204 ANIMAL PHYSIOLOGY, 
impulse gathers force as it proceeds—the avalanche theory of 
Pfliger; but it would seem more natural to refer this effect to 
the greater irritability of the nerve nearer the centers. 
The chemistry of dead nerves throws extremely little light 
on the nature of nervous processes. The latter seem, in fact, 
to be accompanied by chemical changes which almost entirely 
elude our methods of detection and estimation. Relatively to 
the chemical the electrical phenomena are predominant; but 
nerve-force-is not electrical force, nor are we prepared yet to 
teach that it is the equivalent of that or any other force known 
to us. 
The fact that a nerve maintained in a condition approxi- 
mately normal may be stimulated for hours without exhaus- 
tion, has led some to adopt the tempting conclusion that there 
are no invariable chemical accompaniments of nervous excita- 
tion. But in this and all other instances we think that general 
principles must not be readily set aside by special cases, and 
we should ourselves hesitate to adopt any opinion so contrary 
to all that is known of organic processes as this theory implies, 
except on the amplest and clearest evidence; and we lay the 
more stress on this, because we think it is a sample of the sort 
of reasoning that is apt to become over-potent with those that 
derive their conclusions wholly or chiefly from laboratory ex- 
periments, to the neglect of wider observations, which put the 
more limited, and possibly more accurate, ones derived from 
the former source, in a truer light, and enable us to establish 
juster relations. 
UNSTRIPED MUSCLE. 
This form of muscular tissue is characterized by its long 
latent period, its slow wave of contraction, its not passing into 
tetanus, and the progress of the contraction being in either a 
transverse or longitudinal direction, a wave of contraction in 
one cell being capable of setting up a corresponding wave in 
adjoining cells even when no nerve-fibers are distributed to 
them. It is excited, though less readily, by all the kinds of 
stimuli that act upon striped muscle. In the higher groups of 
animals this tissue is chiefly confined to the viscera of the 
chest and abdomen, constituting in the case of some of them 
the greater part of the whole organ. 
The slow but powerful and rhythmical contraction of this 
form of muscle adapts it well to the part such organs play in 
