INTERNAL STRUCTURE OF THE MEDULLA. 461 
Formatio Reticularis—PBehind the olive and the pyramid is the formatio 
reticularis. In the medulla it occupies a position which, to a large extent, corre- 
sponds with that of the lateral column in the spinal cord. In transverse section it 
appears as an extensive area, which is divided into a lateral and a mesial field by 
the root fascicles of the hy poglossal nerve as they traverse the substance of the 
medulla to reach the surface. In the lateral portion which lies behind the olive 
there is a considerable quantity of gray matter, continuous with that in the cord, 
present in the reticular formation; it is, therefore, called the formatio reticularis 
grisea. In the mesial part which les behind the pyramid the gray matter is 
extremely scanty, and the reticular matter here is termed the formatio reticularis 
alba. 
In the formatio grisea the cells which are scattered thickly amongst the intersecting bundles 
of fibres correspond to the strand-cells of the cord. They possess short axons, which serve to 
bind different levels of the medulla to each other. They therefore constitute association fibres. 
Certain compact masses of gray matter are also seen in the formatio grisea. Of these may be 
mentioned (a) the dorsal accessory olivary nucleus, which has been already described, and (b) the 
nucleus lateralis, The nucleus lateralis is seen in the region between the olive and the substantia 
gelatinosa Rolandi. In the upper part of the medulla it gradually becomes diffuse and dis- 
appears. 
Except in the immediate vicinity of the raphe, the formatio alba may be said to 
be devoid of cells. The mesial accessory olive, however, forms an isolated compact 
mass of gray matter within its limits. 
The nerve fibres which traverse the formatio reticularis run both in a transverse 
and a longitudinal direction. The transverse fibres are the deep arcuate fibres. 
The longitudinal fibres are derived from different sources in the two fields. In the 
formatio grisea they represent to a large extent the fibres of the lateral column of 
the cord, after the removal of the direct cerebellar and the crossed pyramidal tracts. 
They consist, therefore, of the fibres of the tract of Gowers and of fibres correspond- 
ing to the lateral basis-bundle of the cord. In the formatio alba the longitudinal 
fibres are the tract of the fillet and the posterior longitudinal bundle, both of which 
have already been sufficiently described. 
Central Canal and the Gray Matter which surrounds it.—The central canal, 
as it proceeds upwards through the closed part of the medulla, is gradually forced 
to assume a more dorsal position, owing to the accumulation of fibres on its ventral 
aspect. First the decussation of the pyramids, and then the decussation of the 
fillet, both of which take place in front of the canal, tend to push it backwards ; 
and the formation of the longitudinal strands in which these intercrossings result 
(viz. the pyramid and the fillet), together with the continuation upwards of the 
anterior basis-bundle, lead to a great increase in the amount of tissue which 
separates it from the anterior surface of the medulla. In the closed part of the 
medulla it is surrounded by a thick layer of gray matter, which is continuous with 
the basal portions of the anterior and posterior horns of gray matter in the cord. 
This central gray matter is sharply defined on each side by the deep arcuate fibres 
which curve forwards and inwards around it. Finally, the central canal opens on 
the dorsal aspect of the medulla into the cavity of the fourth ventricle. The 
central mass of gray matter which surrounds the canal in the closed part of the 
medulla is now spread out in a thick layer on the floor of the fourth ventricle, and 
in such a manner that the portion which corresponds to the basal part of the 
anterior horn of the cord is situated close to the mesial plane, whilst the part which 
represents the base of the posterior horn occupies a more lateral position. This is 
important, because the nucleus or origin of the motor hypoglossal nerve is placed in 
_ the mesial part of the floor, whilst the nuclei of termination of the afferent fibres of 
the vagus, glosso- -pharyngeal, and auditory nerves lie in the lateral part of the 
floor. The gray matter of the ventricular floor is covered by ependyma. 
Three Areas of Flechsig.—In transverse sections, through the upper open part of the 
~ medulla, the root fibres of the hypoglossal and vagus nerves are seen traversing the substance of 
B the medulla. The nucleus of origin of the hypoglossal is placed in the gray matter of the floor 
of the fourth ventricle close to the mesial plane; the nucleus of termination of the + vagus 18 
situated in the gray matter of the ventricular floor immediately to the outer side of the hypo- 
glossal nucleus. From these nuclei the root-bundles of the two nerves diverge from each other 
