oyed by the lesion, paralysis is
produced of the corresponding muscles on the opposite side of the
body. At first the paralysed muscles are flaccid, but spasticity soon
develops. In some cortical lesions, for reasons not yet understood,
the paralysis remains of the flaccid type. The seat and extent of the
paralysis depend upon the area of the cortex destroyed. In rare cases
the whole motor area is destroyed--_cortical hemiplegia_; more
generally the lesion affects one or more groups of muscles, and
occasionally all the muscles of one limb are paralysed--_cortical
monoplegia_. Lesions are often both irritative and destructive, and
lead to paralysis of one or more groups of muscles associated with
spasms and convulsions of the muscles governed by neighbouring areas
of the cortex. Irritation or destruction of the sensory centres may
also exist, giving rise to areas of paraesthesia and anaesthesia.

Lesions in the _centrum ovale_, which destroy the fibres proceeding
from the overlying cortex, produce a corresponding spastic paralysis
on the opposite side of the body. No irritative phenomena are
associated with such a sub-cortical lesion.

Lesions in the region of the _internal capsule_ often produce complete
spastic hemiplegia of the opposite side of the body. When the
posterior part of the capsule is involved, there are, in addition,
hemianaesthesia and hemianopia, and sometimes disturbances of hearing,
smell, and taste.

A lesion of the _crus_ may in like manner produce spastic hemiplegia
and hemianaesthesia of the opposite side, often associated with a lower
neurone paralysis of the third and fourth nerves of the same side
(crossed paralysis). The optic tract, which crosses the crus, may also
be affected, and hemianopia result.

Lesions of the _corpora quadrigemina_ cause interference with the
reaction of the pupil, disturbance of the functions of the oculo-motor
nerve and of mastication, ataxia, and inco-ordination of the movements
of the limbs.

The symptoms produced by lesions of the _pons and medulla_ vary
according to the position of the lesion. If it is unilateral, there
may be spastic hemiplegia and hemianaesthesia of the opposite side; if
it is situated in the lower part of the pons or in the medulla, there
is often also a lower neurone paralysis of one or more of the cranial
nerves on the same side as the lesion (crossed paralysis). Paralysis
of the external rectus of one eye and of the internal rectus of