Page 366 - Clinical Anatomy

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The brain 351

tion and their integration with other forms of sensory information. It also
receives afferents from the thalamus and, when damaged, gives rise to
more complex defects than simple loss of sensation—the inability to recog-
nise somatic stimuli called astereognosis; put a key or a coin in the patient’s
hand — he is aware of the object but is unable to recognise what it is. The
lower part of the parietal lobe in the subject’s dominant hemisphere inter-
acts with the somato-sensory visual and auditory associations and has a
key role in language.

The temporal lobe

This is arbitrarily separated from the occipital lobe by a line drawn verti-
cally downwards from the upper end of the lateral sulcus.
The important cortical areas of the temporal lobe are the following.
1◊◊The auditory cortex. This lies in the superior temporal gyrus on the lateral
and superior surfaces of the hemisphere. Its afferent fibres are from the
medial geniculate body and it is concerned with the perception of auditory
stimuli.
2◊◊The temporal association cortex. The area surrounding the auditory cortex
is responsible for the recognition of auditory stimuli and for their integra-
tion with other sensory modalities. Lesions of this area result in auditory
agnosia, i.e. the inability to recognize or to understand the significance of
meaningful sounds. The cortical region just above and behind this area on
the dominant hemisphere (Wernicke’s area) is of considerable importance
in the sensory aspects of language comprehension. This visual area of the
occipital lobe connects with the temporal lobe and is concerned with visual
recognition. The antero-inferior aspect of the frontal lobe connects with the
medial aspect of the temporal lobe and is concerned with behavior.

The parahippocampal gyrus

The cortex of the most medial part of the undersurface of the temporal lobe
is known as the parahippocampal gyrus, much of which is referred to as the
entorhinal cortex. It receives widespread association cortical afferents and is
a significant source of inputs to the hippocampus. Anteriorly, it is related to
the olfactory cortex of the uncus. Medially, it is in direct continuity with the
layer of in-rolled cortex which is the hippocampus and which is one of the
most important sources of afferents to this structure. The hippocampus
occupies the whole length of the floor of the inferior horn of the lateral ven-
tricle and extends to the amygdala. It sends its efferents into the overlying
layer of white matter known as the alveus. The fibres of the alveus collect on
the medial margin of the hippocampus to form a compact bundle, the
fimbria, which, as it arches under the corpus callosum, becomes known as
the fornix. The fornix passes forwards and then downwards in front of the
interventricular foramen and finally backwards into the hypothalamus to
terminate in the mamillary body. It also gives fibres to the thalamus and the
hypothalamus.
Projection of the hippocampus to the hypothalamus is part of the limbic