NORMAL HISTOLOGY. 



of the separated area is broken up into a coarse net-work containing 

 nerve-cells and intersecting fibres the formatio reticularis. 



Above the level of the decussation of the pyramids, a second or 

 sensory decussation is seen, formed by the crossing fibres pro- 

 ceeding from the nerve-cells within the funiculus gracilis and cunea- 

 tus. On gaining the opposite side, some of the decussating fibres 

 (internal arcuate) turn sharply brainward and pass to higher levels 

 as a longitudinally coursing tract, known as the mesial fillet. Other 

 fibres, the anterior external arcuate, after crossing, appear ven- 



a 



Section of medulla at level of sensory 

 decussation : a, anterior pyramidal 

 tracts ; b, posterior median septum ; 

 c, h, gray matter representing bases of 

 anterior and posterior cornua ; e, iso- 

 lated anterior horns ; f, bundles of sen- 

 sory fibres displacing posterior horn ; 

 g, nucleus gracilis ; /, posterior horn, 

 substantia gelatinosa of Rolando ; k, 

 nucleus cuneaius; /, decussating sen- 

 sory fibres crossing to the opposite 

 median tract ; nt, root-fibres of hypo- 

 glossal nerve. (After Testut-Duval.} 



n 



Diagram of medulla through olivary bodies : a, 

 anterior pyramidal tracts ; b, floor of fourth ven- 

 tricle ; c, remains of gray matter of base of anterior 

 horns, nucleus of hypoglossal nerve ; c', accessory 

 hypoglossal nucleus ; d, decussating fibres of for- 

 matio reticularis ; e, nucleus ambiguus ; g, gray 

 matter of posterior funiculus, including h, which 

 represents base of posterior horn ; z, substantia 

 gelatinosa of Rolando ; j, ascending root of tri- 

 facial nerve ; k, restiform nucleus ; /, funiculus 

 solitarius ; m t root-fibres of pneumogastric nerve ; 

 n, hypoglossal nerve; o, nucleus dentatus of olive; 

 p, q, dorsal and mesial accessory olivary nuclei ; 

 r, external arcuate fibres ; s, sensory portion of 

 anterior pyramid. (After Testut-Duval.*) 



trally and wind over the anterior pyramid and inferior olive to gain 

 the restiform body in their course to the cerebellum. Additional 

 decussating fibres pass from the inferior olivary body of the opposite 

 side to the cerebellum by way of the restiform body. A third set 

 of arcuate fibres, the posterior external, passes from the posterior 

 funiculi of the same side to the cerebellum by way of the restiform 

 body. 



In addition to the longitudinal fibres contributed by the deep 

 arcuate bundles, the continuation of tracts from the antero-lateral 

 column furnish longitudinal fibres of the formatio reticularis. In the 

 upper part of the medulla, a group of longitudinal fibres constitute 

 a distinct tract, the posterior longitudinal bundle. 



