the Inferior 

 Olivary Nucleus (Figs. 332 and 335) 



The central canal has opened into the fourth ventricle, the central gray (in- 

 cluding the central gelatinous substance) now being spread out on its floor. The 

 roof of the ventricle is formed by its chorioid plexus. The most conspicuous 

 new feature is the olive. 



Efferent Peripheral Neurones — The nucleus of the XII is large and occupies 



^ Fibres having a transverse curved or arched course are in general termed arcuate 

 fibres. If they are deeply located, they are internal arcttate fibres, if near the periphery, 

 they are superficial or external arcuate fibres. Obviously the same fibre may be, in 

 different parts of its course, internal arcuate, external arcuate, and longitudinal. 



