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PHYSIOLOGY OF THE NERVOUS SYSTEM. 
altogether, notwithstanding the exciting influ- 
ence of the impression. The true explanation 
seems to be, that the mind can exert upon the 
vesicular matter a power which can prevent 
the exercise of that change, or neutralise the 
change, without which the motor fibres will 
not be affected by a physical stimulus. 
Reflex actions are more manifest in some 
situations than others: thus, in cases of hemi- 
plegia from diseased brain, they are generally 
very obvious in the lower extremity, but to- 
tally absent in the upper. This, the advocates 
of the excito-motory theory ascribe to a paucity 
of excito-motory fibres in the latter limb, and 
to a larger amount of them in the former. Or, 
it has been attributed to the greater and 
more enduring influence of shock upon that 
segment of the cord from which the nerves of 
the upper extremities arise, as nearer the seat 
of lesion, than upon the lumbar segment. But 
another explanation of this important fact may 
be offered, which is equally satisfactory, and 
more accordant with other phenomena. A 
certain disposition of the nerves upon the tegu- 
mentary surface is as necessary for the develope- 
ment of reflex actions as of sensations ; and 
these movements will be more or less easily 
manifested, according as this organization of 
the nerves on the surface is more or less perfect. 
That disposition of the cutaneous nerves 
which renders the surface easily excitable by 
titillation seems most favourable to the deve- 
lopement of these actions. Hence, there is no 
lace where they are more readily excited than 
in the lower extremities by stimulating the soles 
of the feet or the intervals between the toes, 
.both of which situations are highly susceptible 
of titillation. At the isthmus faucium the 
slightest touch on the surface excites a move- 
ment of deglutition; and this touch, at the same 
time, produces a very peculiar sensation of 
tickling, quite distinct from that which may 
be excited at other parts of the pharynx, or 
mouth. When this part of the mucous mem- 
brane is in a state of irritation as an effect of 
coryza, this tickling sensation is present, and 
repeated acts of swallowing are provoked. 
Two facts may be stated here, which illus- 
trate the position above laid down respecting 
the necessity of a certain disposition of the 
nerves on the tegumental surface, for the de- 
velopement of reflex actions. The first is one 
which has been noticed by Volkmann, and 
which I have repeatedly observed, namely, that 
in frogs, and other animals, reflex actions are 
readily excited by stimulating the feet; but 
irritating the posterior roots of the spinal nerves, 
which supply those parts, is not sufficient for 
this purpose. I have already remarked that in 
humerous experiments upon the posterior roots 
of the nerves movements have not been ex- 
cited whilst they have been subjected to irrita- 
tion, except when galvanism was employed, 
which, being diffused, affected the cord itself: 
the recorded statements of most modern ex- 
perimenters agree in the main with this state- 
ment. The second fact is this: in the male 
frog the developement of a papillary structure 
on the skin of the thumb seems to have refe- 
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rence to the excitation of the physical power of 
the cord, to enable the animal to grasp the 
female without the necessity of a prolonged 
exercise of volition. Stimulating the fingers 
will scarcely produce reflex actions, but the 
slightest touch to the enlarged thumb will 
cause the animal to assume the attitude of 
grasping. If the papillae be shaved off the 
thumb, its power of exciting these actions 
is instantly lost. 
When the polarity of the cord is greatly ex- 
cited by strychnine or other substances, or 
when tetanus exists, all parts of the surface 
are equally capable of exciting reflex actions. 
The least touch will cause them, not only in 
the limb touched, but in all that side of the 
trunk, or even throughout the whole body. 
So general is the excitation, that the least im- 
pression made on the peripheral extremity of 
a sensitive nerve in any part of the body is 
instantly converted into muscular spasm, more 
or less general. A slight current of air, in 
tetanus, is sufficient to excite general spasm. 
Miller remarks that, in such states of the 
cord, the reflex actions excited by stimulating 
the nerves themselves are much less than those 
produced by excitation of the surface. 
The readiness with which a physical change, 
induced in one part of the centre, is propagated 
to others, whether above or below it, is due no 
doubt to the vesicular matter. An experiment 
made by Van Deen illustrates this statement. 
If, in an animal poisoned by strychnine, the 
cord be divided in its entire length along the 
median line, leaving only a slight bridge of 
grey matter, stimuli applied to any part of the 
surface will exhibit as extensive reactions as if 
the cord were entire. It is evident that the only 
medium of communication between the oppo- 
site halves must be the small portion of vesi- 
cular matter left undivided. 
Impressions conveyed to the cord by the pos- 
terior roots of any of its nerves, may be reflected 
to the corresponding motor nerves, and cause 
movement, or may extend irregularly along the 
posterior horns of grey matter and stimulate 
the nerves implanted in them, and thus give 
rise to new sensations, which may be referred 
to other and even distant parts of the body or 
to new motions. 
The hypothesis under consideration affords 
us an explanation, more satisfactory than any 
other, of the paralytic state of the sphincter 
ani in brain disease, already referred to, as well 
as in that of the spinal cord. This muscle is 
certainly chiefly under the influence of the will. 
In ordinary cases of diseased brain, where the 
lesion is confined to one side, the centre of 
volition is not sufficiently impaired to affect its 
influence upon the sphincter. In graver lesions, 
however, although the will may still continue 
to exert its control upon one side of the body, 
it loses its power over the sphincter, which is 
not excitable by any stimulus. In disease of 
the spinal cord, there is paralysis of the sphinc- 
ters if the lesion involve asufficient portion of the 
cord’s substance, in whatever region of the cord it. 
may exist. Even when the lesion is situate high 
up in the neck, or in the dorsal region, leaving 
