STRUCTURE OF THE MEDULLA OBLONGATA. 



51 



minent (eminentia teres} opposite the fovea but becomes gradually less so above and 

 below. Extending from the superior fovea to the upper end of the ventricle, where 

 this narrows to the Sylvian aqueduct, is a shallow depression (locus cmruleus) distin- 

 guished in the adult by its dark grey or slaty tint, which is due to a subjacent tract of 

 pigmented nerve-cells (substantia ferruginea). The trophic fibres in the fifth nerve 

 have been supposed to be derived from these cells. 



INTERNAL STRUCTURE OF THE MEDULLA OBLONGATA. 



The internal structure of the medulla, like the external form, will be best under- 

 stood by tracing its several parts upwards from the spinal cord ; and this can be 



Fig. 42. SECTION OF THE MEDULLA OBLONGATA AT THE MIDDLE OF 

 THE DECUSSATION OF THE PYRAMIDS (Lockhart Clarke). 



/, anterior ; f.p., posterior fissure ; a p., pyramid ; a, remains 

 of part of anterior cornu, separated by the crossing bundles from 

 the rest of the grey matter ; I, continuation of lateral column of 

 cord ; R, continuation of substantia gelatinosa of Rolando ; pc. , 

 continuation of posterior cornu of grey matter ; f.g., funiculus 

 gracilis. 



most readily done by a comparison of the appear- 

 ances of successive transverse sections. 



Lower or closed part of the medulla oblcm- 

 gata. The first changes are produced, in its internal 

 structure as in its external form, by the passage of the 

 fibre-bundles of the lateral pyramidal tract obliquely 



through the grey matter of the anterior horn, and across the anterior median fissure 

 toihe pyramid of the opposite side (fig. 42). By this abrupt passage of a large 

 number of white fibres through it, the anterior horn is broken up, and one part, the 

 caput cornu (a), is entirely separated from the rest of the grey matter; whilst only 



Fig. 43. SECTION OF THE MEDULLA OBLONGATA 



IN THE REGION OF THE SUPERIOR PYRAMIDAL 



DECUSSATION. (Schwalbe. ) \ 



a.m./, anterior median fissure ; f.a., superficial 

 arciform fibres emerging from the fissure ; py, pyra- 

 mid ; n.ar., nucleus of the arciform fibres ; f.a'., 

 deep arciform fibres becoming superficial ; o., lower 

 end of olivary nucleus ; o', accessory olivary nucleus ; 

 n.l., nucleus lateralis ; f.r.', formatio reticularis ; 

 f.a. 2 , arciform fibres proceeding from formatio reti- 

 cularis ; y, substantia gelatinosa of Rolando ; a. V., 

 ascending root of fifth nerve ; n.c. , nucleus cuneafcus : 

 rt.c'., external cuneate nucleus ;/.c., funiculus cune- 

 atus ; n.g., nucleus gracilis ; /.</., funiculus gracilis ; 

 p.m./., posterior median fissure; c.c., central canal 

 surrounded by grey matter, in which are, n.XI. 

 nucleus of the spinal accessory, and, n.XIL, nucleus 

 of the hypoglossal ; s. d. , decussation of fillet or su- 

 perior pyramidal decussation. 



the base of the horn remains, as a small 

 portion of grey matter close to the antero- 

 lateral aspect of the central canal. 



The separated portion of the anterior 



horn becomes pushed over to the side by the development of the pyramid and the 

 interpolation higher up of the olivary body between them, so that it comes to lie 

 close to the separated caput cornu posterioris (see below). The greater part of 

 the grey substance is broken up into a formatio reticularis (fig. 43,/.r.), i.e. a compara- 



E 2 



