CH. XLIV.] 



INTERIOR OF THE BULB. 



633 



body end hi the nucleus gracilis, and those from the upper part 

 of the body in the nucleus cuneatus. These nuclei form a most 

 important position of relay in the course of the afferent fibres 

 from cord to brain. The new fibres arising from the cells of 

 these nuclei pass in a number of different directions, and break 

 up the rest of the grey matter into what is called the formatio 

 reticularis. The fibres may be divided into three main groups ; 

 they are termed arcuate fibres. 



n.c 



.==2? 



a.m.f. /a. W 



Fig. 467. Transverse section of the medulla oblongata in the region of the superior 

 decuiwation. a.mj., anterior median fissure ; /.., superficial arcuate fibres emerging 

 from the rl.-wure ; py., pyramid ; n.a.r., nuclei of arcuate fibres ; /.a 1 , deep arcuate 

 fibres becoming superficial ; o, ', lower end of olivary nucleus ; n. I, nucleus lateralis ; 

 f.r., formatio reticularis; /.a 1 , arcuate fibres proceeding from the formatio reticu- 

 laris; g, substantia gelatinoea of Rolando; a. P., ascending root of fifth nerve; 

 /.., funioulus cun^atus ; n.e., nucleus cuneatus ; n.c.', external cuneate nucleus ; 

 n.y.. nucleus gracilis; /..</.. funiculus gracilis; p.m.f., posterior median fissure; 

 c.c., central canal surrounded by grey matter, in which are n.Xf., nucleus of the spinal 

 accessory, and . A"//., nucleus of the hypoglossal ; t.d., superior decussation (decussa- 

 tion of fillet} . (Modified from Schwalbe. ) 



1. The external arcuate fibres (fa, /a 1 ) course round the ven- 

 tral surface of the bulb. 



2. Some of these turn round sharply (/a 8 ) to the restiform 

 body of the same side. 



3. The internal arcuate fibres are those which pass into the 

 formatio reticularis and cross with their fellows at the median 

 raph, forming what is sometimes called the superior pyramidal 

 deputation (s.d.), but which should be more properly called the 

 decussation of the fillet. The fillet fibres, after having crossed to 



