PHYSIOLOGY. 
acted on only by tbe rays of light, which 
are, not perceived by any other sense : the 
auditory nerve, which alone perceives 
sounds, is rather firmer, &c. 
That the mind is very immediately and es- 
sentially connected with the brain, and that 
the animal functions of sensation and vo- 
luntary motion, are no less intimately de- 
pendent on the same organ, may be proved 
by such an abundance of physiological and 
pathological phenomena, that no doubt can 
be entertained of the fact. An injury of 
this organ suspends or annihilates the whole, 
or a part, of the mental operations, and 
puts an end to all feeling and motion : die 
organs of the body remain entire, the nerves 
connecting these with the sensorium are un- 
injured ; but the perceptive faculty is lost. 
Again, an injury of one side of the brain of- 
ten causes a loss of feeling and motion in 
one side only of the body ; which, in coij- 
sequence of principles inexplicable by us, 
always affect the opposite half to the in- 
jury. That the sensible impressions made 
on our organs are conveyed by the nerves 
to the brain ; and that the latter part is the 
seat of the sensation, although it is referred 
by the mind to the part itself ; is proved by 
cutting or tying a nerve : in which' case, 
the usual impression causes no perception. 
The truth of this assertion, which will 
hardly meet with credit among the unin- 
formed, is illustrated by what happens to 
persons whose limbs have been amputated ; 
they are constantly complaining of pains in 
the toes or fingers of the limbs they have 
lost. Here the middle of the nerve is irri- 
tated, but the pain is referred by the mind 
to its extremities. 
Yet, although the influence of the brain 
be thus essential, in the business of sensa- 
tion and voluntary motion ■ and an unim- 
paired state of the nerves passing between 
the organ and the sensorium, be conse- 
quently an indispensible condition in those 
functions, other departments of the animal 
economy are not so immediately subject to 
the power of the brain. The processes of 
digestion, absorption, circulation, secretion, 
and nutrition, those,,in short, which consti- 
tute the internal life, still go on when inju- 
ries of the brain have suspended the ani- 
mal functions : nay, they may survive for 
months or even years. The ligature of the 
nerves of a part does not destroy its circu- 
lation or nutrition ; although these pro- 
cesses may perhaps be impaired. How, 
then, will it be said. Does an injury of the 
brain so often prove fatal ? The individual 
ought still to live internally, aitliougli liii 
external life has been annihilated. But 
here we notice a function that partakes of 
both : namely respiration. The dilatation 
of the chest can only be performed by 
means of muscles, whose principle of action 
comes from the brain : as the injury of the 
latter organ has paralysed these, the blood 
can no longer receive those changes which 
it undergoes in respiration, and thereby be- 
comes unfit to stimulate tlie heart to ac- 
tion, or to keep up the powers of life in any 
of the organs of the body. 
That the nerves are, as we have describ- 
ed, the medium of connection between the 
mind and its organs, is clear ; but how th-eir 
offices are performed is a much more ob-? 
scure question. It has occupied the atten- 
tion, and engaged the experiments of phy- 
siologists, in all ages ; but nature has not 
hitherto lifted the veil, and the subject re- 
mains nearly in its original obscurity. An 
oscillatory or vibratory motion of the 
nerves ; or a nervous fluid contained in or 
adhering to these organs, have been assumed 
in explanation of the facts. According to 
some, the latter is a liquid contained in 
tubes; while others liken it to caloric, 
light, oxygen, the electric or magnetic 
fluids. The partisans of the latter opinions 
consider, that the recent discoveries of gal- 
vanism add much weight to their argu- 
ments. See G.vlv.vnism. 
A supposed central point, to which all 
sensations are carried, and from which all 
motions emanate, is called the sensorium 
commune ; and is considered as ftie seat of 
the soul. Des Cartes placed this in the 
pineal gland, others in the corpus callosum, 
pons Varolii, corpora striata. Sec. The 
learned Soemmerring has lately endeavour- 
ed to show, that the seat of the spul must 
be in the water of the ventricles, as he has 
succeeded in tracing the origins of all the 
nerves from the sides of these cavities. The 
records of morbid anatomy refute many of 
these opinions, as they show the parts con- 
sidered as sensoria to have been diseased 
and destroyed without any impairment of 
the mental faculties. 
The curious and complicated structure 
of the brain has led some to suppose, that 
particular powers resided in certain emi- 
nences or depre.ssions of the brain ; and this 
is the foundation of the peculiar notions of 
Dr. Gall, whose speculations have attracted 
so much notice. He contends, that the in-- 
equaiities of the brain’s surface are the seat 
of the mental powers, and of the varioui 
