THE SPINAL CORD.—GENERAL. 485 
ening the latent period and extending the range of muscular 
action, which, it has been shown, is not due to changes in the 
nerves themselves. <A very slight stimulus suffices in this in- 
stance to cause the whole body of a decapitated frog to pass 
into a tetanic spasm. We must suppose that the processes 
usually confined to certain groups of central cells have in such 
a case involved others, or that the “resistance” of the centers 
of the cord has been diminished, so that many more cells are 
now involved; hence many more muscles called into action. 
Normally there is resistance to the passage of an impulse to the 
opposite side of the cord, as is shown by the fact that when a 
slight stimulus is applied to the leg of one side the reflex is 
confined to this member. 
It is evident, then, that the reflex resulting is dependent on 
(1) the location of the stimulus, (2) its intensity and duration, 
(3) its character, and (4) the condition of the spinal cord at the 
time. Occasionally on irritating one fore-limb the opposite 
hind one answers reflexly. Such is a “crossed reflex,” and is 
the more readily induced in animals the natural gait of which 
involves the use of one fore-leg and the opposite hind-limb 
together. 
Reflexes are often spoken of as purposive, and suggest at 
first intelligence in the cord; but such phenomena are explained 
readily enough without such a strained assumption. 
Evolution, heredity, and the law of habit, apply here as else- 
where. The relations of an animal to its environment must 
necessarily call into play certain nervo-muscular mechanisms, 
which from the law of habit come to act together when a 
stimulus is applied. Naturally those that make for the welfare 
of the animal are such as are most used under the influence of 
the intelligence of the animal—i. e., of the domination of the 
higher cerebral centers, so that when the latter are removed it 
is but natural that the old mechanisms should be still employed. 
Moreover, the reflex movements are not always beneficial, as 
when a decapitated snake coils itself around a heated iron 
under reflex influence, which is readily enough understood if 
we remember the habit of coiling around objects, and what 
this involves—viz., organized tendencies. 
Inhibition of Reflexes.—It can be shown in the case of a frog 
that still retains its optic lobes and the parts of the brain pos- 
terior to them that, when these are stimulated at the same time 
as the leg, the reflex, if it occurs at all, is greatly delayed. 
On the other hand, in the case of dogs, from which a part 
