THE PONS VAROLII. 



1081 



to the descending or spinal root of the fifth nerve. A second and more compact gangHon, the 

 motor nucleus of the trigeminus, Hes to the inner side and slightly farther back. It contains large 

 multipolar cells, extends to a somewhat higher level than the sensory nucleus, and is separated 

 from the latter by a strand of fibres which arch over the motor nucleus and then pass mesially 

 beneath the ventricular floor to the raphe, where they cross to the motor nucleus of the oppo- 

 site side. These fibres are part of the crossed constituents of the motor trigeminal root. Additional 

 components of the latter, the descending or mesencephalic root, are seen in the interval between 

 the superior cerebellar peduncle and the lateral angle of the ventricle. The motor root itself is 

 represented by several inconspicuous and broken strands of fibres that emerge from the motor 

 nucleus and lie close to the inner side of the large sensory root. 



Lateral to the sensory nucleus and root of the fifth, and therefore beyond the conventional 

 limits of the pons, the section includes the three large fibre-tracts of the three cerebellar 

 peduncles. The most anterior of these is the middle peduncle into which the corresponding 

 ventral part of the pons is continued. The next and middle tract, joining the tegmentum to the 



Fourth ventricle 



Fig. 936. 



Superior cerebellar penduni-le 



Substantia ferrUjjinea, 

 Tegmental area 



Lateral fillet 



Nucleus 01 lateral fillet /^ 



Lingula overlying superior medullary velum 



I kor of fourth ventricle 



Mesencephalic root of trigeminus 



Posterior longitudinal 

 fasciculus 



Nucleus centralis superior 



Gray matter 



of pontine 



nucleus 



Pyramidal tracts 



Raphe 



Transverse section of pons at level K, Fig. 919. showing fourth ventricle closed by superior cerebellar penduncles 

 and superior medullary velum. X 3. Preparation by Professor Spiller. 



outer side of the sensory trifacial nucleus, is the now obliquely cut inferior peduncle or resti- 

 form body. The third and dorsal tract is part of the superior peduncle, which being crescentic 

 in cross-section, is here represented by its ventral edge. The three peduncles are thus 

 intimately related as they pass into the central core of white matter of the cerebellum. 



In sections passing at levels above the middle cerebellar peduncle (Fig. 936), the ventro- 

 lateral surface of the pons is free and unattached and passes over the rounded dorso-lateral 

 border onto the free posterior surface of the projecting part of the pons. Behind, the latter is 

 blended with the robust arms, the superior cerebellar pedimcles, that form the lateral walls of 

 the upper part of the narrowing fourth ventricle. This latter space is roofed in by the superior 

 medullary velum which stretches across the ventricle between the superior peduncles and on its 

 upper surface supports the thin lamina of cerebellar cortical gray matter belonging to the lingula 

 of the superior worm. 



The floor of the ventricle is grooved in the mid-line by a furrow bounded on each side by 

 an elevation— the upward prolongation of the eminentia teres. The depression at the lateral 

 angle of the ventricular floor is the upper part of the fovea superior. 



Beneath the latter are grouped the deeply pigmented nerve-cells of the substantia ferrug in e a 

 that, seen through the intervening layer of tissue, confer the characteristic bluish tint of the 



