1.)

[27] I do not need here to go into the question whether, as Johannes
Mueller assumed, this is an original attribute of nerve-structure, or
whether, as Wundt suggests, it is due simply to the fact that certain
kinds of nervous fibre have, in the course of evolution, been slowly
adapted to one kind of stimulus.

[28] I here refer to what is commonly supposed to be the vague innate
difference of sensation according to the local origin, before this is
rendered precise, and added to by experience and association.

[29] The illusory character of this simple mode of perception is seen
best, perhaps, in the curious habit into which we fall of referring a
sensation of contact or discomfort to the edge of the teeth, the hair,
and the other insentient structures, and even to anything customarily
attached to the sentient surface, as dress, a pen, graving tool, etc. On
these curious illusions, see Lotze, _Mikrokosmus_, third edit., vol. ii.
p. 202, etc.; Taine, _De l'Intelligence_, tom. ii. p. 83, _et seq._

[30] Quoted by G.H. Lewes, _Problems of Life and Mind_, third series, p.
335. These illusions are supposed to involve an excitation of the
nerve-fibres (whether sensory or motor) which run to the muscles and
yield the so-called muscular sensations.

[31] It is brought out by Griesinger (_loc. cit._) and the other writers
on the pathology of illusion already quoted, that in the case of
subjective sensations of touch, taste, and smell, no sharp line can be
drawn between illusion and hallucination.

[32] For a fuller account of these pathological disturbances of
sensibility, see Griesinger; also Dr. A. Mayer, _Die Sinnestaeuschungen_.

[33] Helmholtz, _op. cit._, p. 600, _et seq._ These facts seem to point
to the conclusion that at least some of the feelings by which we know
that we are expending muscular energy are connected with the initial
stage of the outgoing nervous process in the motor centres. In other
pathological conditions the sense of weight by the muscles of the arms
is similarly confused.

[34] Wundt (_Physiologische Psychologie_, p. 653) would exclude from
illusions all those errors of sense-perception which have their
foundation in the normal structure and function of the organs of sense.
Thus, he would exclude the effects of colour-contrast, _e.g._ the
apparent modification of two colours in, juxtaposition towards their
common boundary, which probably arises (according to E. Hering) from
some mutual i