eing an axon growing from a sense cell. This is
the rule in invertebrates, but in vertebrates the sensory axon is
regularly an outgrowth of a {191} nerve cell, and only in the nose do
we find sense cells providing their own sensory nerve.

[Illustration: Fig. 27.--Sense cells and nerve cells of the retina.
Light, reaching the retina from the interior of the eyeball (as shown
in Fig. 28), passes through the nearly transparent retina till stopped
by the pigment layer, and then and there arouses to activity the tips
of the rods and cones. The rods and cones pass the impulse along to
the bipolar cells and these in turn to the optic nerve cells, the
axons of which extend by way of the optic nerve to the thalamus in the
brain. (Figure text: pigment layer, rods, cones, light, bipolar Cells,
optic Nerve Cells)]

In the eye, the sense cells are the rods and cones of the retina.
These are highly sensitive to light, or, it may be, to chemical or
electrical stimuli generated in the pigment of the retina by the
action of light. The rods are less highly developed than the cones.
Both rods and cones connect at their base with neurones that pass the
activity along through the optic nerve to the brain.

The internal ear contains sense cells of three rather similar kinds,
all being "hair cells", Instead of a single {192} sensitive tip, each
cell has a number of fine hair-tips, and it is these that first
respond to the physical stimulus. In the cochlea, the part of the
inner ear concerned with hearing, the hairs are shaken by sound
vibrations that have reached the liquid in which the whole end-organ
is immersed. In the "semicircular canals", a part of the inner ear
that is concerned not with sound but with rotary movements of the
head, we find hair cells again, their hair-tips being matted together
and so located as to be bent, like reeds growing on the bottom of a
brook, by currents of the liquid filling the canals. In the
"vestibule", the central part of the inner ear, the hair-tips of the
sense cells are matted together, and in the mat are imbedded little
particles of stony matter, called the "otoliths". When the head is
inclined in any direction, these heavy particles sag and bend the
hairs, so stimulating them; and the same result occurs when a sudden
motion up or down or in any direction is given to the head. Around the
base of the sense cells, in any of these parts of the internal ear,
are twined the fine endings of sensory