500 THE NERVOUS SYSTEM. 
eyeball, viz. the sphincter iridis and the musculus ciliaris. The nucleus of origin is 
placed in the ventral part of the Sylvian gray matter subjacent to the superior 
x a quadrigeminal body (Fig. 366, 
pe p. 495). In length it measures 
= > from 5 to 6 mm. — Its lower 
____—_Bylvian gray end is partially continuous 
j aero with the nucleus of the troch- 
(~~) aqueduct lear nerve, whilst its upper end 
4 Mesencephalic 
| root of fifth  @Xtends upwards for a short 
eee Re distance beyond the mesen- 
z Fourth nerve f ; 
J — leaving cephalon into the gray matter 
tuclous on the lateral wall of the third 
yoo a ventricle. Its relation to the 
ole posterior longitudinal bundle is 
--Decussation of VEN More intimate than that 
_—the superior of the trochlear nucleus. _ It is 
peduncles ° Glosely applied to the dorsal and 
inner aspect of this strand ; 
many of its cells occupy a posi- 
tion in the intervals between 
the nerve bundles of the tract, 
and some even are seen on its 
ventral or tegmental aspect. 
Fic. 372.—SEcrioN THROUGH THE INFERIOR QUADRIGEMINAL Bopy The axons of the nuclear cells 
AND THE TEGMENTUM OF THE MESENCEPHALON AT THE leave the nucleus in numerous 
LEVEL OF THE LOWER PART OF THE NUCLEUS OF THE TROCH- : oa : 
bundles, which describe a series 
LEAR NERVE (ORANG). 
of curves as they proceed for- 
wards through the posterior longitudinal bundle, the tegmentum, red nucleus, 
and inner margin of the substantia nigra, to finally emerge from the brain-stem 
along the bottom of the sulcus oculo-motorius on the inner aspect of the crus cerebri. 
The cells of the oculo-motor nucleus are not uniformly distributed throughout it. They are 
grouped into several more or less distinct collections or clumps, some of which possess cells which 
differ in size and appearance from the others. These cell-clusters are very generally believed to 
possess a definite relation to the several branches of the nerve and the muscles which they 
supply. Perlia recognises no less than seven such cell-clusters in each nucleus, with a small 
median nucleus placed accurately on the middle line, and from which fibres for both nerves 
spring. Whilst the majority of the fibres in the oculo-motor nerve arise from the cell-groups 
which le on its own side of the mesial plane, it has been satisfactorily established that a certain 
proportion of its fibres are derived from the nucleus of the opposite side, thus forming a crossed 
connexion and giving rise to a median decussation. These crossed fibres are supposed by some 
to supply the internal rectus muscle ; and if this be the case, the harmonious action of the external 
and internal recti muscles in producing the conjugate movements of the eyeballs could be under- 
stood without accepting in full the views of Duval and Laborde (p. 497). 
The oculo-motor nucleus is connected—(1) with the occipital part of the cerebral cortex by 
fibres which reach it through the optic radiations; (2) with the trochlear and abducent nuclei 
(and probably with other nuclei) by fibres which come to it through the posterior longitudinal 
bundle ; (3) possibly with the facial nerve by fibres which pass out from it into the posterior 
longitudinal bundle (p. 496) ; (4) with the visual system by fibres which enter it from the cells of 
the superior quadrigeminal body. 
DEVELOPMENT OF THE MESENCEPHALON. 
Even in the early embryo the mesencephalon constitutes the smallest section of the 
brain-tube, although the disproportion in size between it and the other primitive sub- 
divisions of the brain is not nearly so marked as in the adult. Owing to the cephalic 
flexure, the mid-brain for a time occupies the highest part of the summit of the head. 
Later on it becomes completely covered over by the expanding cerebral hemispheres. 
The corpora quadrigemina are derived from the alar laminee of the lateral walls of the 
brain-tube, whilst the basal laminz thicken and ultimately form the tegmenta and crustze 
of the two crura cerebri. The original cavity of the mid-brain is retained as the aqueduct 
of Sylvius. 
For a considerable time the cavity of the mesencephalon remains relatively large, and 
the lower part of its dorsal wall is carried downwards in the form of a diverticulum or 
recess, which overlaps the cerebellar plate. About this time, also, the dorsal wall shows a 
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