THE NERVOUS SYSTEM 489 



1. Through the pyramidal decussation. 



2. Through the sensory decussation. 



3. Through the lower part of the olivary nucleus. 



4. Through the middle of the oliwary nucleus. 



5. Through the entrance of the cochlear nerve. 



6. Through the entrance of the vestibular nerve. 



7. Through the roots of the sixth and seventh cranial nerves. 



8. Through the roots of the fifth cranial nerve. 



The methods of Nissl, Cajal and glia stains should also be used when practic- 

 able. 



PRACTICAL STUDY 



I. Transverse Section of the Medulla through the Decussation of the F^yram- 

 idal Tracts (Motor Decussation) (Figs. 332 and 333) 



The most conspicuous features of this section are the decussation of the pyra- 

 mids, the larger size of the dorsal horn and the beginning of the gray reticular 

 formation. Surrounding the central canal is the central gray. 



Efferent Peripheral Neurones. — Nuclei of first cervical spinal nerve in ventral 

 gray, and root fibers passing out to emerge on ventral aspect. Nuclei of XI, in 

 mesial position in central gray, or in ventral gray. Axones pass out laterally 

 from latter and emerge on the lateral surface. The mesial or deep nuclei are 

 best reckoned with nerve X. 



Afferent Roots, their Terminal Nuclei and Secondary Tracts. — Some afferent 

 fibres of the first cervical spinal nerve are still entering at this level. 



Ascending afferent roots: The dorsal funiculus comprising the fasciculus 

 cuneatus and fasciculus gracilis remain as in the cord. Collaterals and terminals 

 from them can be seen entering the subjacent gray. 



Descending afferent roots: Some of the fine fibres between the enlarged dorsal 

 horn and the periphery, occupying the position of the zone of Lissauer in the 

 cord, are descending afferent root fibres of the V-cranial nerve or traclus spinalis 

 trigemini {spinal V). Collaterals and terminals from these fibres terminate in 

 the gelatinous substance of Rolando and also traverse it to form a plexus of 

 medullatcd fibres in its inner side very similar to the cord. The axones of the 

 dorsal horn cells (or terminal nucleus of the V) form the secondary tracts of the 

 V which cannot be distinguished (p. 512 and Fig. 342). 



Secondary tracts, forming parts of afferent suprasegmental paths: These form 

 a mass of fibres along the lateral periphery of the medulla which consists of (a) 

 the dorsal spino-cerebellar (b) the ventral spino-cerebellar and (c) the spino- 

 thalamic tracts. 



Intersegmental Neurones. — The neurone bodies are, as in the cord, scattered 

 throughout the gray. The continuation of the ventro-lateral intersegmental 

 tracts of the cord (and the colliculo-spinal tract) is the U-shaped mass of fibres 

 around the ventral gray. This mass consists of the long descending, the shorter 

 ascending and descending intersegmental tracts, and the colliculo-spinal; i.e., 

 (a) rubro-spinal (in lateral arm of U), (b) Deitero-spinal (lateral and mesial), 



