to distinguish
between lesions implicating the upper and the lower neurones.

#Sensory Functions and Mechanism.#--Three kinds of sensory impulses
pass from the periphery to the brain; (1) deep, or muscular
sensibility, (2) protopathic sensibility, and (3) epicritic
sensibility.

_Deep sensibility_ includes the recognition of (_a_) deep pressure,
say by the blunt end of a pencil; (_b_) the position of a joint on
passive movement (joint sense); (_c_) active muscular contraction
(kinesthetic sense). The fibres that convey these impulses to the
spinal cord pass in the afferent nerves from the muscles, tendons, and
bones, and so long as these nerves are intact these sensations are
retained, even if the surface of the skin is quite anaesthetic.

_Protopathic sensibility_ is of a lower order than epicritic. It
consists in the recognition of painful cutaneous stimuli and of
extreme degrees of heat and cold. The fibres concerned are
non-medullated and regenerate comparatively quickly after injury, so
that protopathic sensibility is regained before epicritic.

_Epicritic sensibility_ is the most highly specialised and permits of
the recognition of light touch, _e.g._, with a wisp of cotton wool, of
fine differences of temperature, and of discriminating as separate the
points of a pair of compasses 2 cm. apart. These sensations are
carried by medullated nerve fibres, and are slow to return after
injury to the nerves.

The sensory nerve fibres conveying these different impulses pass to
the ganglionic cells of the posterior nerve roots. From each of these
cells a process passes into the cord and bifurcates into an ascending
and a descending branch. In the cord the fibres rearrange themselves
and pass to the brain by a double path. Those that convey sensations
of pain and of temperature pass by the spino-thalamic route by way of
the tract of Gowers and the fillet to the optic thalamus; those that
are concerned with the muscular sense, the joint sense, and tactile
discrimination pass up the posterior columns in the tracts of Goll and
Burdach to the nuclei gracilis and cuneatus in the medulla, whence
they pass to the optic thalamus.

From the cell station in the optic thalamus the fibres proceed to the
_cortical sensory centres_, that for tactile sensation being situated
in the post-central (ascending parietal) gyrus; that for muscular and
stereognostic sense lying probably in the adjacent portions of the
parietal lobe.

In a