512 



THE ORGANS 



tracts of V (three-neurone arc). The nature of its connections with efferent 

 paUial fibres is not known. Many collaterals are also received from the 

 mesencephalic root. (Fig. 3-14.) 



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

 fibres of the \' pass through the pons and enter the tegmentum where they 

 divide into short ascending and long descending arms. The former, together 

 with collaterals, terminate in the cephalic end of the terminal nucleus of the V. 

 This is broken up into groups of cells which lie dorso-lateral to the entering 

 fibres and is sometimes known as the "principal sensory" nucleus of the V. 

 The long descending arms pass down to the cord as the spinal V, giving off collat- 

 erals and terminals to the nucleus en route. (Fig. 344.) A third source of 



C'/f /• elf 



rn'n. I'. 



Fig. 344. — ^Diagram of Origin of Fifth Cranial Nerve. (Schafer.) G, Gasserian 

 ganglion; a, h, c, the three divisions of the nerve; m.n.V, principal motor nucleus; p.s.n.V, 

 principal terminal "sensory" nucleus; d.s.n.V, terminal nucleus of spinal root; d.s.V, 

 descending or spinal root; c.V and c'.V, secondary trigeminal tracts (axones of cells in 

 terminal nuclei); r, median raphe; 7>i'.n.V, mesencephalic nucleus. 



fibres of the V is a series of cells extending upward into the roof of the mesen- 

 cephalon. The axones of these cells form the mesencephalic root of the V. There 

 is reason to suppose, from their peculiar location and for other reasons, that 

 these are afferent peripheral neurones which have remained within the neural 

 tube. From the region of the terminal nucleus of the V, a transverse bundle 

 passes to the opposite side in the floor of the fourth ventricle. This is considered 

 a secondary decussating trigeminal tract which forms an ascending tract in the 

 dorsal part of the reticular formation. Fibres of secondary tracts give off 

 collaterals to v.arious efferent nuclei and probably axones of some cells of the 

 terminal nuclei become intersegmental fibres in the reticular formation. Second- 



/ 



