ay already suggested.

[Illustration: Fig. 31.--A small sample of the sense cells of the
cochlea. The hairs of the sense cells are shaken by the vibration of
the water, and pass the impulse back to the end-brushes of the
auditory axons, The tectorial membrane looks as if it might act as a
damper, but may be concerned, as "accessory apparatus," in the
stimulation of the hair cells. The basilar membrane consists in part
of fibers extending across between the ledges of bone; these fibers
are arranged somewhat after the manner of piano strings, and have
suggested the "piano theory" of hearing, to be mentioned later in the
chapter. (Figure text: water space, membrane, Tectorial membrane, bone,
soft tissue, basilar membrane, auditory axons to brain stem, nerve
cells of auditory nerves, auditory hair cells with end brushes of
auditory axons)]

Further study of the accessory apparatus of the eye and ear can be
recommended as very interesting, but the little that has been said
will serve as an introduction to the study of sensation.

{197}

Analysis of Sensations

Prominent among the psychological problems regarding sensation is that
of analysis. Probably each sense gives comparatively few elementary
sensations, and many blends or compounds of these elements. To
identify the elements is by no means a simple task, for under ordinary
circumstances what we get is a compound, and it is only by carefully
controlling the stimulus that we are able to get the elements before
us; and even then the question whether these are really elementary
sensations can scarcely be settled by direct observation.

Along with the search for elementary sensations goes identification of
the stimuli that arouse them, and also a study of the sensations
aroused by any combination of stimuli. Our task now will be to ask
these questions regarding each of the senses.

The Skin Senses

Rough and smooth, hard and soft, moist and dry, hot and cold, itching,
tickling, pricking, stinging, aching are skin sensations; but some of
these are almost certainly compounds. The most successful way of
isolating the elements out of these compounds is to explore the skin,
point by point, with weak stimuli of different kinds. If a blunt metal
point, or the point of a lead pencil, a few degrees cooler than the
skin, is passed slowly over the skin, at most points no sensation
except that of contact arises, but at certain points there is a clear
sensation of cold