main mass of the motor fibres crosses the middle line,
and enters the lateral column of the spinal cord as the _crossed
pyramidal tract_. The remaining fibres pass down as the _direct
pyramidal tract_, and decussate in the cord near their termination.

The fibres forming the second path pass through the red nucleus in the
cerebral peduncle (crus cerebri) and thence by way of the rubro-spinal
tract in the lateral column of the cord.

The existence of this double motor path explains how after a
hemiplegic stroke in which the pyramidal tract is destroyed while the
rubro-spinal tract escapes, the patient is able to perform such
primitive movements as are involved in walking or standing, while he
is unable to carry out finer movements that require higher education.

The pyramidal and rubro-spinal tracts, in addition to conveying motor
impulses, convey impulses that influence muscle tonus and the deep
reflexes. The pyramidal tract conveys impulses that inhibit muscle
tonus, while the rubro-spinal tract is the path by which excitatory
impulses travel. When the inhibitory influences are cut off, as in a
lesion of the internal capsule, the paralysed muscles become spastic,
and the deep reflexes are exaggerated. When the excitatory impulses
are also lost, as in a total transverse lesion of the cord, the
paralysed muscles are flaccid and the deep reflexes disappear. In
destructive lesions of the lower neurones, the muscles are always
flaccid.

The axons passing from the cerebral cortex terminate at different
levels in the cord by breaking up into dendrites which arborise around
the cells on the grey matter of the posterior horns--this system of
cells, axons, and dendritic processes forming an _upper neurone_. From
this synapsis the _lower neurone_ proceeds, its axons travelling to
the anterior horn and arborising around the motor cells. The axis
cylinders pass out in the anterior nerve roots to the spinal nerves
and are continued in them to their distribution in voluntary muscles.

If the continuity of any group of these lower neurones is interrupted,
not only do the nerve fibres degenerate, but the nutrition of the
muscles supplied by them is interfered with and they rapidly
degenerate and waste, and after an interval show the reaction of
degeneration. In addition, the reflex arc is disturbed, and reflexes
are lost. As these changes do not occur in lesions of the upper
neurones, an appreciation of the differences enables us