Page 364 - Clinical Anatomy

ECA6 7/18/06 6:54 PM Page 349

The brain 349

The thalamus (see Figs 246, 248)
The thalamus is an oval mass of grey matter which forms the lateral wall of
the 3rd ventricle; it extends from the interventricular foramen rostrally to the
midbrain caudally. Laterally, it is related to the internal capsule (and through
it to the basal ganglia), and dorsally to the floor of lateral ventricle. Medially,
it is frequently connected with its fellow of the opposite side through the
massa intermedia (interthalamic connexus). Posteriorly, it presents three dis-
tinct eminences, the pulvinar, and the medialand lateral geniculate bodies, these
latter are the thalamic relay nuclei of hearing and vision respectively.
The thalamus is the principal sensory relay nucleus which projects
impulses from the main sensory pathways onto the cerebral cortex. It does
this via a number of thalamic radiations in the internal capsule.
The blood supply of the thalamus is derived principally from the poste-
rior cerebral artery through its thalamostriate branches, which pierce the
posterior perforated substance to supply also the posterior part of the inter-
nal capsule. Thalamic damage by occlusion of this blood supply results in
contralateral sensory loss of face and body.

The cerebral hemispheres

The cerebral hemispheres which, in man, have developed out of all propor-
tion to the rest of the brain, comprise the cerebral cortex, the basal ganglia,
and their afferent and efferent connections. The lateral ventricles, contain-
ing CSF, are at their centre.

The cerebral cortex
The cortex of the cerebral hemispheres is divided on topographical and
functional grounds into four lobes—frontal, parietal, temporal and occipital
(Fig. 247).

Frontal lobe
This includes all the cortex anterior to the central sulcus of Rolando. Its
important cortical areas are as follows:
1◊◊The motor cortex. The primary motor area occupies a large part of the
precentral gyrus. It receives afferents from the premotor cortex, thalamus
and cerebellum and is concerned with voluntary movements. Stimulation
of this area results in discrete muscle movements. Details of localisation of
function in the motor cortex are considered on page 356.
2◊◊The premotor cortex. This lies anterior to the precentral gyrus and the
adjoining lower part of the frontal gyri. It too is concerned with voluntary
movement, but its stimulation results in less focussed movements of
groups of muscles with a common function.
3◊◊Eye motor field. This lies in the middle frontal area anterior to the premo-
tor cortex. Lesions of this area result in impaired eye movement with devia-
tion of gaze to the side of the lesion.