SOUTHERN CALIFORNIA ACADEMY OF SCIENCES 77 
Another substance characteristic of neurone katabolism is a 
yellowish pigment which apparently is normally retained with- 
in the neurone. According to Obersteiner and others this pig- 
ment is first found in the neurones of the spinal ganglia at about 
the sixth year. About two years later the cells of the cord 
contain this pigment. Throughout life it increases in quantity 
and in distribution. Rosin has made an especial study of this 
pigment. The bodies of Nissl are absent from the part of the 
cell occupied by the pigment. It is not identical with the pig- 
ments found in the locus ciruleus, the substantia nigra, ete. 
It stains with osmie acid but fails to give other reactions char ac- 
teristic of fats. In old age many nerve cells are practically 
‘filled with this substanee. In such cells the nucleus is found 
to be distorted and displaced or even dissolved or extruded. 
Eventually the cell membrane and protoplasm also disappear, 
leaving only the metaplastic granules of pigment in the midst 
ct some detritus. 
It must not be forgotten that the normal neurone is always 
in a state of activity. There is no such thing as absolute rest. 
Neither in the neurone, nor in other cells, nor in any living 
being, is rest or quiet found. There are only alternate periods 
during which now anabolism and now katabolism is most 
predominant. What we eall rest is. simply a period of cell- 
parsimony ; what we eall activity, a period of cell extravagance. 
The great increment of katabolic processes which results from 
changes in the environment of the neurone is a nerve impulse. 
That is, a nerve impulse is simply the kinetic energy produced 
by the liberation of the potential energy stored in the cell in 
the form of the extreme complexity and instability of its mole- 
ecules, especially of the molecules of the Nissl bodies. Any 
sufficient change in the environment of the neurone initiates the 
dissociation of these molecules with a corresponding evolution 
of energy. 
Goldscheider has used the term ‘‘neurone threshold’’ to ex- 
press the degree of excitation which is just sufficient to initiate 
a nerve impulse. Neurones vary in their threshold value with 
their location, strueture and function. The threshold of the 
same neurone varies with its nutrition, development and con- 
ditions of stimulation. Attention is a semi-volitional or wun- 
conscious lowering of the threshold of certain neurone systems. 
By listening attentively, for example, sounds are apparent 
which would otherwise be unheard. The threshold value of 
association areas may be lowered to impulses which are es- 
pecially weleome, hence the significance of the saying: ‘‘'The 
wish is father to the thought.’’ Every discharge of nervous 
impulse renders the cell more susceptible to subsequent excita- 
