crenated
section of the olivary nucleus, the surface bulging of which forms the
olivary body.

[Illustration: From Cunningham, _Text-book of Anatomy._

FIG. 3.--Back View of the Medulla, Pons and Mesencephalon of a
full-time Human Foetus.]

[Illustration: From Cunningham, _Text-book of Anatomy._

FIG. 4.--Transverse Section through the Human Medulla in the Lower
Olivary Region.]

The grey matter of the medulla oblongata, which contains numerous
multipolar nerve cells, is in part continuous with the grey matter of
the spinal cord, and in part consists of independent masses. As the
grey matter of the cord enters the medulla it loses its crescentic
arrangement. The posterior cornua are thrown outwards towards the
surface, lose their pointed form, and dilate into rounded masses named
the grey tubercles of Rolando. The grey matter of the anterior cornua
is cut off from the rest by the decussating pyramids and finally
disappears. The _formatio reticularis_ which is feebly developed in
the cord becomes well developed in the medulla. In the lower part of
the medulla a central canal continuous with that of the cord exists,
but when the clavae on the opposite sides of the medulla diverge from
each other, the central canal loses its posterior boundary, and
dilates into the cavity of the fourth ventricle. The grey matter in
the interior of the medulla appears, therefore, on the floor of the
ventricle and is continuous with the grey matter near the central
canal of the cord. This grey matter forms collections of nerve cells,
which are the centres of origin of several cranial nerves. Crossing
the anterior surface of the medulla oblongata, immediately below the
pons, in the majority of mammals is a transverse arrangement of fibres
forming the _trapezium_, which contains a grey nucleus, named by van
der Kolk the _superior olive_. In the human brain the trapezium is
concealed by the lower transverse fibres of the pons, but when
sections are made through it, as L. Clarke pointed out, the grey
matter of the superior olive can be seen. These fibres of the
_trapezium_ come from the cochlear nucleus of the auditory nerve, and
run up as the lateral fillet.

The _Pons Varolii_ or BRIDGE is cuboidal in form (see fig. 2): its
anterior surface rests upon the dorsum sellae of the sphenoid, and is
marked by a median longitudinal groove; its inferior surface receives