723c 
them serves to establish a communication be- 
tween the centre of intellectual action, and the 
centres of volition and sensation. It is through 
this connection that the intellect and the will 
are capable of mutually affecting each other, 
the intellect prompting or exciting the will; 
and the will, on the other hand, controlling or 
applying the powers of the intellect. The 
faculty of attention, and, therefore, in a certain 
degree, that of memory, are dependent on the 
influence of the centre of volition upon the 
centre of intellectual action. Every one is sen- 
sible of a power which he possesses of fixing 
his attention on any given subject, as distinct 
as that by which he can contract any particular 
muscle. The association of the intellectual 
centre with that of sensation is necessary to en- 
sure the full perception of sensitive impressions. 
The experience of each individual can supply 
him with numberless instances in which, while 
the mind was employed upon some other ob- 
ject of interest, an impression was made upon 
some one of the organs of sense, and indistinctly 
Jelt, but not fully perceived. When the mind 
has become disengaged, the fact that an impres- 
sion had been made is recalled, without any 
ability to recollect its precise nature, And in 
many lunatics the centre of intellectual action 
is so impaired as to destroy or greatly reduce 
the power of perception, whilst there is abun- 
dant evidence to shew that the affections of the 
organs of sense make a sufficient impression on 
the centre of sensation, although in such cases 
this centre may likewise participate in the general 
hebetude. 
Perfect power of speech, that is, of expressing 
our shonchas in suitable language, depends 
upon the due relation between the centre of 
volition and that of intellectual action. The 
latter centre may have full power to frame the 
thought; but, unless it can prompt the will to 
a certain mode of sustained action, the organs 
of speech cannot be brought into play. A loss 
of the power of speech is frequently a precursor 
of more extensive derangement of sensation and 
motion. In some cases the intellect seems 
clear, but the patient is utterly unable to ex- 
press his thoughts ; and in others there is more 
or less of mental confusion. The want of con- 
sent between the centre of intellectual action 
and of volition is equally sppernt in cases of 
this description, from the inability of the patients 
to commit their thoughts to writing. 
The hemispheres of the brain, as has been 
already stated, are insensible to pain from me- 
chanical division or irritation; in wounds of 
the cranium in the human subject, pieces of the 
brain which had protruded have been removed 
without the knowledge of the patient. Never- 
theless, pain is felt in certain lesions of the 
brain, even when seated in the substance of the 
hemispheres, or in the optic thalami or corpora 
striata. This results from the morbid irritation 
extending to other parts with which nerves are 
connected, as the medulla oblongata; or in 
which nerves are distributed, as the membranes. 
The nearer a cerebral lesion is to the membranes 
or to the medulla oblongata, the more likely is 
it to excite pain. Headaches, of whatever na- 
round, and then erect when standing st 
PHYSIOLOGY OF THE NERVOUS SYSTEM. 
ture, must be referred to pe hig sites at 
their centres or at their iphery, of those — 
nerves which are y anrceen 4 "be dura mater 
or in the scalp. The branches of the fifth pair, 
of the occipital nerve, and the auricular brane 
of the cervical plexus, are those most frequently 
affected. hal 
Certain sensations are referred to the he 
ig may occur from a moray state, or maj 
roduced by changes 0 ition in 
we Such are vertigo, a say ness, Or 
of a weight in the head, a feeling of a tig 
cord round the head. These are, no dou 
truly subjective, arising from altered states 
the distribution or in the quality of the 
sent to the brain. A sensation of a»rusl 
blood to the head is often consequent ups 
excessive hemorrhage, or accompanies. d 
of extreme debility from any cause. — ; 
doubtless, owing in great part to the feeble tom 
of the arteries, resisting im ctly the flow 
blood to the head, and allowing it to i 
the nervous matter too much. It is well ku 
that, by turning round quickly on one’ 
axis, the sense of vertigo may be produced 
confused feeling in the head, and an inabi 
to maintain the balance of the body, accom 
nied by an appearance as if external 
were revolving. Ifthe eyes be kept shut, 
uneasy feeling of the head will take, plac 
no true vertigo. To obtain this feeling 
fectly, the eyes must be open, and. objects ] 
sented to them. And Purkinje has shewn 
the direction in which external objects 
to revolve is influenced by the position 
body and of the head while turning round, 
by the position of it afterwards, when the. 
rimenter has ceased to move round... I 
experimenter have kept his head in the ve 
position while moving round, and : 
when standing still, the objects appear to 
volve in the horizontal direction. “at the | 
be held with the occiput upwards while tu 
oa 
— 
objects seem to rotate in a vertical plan 
a wheel placed vertically revolving roun 
axis.* It is highly probable that these : 
tions, as well as those which arise spontane 
are due to some irregular distribution of | 
to various parts of the brain, . A sense ¢ 
diness frequently precedes fainting, and — 
tributable to the temporary deficiency im 
supply of blood to the head. _ If the hon 
age be immediately adopted, or the 
laid with the head inclined downware 
faint may be prevented. The sense of 
ness which is experienced upon rising fro 
horizontal position after illness, is doubt 
the same kind. Anemic patients expe 
this feeling of giddiness even in the hor 
position; and both it and the headach 
delirium, which accompany this state of 
lessness, may be relieved by placing the 
on an inclined plane with the head down 
The mind possesses a remarkable p 
exciting and of exalting painful sensatit 
various parts of the body. If the attentic 
* Miiller’s Physiology, by Baly, vol. i. p. 8 
