THE NERVOUS SYSTEM 



495 



tne swelling in the floor of the ventricle each side of the median line known as 

 the eminentia or trigonum hypoglossi. The root fibres of the XII pass lateral to 

 the medial lemniscus, between the olive and pyramid, and then emerge at the 

 groove between olive and pyramid. 



The dorsal nucleus of the X occupies a swelling lateral to the preceding and 

 known as the ala cinerea. Some of the root fibres of the X are axones from this 

 nucleus. They probably innervate (via sympathetic neurones) some, at least, of 

 the smooth muscles, heart (and 

 glands ?), innervated by the vagus 

 (X). The dorsal nucleus appears 

 to be relatively deficient in termi- 

 nals. What it does receive ap- 

 pears to come from the secondary 

 vago-glossopharyngeal and tri- 

 geminal tracts. 



The bodies of another group 

 of peripheral efl'erent neurones 

 form the nucleus ambiguiis, often 

 difficult to distinguish, in the 

 reticular formation. Their ax- 

 ones pass obliquely dorsally and 

 mesially, join the other root fibres 

 of the X, and then bending ab- 

 ruptly, pass with them to the 

 lateral surface of the medulla. 

 Some pass across the median line 

 and leave by the root of the op- 

 posite side. They probably in- 

 nervate the striated muscles of 

 the pharynx, larynx (and oesoph- 

 agus ?). The nucleus receives 

 various terminals, some at least 

 appearing to come from the 

 secondary trigeminal tracts and 

 from the lateral part of the retic- 

 ular formation. (Fig. 336.) 



Afferent Peripheral Roots, their Terminal Nuclei and Secondary Tracts. — 

 Other root fibres are the afferent fibres of the X w^hich form a common root with 

 the preceding. Sometimes they can be seen joining the fasciculus solitarius of 

 which they form a part. Some fibres or collaterals may enter the adjacent gray 

 {terminal nucleus of the X) (see Fig. 336). 



If the roots of the X do not show well in the section, defer their study until 

 the following section where the IX shows similar relations. 



The spinal V is partly pierced and partly covered by transverse fibres, princi- 

 pally oUvo-cerebellar fibres (see below) . Its terminal nucleus is less conspicuous. 

 Two new bundles of descending root fibres have appeared; one is the fasciculus 

 solitarius composed of the afferent root fibres of the X, IX (including gustatory 



Fig. 336. — Diagram of Origin of Cranial 

 Nerves X and XII. (Schafer.) pyr, Pyramid; 

 0, olivary nucleus; r, restiform body; d.V, spinal 

 root of fifth nerve; n.XII, nucleus of hypo- 

 glossal; XII, h>=poglossal nerve; d.n.X.XI, 

 dorsal nucleus of vagus; n.amb, nucleus am- 

 biguus;/.^., solitary fasciculus (descending root 

 of vagus and glosso-pharyngeal) ; f.s.n, nucleus 

 of solitary fasciculus; X, motor fibre of vagus 

 from nucleus ambiguus; g, ganglion cell of 

 sensory root of vagus sending central arm into 

 solitary fasciculus {f.s.) and collateral to its 

 nucleus {f.s.n.); f.s.n, cell of nucleus of solitary 

 fasciculus sending axone as internal arcuate 

 fibre to opposite side of cord (secondary vagus 

 and glossophar^Tigeal tract.) This course of 

 the secondary tract is doubtful. 



