212 ANIMAL PHYSIOLOGY. 
line and large nuclei of nerve-cells, suggest the probability 
that they play a very important part, and such is found to be 
the case. Indeed, in some sense the rest of the nervous system 
may be said to exist for them. 
Probably nerve-cells do sometimes act as mere conductors 
of nervous impulses originating elsewhere, but such is their 
lowest function. Accordingly, it is found that the nature of 
any reflex action depends most of all on the behavior of the 
central cells. 
It can not be too well borne in mind that nerves are con- 
ductors and such only.. They never originate impulses. 
The properties considered in the last chapter are common to 
all kinds of nerves known; and though we must conceive that 
there are some differences in the form of impulses, these are to 
be traced, not to the nerve primarily, but to the organ in which 
it ends peripherally or to the central cells. 
To return to reflex action, it is found that the muscular re- 
sponse to a peripheral irritation varies with the point stimu- 
lated, the intensity of the stimulus, etc., but is, above all, de- 
termined by the central cells. 
Nerve influence may be considered as following lines of 
least resistance, and there is much evidence to show that an 
impulse having once taken a certain path, it is easier for it to ' 
pass in this direction a second time, so that we have the founda- 
tion of the laws of habit and a host of interesting phenomena 
in this simple principle. 
It is found that, in a frog deprived of its brain and sus- 
pended by the under jaw, there is no movement unless some 
stimulus be applied; but if this be done under suitable condi- 
tions, instructive results follow, which we now proceed to indi- 
cate briefly. The experiments are of a simple character, which 
any student may carry out for himself. 
Experimental_—Preparing a frog by cutting off the whole 
of the upper jaw and brain-case after momentary anesthesia, 
suspend the animal by the lower jaw and wait till it is perfect- 
ly quiet. Add to water in a beaker sulphuric acid till it tastes 
distinctly but not strongly sour, to be used as a stimulus. 1. 
Apply a small piece of. bibulous paper, moistened with the acid, 
to the inner part of the thigh of the animal. The leg will be 
drawn up and the paper probably removed. Remove the paper 
and cleanse the spot. 2. Apply a similar piece of paper to the 
middle of the abdomen; one or both legs will probably be 
drawn up, and wipe off the offending body. 3. Let the foot of 
