unilateral lesion of the cord, pain and the temperature sense may
be disturbed in one limb, and motor power and tactile sensibility in
the other, as the fibres that convey impressions of pain, and those
that subserve the discrimination of temperature, pass up and decussate
in the cord a few segments above their point of entrance.

[Illustration: FIG. 180.--Diagram of the Course of Motor and Sensory
Nerve Fibres.]

#Effects of Lesions of the Motor and Sensory Mechanisms.#--Lesions of
the _motor mechanism_ differ in their fundamental characters according
as they affect the upper or the lower neurones. The signs also vary
according as the affected area is _destroyed_ or merely _irritated_,
say by the pressure of a tumour. Irritative lesions in general produce
muscular spasms or convulsions, while destructive lesions cause
paralysis. The essential differences in the effects of destructive
lesions of upper and lower neurones may be indicated thus:--

_Upper Neurone Lesion._ _Lower Neurone Lesion._

Spastic paralysis of voluntary Flaccid paralysis of voluntary
muscles. muscles.
No marked wasting of paralysed Marked wasting of paralysed
muscles. muscles.
No reaction of degeneration. Reaction of degeneration.
Exaggeration of reflexes. Loss of reflexes.

Irritative lesions of the sensory mechanism cause numbness and
tingling (paraesthesia); more extensive paralytic lesions produce
anaesthesia, astereognosis, loss of muscle sense, loss of pain, or
inability to distinguish temperature, according to the tracts that are
affected.

_Lesions of the Upper Motor Neurone_ may occur in any part of its
course. _Localised lesions of the motor cortex_ of an irritative kind,
for example, a patch of meningitis, a tumour, meningeal haemorrhage, or
a spicule of bone, produce spasms in those groups of muscles on the
opposite side of the body that are supplied by the centres
implicated--Jacksonian epilepsy. The cortical discharge may overflow
into neighbouring centres and cause more widespread convulsive
movements, or, if strong and long-continued, may even lead to general
convulsions. Consciousness is usually lost before the whole of one
side becomes implicated in the spasms; always before they spread to
the opposite side. Contracture may occur in the muscles affected after
the spasms cease.

If an area of the cortex is destr