THE GRAY MATTER, 



609 



root and a larger sensory root, the latter bearing the large ganglion of Gas- 

 ser ; and the origin of the nerve is in many ways complex. Both roots 

 may be traced in an oblique direction (Fig. 135, F.), inward and toward 

 the dorsal surface, through the pons to the reticular formation beneath the 

 floor of the front part of the fourth ventricle, the smaller motor root taking 

 up a position median to the larger sensory root. 



Here the motor root comes into connection with a collection of nerve- 

 cells (Figs. 138 and 135, V. m.), which may be regarded as its nucleus ; but 

 this is not the whole nucleus of the motor root. From the level of the 

 nucleus there stretches forward as far as the level of the anterior corpora 

 quadrigemina a bundle of longitudinal fibres which, since it is usually traced 

 from the front backward until it passes into the root of the nerve, is spoken 

 of as the descending root of the fifth nerve. 



This descending root begins as a few scattered bundles of fibres at the 

 level of the anterior corpora quadrigemina, in the peripheral lateral part 

 of the central gray matter surrounding the aqueduct, dorsal, and lateral 

 (Fig. 137, V. d), to the nucleus of the third nerve (Fig. 137, III. n.). 

 From thence the fibres pass backward, augmenting in number, and soon 

 form a compact bundle, semilunar in transverse section, lying lateral to the 

 fourth nerve as this is rising dorsally (Fig. 136, V. d.~) ; still increasing in 



Through the Fore Part of the Pons. (Sherrington.) In the line 113, Fig. 131. Py. pyramidal 

 fibres ; F. C. fibres from the frontal cortex ; 8. P. superior peduncle of the cerebellum ; Fm. me- 

 dian portion; Fl. lateral portion of the fillet; 1. posterior longitudinal bundles; P. C. Q. posterior 

 corpora quadrigemina ; y. fibres which become detached from the fillet, and further forward form 

 (the innermost) part of the pes of the crus ; 1. c. locus cseruleus ; n. P. Q. nucleus of the posterior 

 corpora quadrigemina the outline is made too sharp; IV. bundles of the fourth nerve decussat- 

 ing, IV. n. its nucleus ; V. d. descending root of the fifth nerve ; Aq. the aqueduct ; c. g. the region 

 of central gray matter. 



number in their course backward, they gradually assume a more ventral 

 position as the aqueduct opens into the fourth ventricle. All along its 

 course this descending root has attached to it large (70 A* or more in diam- 

 eter), sparse spheroidal nerve-cells of striking appearance ; these, however, 

 seem too few to give origin to all of the fibres, and there are some reasons 



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