axons, which are excited by the
activity of the sense cells, and pass the activity on to the brain.

Accessory sense-apparatus.

Every sense except the "pain sense" has more or less of this. The
hairs of the skin are accessory to the sense of touch. A touch on a
hair is so easily felt that we often think of the hairs as sensitive;
but really it is the skin that is sensitive, or, rather, it is the
sensory axon terminating around the root of the hair in the skin. The
tongue can be thought of as accessory apparatus serving the sense of
taste, and the breathing apparatus as accessory to the sense of smell,
"tasting" being largely a tongue movement that brings the substance to
the taste cells, and "smelling" of anything being largely a series of
little inspiratory movements that carry the odor-laden air to the
olfactory part of the nasal cavity.

{193}

But it is in the eye and the ear that the highest development of
accessory sense apparatus has taken place. All of the eye except the
retina, and all of the ear except the sense cells and the sensory
axons, are accessory.

[Illustration: Fig. 28.--Horizontal cross section through the right
eyeball. (Figure text: cornea, ciliary muscle, retina, choroid.
sclerotic, Optic Nerve)]

The eye is an optical instrument, like the camera. In fact, it is a
camera, the sensitive plate being the retina, which differs indeed
from the ordinary photographic plate in recovering after an exposure
so as to be ready for another. Comparing the eye with the camera, we
see that the eyeball corresponds to the box, the outer tough coat
{194} of the eyeball (the "sclerotic" coat) taking the place of the
wood or metal of which the box is built, and the deeply pigmented
"choroid" coat, that lines the sclerotic, corresponding to the coating
of paint used to blacken the inside of the camera box and prevent
stray light from getting in and blurring the picture. At the front of
the eye, where light is admitted, the sclerotic is transformed into
the transparent "cornea", and the choroid into the contractile "iris",
with the hole in its center that we call "the pupil of the eye".

[Illustration: Fig. 29.--Diagram to show the course of the sound waves
through the outer and middle ear and into the inner ear. The arrow is
placed within the "meatus," and points in the direction taken by the
sound waves. See text for their further course. (Figure text: cochlea,
vestibule, semicircular canal, ossicl