452 THE NERVOUS SYSTEM. q 
verse bundles of fibres termed the strie acustice. They appear to emerge from 
the mesial groove and they are carried outwards over the upper part of the resti- 
form body. They somewhat resemble the superficial arcuate fibres on the lateral 
‘surface of the medulla, and their connexions will be discussed at a later stage. 
The strize acustice divide each lateral half of the ventricular floor into upper and 
lower portions, which very nearly correspond to the subdivisions of this area formed 
by the medulla and the pons. Except for this break on the surface, the medullary 
and pontine portions of the floor of the fourth ventricle are quite continuous with 
each other. 
On the lower medullary district of the ventricular floor a small triangular 
depression placed immediately below the strize acusticwe catches the eye. This is 
termed the fovea inferior. It is shaped somewhat lke an arrow-head. The apex 
or point looks towards the striz, whilst the lateral angles of the base are prolonged 
downwards in the form of diverging grooves (Fig. 333, p. 447). Of these, the inner 
groove runs towards the opening of the central canal at the calamus scriptorius, 
whilst the outer groove runs towards the lateral boundary of the floor. In this 
manner the portion of the floor which hes below the strize acustice is mapped out 
into three triangular areas. The mesial subdivision is slightly elevated and is 
termed the trigonum hypoglossi, because, subjacent to this area, is the nucleus of 
origin of the hypoglossal or twelfth cranial nerve. The intermediate area between 
the two diverging grooves which proceed from the base of the fovea inferior is the 
trigonum vagi (ala cinerea), so called because the terminal nucleus of the vagus or 
tenth and the glosso-pharyngeal or ninth cranial nerves lies subjacent to it. The 
external area is the trigonum acustici. The base of this area is directed upwards 
and runs continuously into an eminence—the acustic area (area acustica)—over 
which the striz acustice pass. Subjacent to this district of the floor of the 
ventricle lies the large terminal chief nucleus of the vestibular division of the 
auditory or eighth cranial nerve. 
On the part of the floor of the ventricle which les above the striz acustice, and 
which corresponds to the dorsal surface of the pons, there is also a slight depression 
termed the fovea superior. Between it and the median groove is a marked pro- 
minence called the eminentia teres. Inferiorly this elevation passes downwards 
and becomes continuous with the trigonum hypoglossi, whilst above it is carried 
upwards towards the opening of the aqueduct of Sylvius. In both directions it 
becomes gradually less prominent, but still it forms a distinct elongated elevation, 
which stretches along the whole length of the median groove. As already stated, 
the area acustica extends upwards into the pontine part of the ventricular floor and 
forms an elevated region in the outermost part of its widest portion, below and to 
the outer side of the fovea superior. Proceeding upwards from the fovea superior 
to the opening of the Sylvian aqueduct there is a shallow depression termed the 
locus ceruleus, seeing that it usually presents a faint slate gray colour. When the 
ependyma is scraped away from the surface of this part of the floor, the colour is 
seen to be due to the substantia ferruginea,—a name applied to a linear group of 
strongly pigmented cells, which lies in the lateral part of the gray matter covering 
this portion of the ventricular floor. When transverse sections are made through 
the upper part of the pons, the substantia ferruginea appears on the cut surface 
as a small black spot. 
INTERNAL STRUCTURE OF THE MEDULLA. 
The internal structure of the medulla differs in a marked degree from that of 
the spinal cord; indeed, in its upper part it presents very little in common with the 
latter. The various strands of the cord either come to an end within the medulla 
or undergo changes in their relative position, whilst the gray matter is much modi- 
fied and new masses are added. Like the cord, however, the medulla consists of 
two nearly symmetrical right and left halves. When transverse sections are made 
through it at different levels each lateral half is seen to be partly marked off from 
the other in the lower closed part of the medulla by the anterior and posterior 
inedian fissures, whilst in the upper open part of the medulla the subdivision is 
ill 
