DEVELOPMENT OF THE MEDULLA. 487 
cord we have recognised two thick lateral walls connected in front and behind by narrow 
mid-ventral and mid-dorsal laminz. The same parts are seen in the developing medulla. 
Owing, however, to the expansion of the cavity in this portion of the tube the mid-dorsal 
lamina is stretched out into an extensive and thin epithelial membrane, which forms the 
dorsal wall or roof of the ventricle at this level. The thick lateral walls have also fallen 
away from each other, and are joined in front by the narrow mid-ventral lamina. On 
section, therefore, the medullary part of the 
neural tube presents a triangular figure—the 
base, which is directed backwards, being 
formed by the thin epithelial expansion derived 
from the mid-dorsal lamina, the apex by the 
narrow mid-ventral lamina, and the sides by 
the thick lateral walls of the tube. Further, 
each lateral wall consists of an alar or 
dorsal and a basal or ventral lamina. This 
subdivision is more clearly indicated than in 
the cord, and on the inner surface of the 
lateral wall a strongly-marked longitudinal 
furrow marks the line of junction of the two 
lamine. The histological development of 
these several parts of the wall of the medul- 
lary portion of the neural tube proceeds in a 
manner very similar to that already detailed 
in the case of the cord. No neuroblasts are 
formed in the mid-ventral and mid-dorsal A 4 HYPOGLOSSALNY 
laminee ; the entire neuroblastic formation is GANGUGN 
e F EXPAW 
confined to the basal and alar lamine. a) 
Within the basal lamina, likewise, are col- ————eRHomBic LIP. 
lected the neuroblasts which form the nuclei = =u 
of origin of the efferent nerves ; whilst within =~ Ss 
the alar lamina are developed the neuroblasts 
which constitute the nuclei of termination 
for the fibres of the afferent nerves. 
As development proceeds, the two laminze 
of the lateral wall fall outwards to a still 
greater extent, so that they come to lie very 
nearly in the same horizontal plane. In this 
manner their originally mesial or ventricular 
surfaces come to form the floor of the fourth 
ventricle. Even in the adult the groove, 
which separates the basal and alar laminie 
so clearly from each other in the early con- 
dition, is more or less distinctly perceptible 
on the ventricular floor. It is represented 
by the fovea inferior and by the fovea 
superior. Between these depressions and 
the mesial groove on the floor of the fully- 
developed fourth ventricle there is an elon- 
gated elevation, which, in its lower part, 
forms the trigonum hypoglossi, above this c 
the emenentia teres, whilst higher up it is Bb areter vate yeh 2A Unease sailtcas tt deaf 
continued towards the commencement of the me MEDUMEA OBEONGAMA (from Hist /aliehtly 
Sylvian aqueduct. This clearly-marked and modified). 
bulging mesial strip of the ventricular floor 
corresponds to the basal lamina, whilst the part of the floor which lies to the outer side of 
it and the two fovez is derived from the alar lamina. The latter, therefore, includes 
the trigonum vagi, the area acustica, and the locus cceruleus. 
The further development of the medulla takes place on the ventral aspect of the two 
lamin by the deposition of new parts on those which are already formed. An oval 
bundle of longitudinal fibres makes its appearance on the outer surface of the alar 
lamina, at the point where this joins the basal lamina. This is the early fasciculus 
solitarius. It is composed of afferent fibres from the glosso-pharyngeal and vagus nerves. 
These, on reaching the surface of the medulla, turn downwards upon it. At first the 
<vOOR Or veENTRICre IV. : 
mT TTA 
RHOMBIC LIP 
FASC.SOL. 
HYPOGLOSSAL 
