Page 380 - Clinical Anatomy

ECA6 7/18/06 6:54 PM Page 365

The cranial nerves 365

olfactory mucosa in the upper part of the superior nasal concha and
septum, through the cribriform plate of the ethmoid bone to end by synaps-
ing with the dendrites of mitral cells in the olfactory bulb. The mitral cells in
turn send their axons back in the olfactory tract to terminate in the cortex of
the uncus, the adjacent inferomedial temporal cortex and the region of the
anterior perforated space. The further course of the olfactory pathway is
uncertain in man, but it is now clear that the hippocampus–fornix system is
not directly concerned with olfaction.

Clinical features

1◊◊The sense of smell is not highly developed in man and is easily dis-
turbed by conditions affecting the nasal mucosa generally (e.g. the common
cold). However, unilateral anosmia may be an important sign in the diag-
nosis of frontal lobe tumours. Tumours in the region of the uncus may give
rise to the so-called ‘uncinate’ type of fit, characterized by olfactory halluci-
nations associated with impairment of consciousness and involuntary
chewing movements.
2◊◊Bilateral anosmia due to interruption of the 1st nerve is common after
head injuries, particularly in association with anterior cranial fossa frac-
tures, when leakage of C.S.F. through the cribriform plate may present as
rhinorrhoea.

The optic nerve (II) and the visual pathway
(For a description of the eye itself see the section on special senses, page
388.)
The optic nerve is the nerve of vision. It is not a true cranial nerve but
should be thought of as a brain tract which has become drawn out from the
cerebrum. Embryologically, it is developed, together with the retina, as a
lateral diverticulum of the forebrain. Devoid of neurilemmal sheaths, its
fibres, like other brain tissues, are incapable of regeneration after division.
From a functional point of view the retina can be regarded as consisting
of three cellular layers: a layer of receptor cells — the rods and cones — an
intermediate layer of bipolar cells, and a layer of ganglion cells, whose axons
form the optic nerve (Fig. 255). From all parts of the retina these axons con-
verge on the optic disc whence they pierce the sclera to form the optic nerve.
The optic nerve passes backwards and medially to the optic foramen
through which it reaches the optic groove on the dorsum of the body of the
sphenoid. Here, all the fibres from the medial half of the retina (i.e. those
concerned with the temporal visual field) cross over in the optic chiasma to
the optic tract of the opposite side, while the fibres from the lateral half of
the retina (nasal visual field) pass back in the optic tract of the same side.
The great majority of the fibres in the optic tract end in the six-layered lateral
geniculate body of the thalamus, but a small proportion, subserving pupil-
lary, ocular and head and neck reflexes, bypass the geniculate body to reach
the superior colliculus and pretectal area. From the lateral geniculate body